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  • 1. Bloch, Natasha
    et al.
    Corral-López, Alberto
    Stockholm University, Faculty of Science, Department of Zoology.
    Buechel, Severine D.
    Stockholm University, Faculty of Science, Department of Zoology.
    Kotrschal, Alexander
    Stockholm University, Faculty of Science, Department of Zoology.
    Kolm, Niclas
    Stockholm University, Faculty of Science, Department of Zoology.
    Mank, Judith E.
    Early neurogenomic response associated with variation in guppy female mate preference2018In: Nature Ecology & Evolution, E-ISSN 2397-334X, Vol. 2, no 11, p. 1772-1781Article in journal (Refereed)
    Abstract [en]

    Understanding the evolution of mate choice requires dissecting the mechanisms of female preference, particularly how these differ among social contexts and preference phenotypes. Here, we studied the female neurogenomic response after only 10 min of mate exposure in both a sensory component (optic tectum) and a decision-making component (telencephalon) of the brain. By comparing the transcriptional response between females with and without preferences for colourful males, we identified unique neurogenomic elements associated with the female preference phenotype that are not present in females without preference. A network analysis revealed different properties for this response at the sensory-processing and the decision-making levels, and we show that this response is highly centralized in the telencephalon. Furthermore, we identified an additional set of genes that vary in expression across social contexts, beyond mate evaluation. We show that transcription factors among these loci are predicted to regulate the transcriptional response of the genes we found to be associated with female preference.

  • 2.
    Buechel, Séverine D.
    et al.
    Stockholm University, Faculty of Science, Department of Zoology.
    Booksmythe, Isobel
    Kotrschal, Alexander
    Stockholm University, Faculty of Science, Department of Zoology.
    Jennions, Michael D.
    Kolm, Niclas
    Stockholm University, Faculty of Science, Department of Zoology.
    Artificial selection on male genitalia length alters female brain size2016In: Proceedings of the Royal Society of London. Biological Sciences, ISSN 0962-8452, E-ISSN 1471-2954, Vol. 283, no 1843, article id 20161796Article in journal (Refereed)
    Abstract [en]

    Male harassment is a classic example of how sexual conflict over mating leads to sex-specific behavioural adaptations. Females often suffer significant costs from males attempting forced copulations, and the sexes can be in an arms race over male coercion. Yet, despite recent recognition that divergent sex-specific interests in reproduction can affect brain evolution, sexual conflict has not been addressed in this context. Here, we investigate whether artificial selection on a correlate of male success at coercion, genital length, affects brain anatomy in males and females. We analysed the brains of eastern mosquitofish (Gambusia holbrooki), which had been artificially selected for long or short gonopodium, thereby mimicking selection arising from differing levels of male harassment. By analogy to how prey species often have relatively larger brains than their predators, we found that female, but not male, brain size was greater following selection for a longer gonopodium. Brain subregion volumes remained unchanged. These results suggest that there is a positive genetic correlation between male gonopodium length and female brain size, which is possibly linked to increased female cognitive ability to avoid male coercion. We propose that sexual conflict is an important factor in the evolution of brain anatomy and cognitive ability.

  • 3.
    Buechel, Séverine D.
    et al.
    Stockholm University, Faculty of Science, Department of Zoology.
    Boussard, Annika
    Stockholm University, Faculty of Science, Department of Zoology.
    Kotrschal, Alexander
    Stockholm University, Faculty of Science, Department of Zoology.
    van der Bijl, Wouter
    Stockholm University, Faculty of Science, Department of Zoology.
    Kolm, Niclas
    Stockholm University, Faculty of Science, Department of Zoology.
    Brain size affects performance in a reversal-learning test2018In: Proceedings of the Royal Society of London. Biological Sciences, ISSN 0962-8452, E-ISSN 1471-2954, Vol. 285, no 1871, article id 20172031Article in journal (Refereed)
    Abstract [en]

    It has become increasingly clear that a larger brain can confer cognitive benefits. Yet not all of the numerous aspects of cognition seem to be affected by brain size. Recent evidence suggests that some more basic forms of cognition, for instance colour vision, are not influenced by brain size. We therefore hypothesize that a larger brain is especially beneficial for distinct and gradually more complex aspects of cognition. To test this hypothesis, we assessed the performance of brain size selected female guppies (Poecilia reticulata) in two distinct aspects of cognition that differ in cognitive complexity. In a standard reversal-learning test we first investigated basic learning ability with a colour discrimination test, then reversed the reward contingency to specifically test for cognitive flexibility. We found that large-brained females outperformed small-brained females in the reversed-learning part of the test but not in the colour discrimination part of the test. Large-brained individuals are hence cognitively more flexible, which probably yields fitness benefits, as they may adapt more quickly to social and/or ecological cognitive challenges. Our results also suggest that a larger brain becomes especially advantageous with increasing cognitive complexity. These findings corroborate the significance of brain size for cognitive evolution.

  • 4. Chen, Yu-Chia
    et al.
    Harrison, Peter W.
    Kotrschal, Alexander
    Stockholm University, Faculty of Science, Department of Zoology, Ethology. Uppsala University, Sweden.
    Kolm, Niclas
    Stockholm University, Faculty of Science, Department of Zoology, Ethology. Uppsala University, Sweden.
    Mank, Judith E.
    Panula, Pertti
    Expression change in Angiopoietin-1 underlies change in relative brain size in fish2015In: Proceedings of the Royal Society of London. Biological Sciences, ISSN 0962-8452, E-ISSN 1471-2954, Vol. 282, no 1810, article id 20150872Article in journal (Refereed)
    Abstract [en]

    Brain size varies substantially across the animal kingdom and is often associated with cognitive ability; however, the genetic architecture underpinning natural variation in these key traits is virtually unknown. In order to identify the genetic architecture and loci underlying variation in brain size, we analysed both coding sequence and expression for all the loci expressed in the telencephalon in replicate populations of guppies (Poecilia reticulata) artificially selected for large and small relative brain size. A single gene, Angiopoietin-1 (Ang-1), a regulator of angiogenesis and suspected driver of neural development, was differentially expressed between large-and small-brain populations. Zebra fish (Danio rerio) morphants showed that mild knock down of Ang-1 produces a small-brained phenotype that could be rescued with Ang-1 mRNA. Translation inhibition of Ang-1 resulted in smaller brains in larvae and increased expression of Notch-1, which regulates differentiation of neural stem cells. In situ analysis of newborn large-and small-brained guppies revealed matching expression patterns of Ang-1 and Notch-1 to those observed in zebrafish larvae. Taken together, our results suggest that the genetic architecture affecting brain size in our population may be surprisingly simple, and Ang-1 may be a potentially important locus in the evolution of vertebrate brain size and cognitive ability.

  • 5.
    Corral-López, Alberto
    et al.
    Stockholm University, Faculty of Science, Department of Zoology.
    Bloch, Natasha I.
    Kotrschal, Alexander
    Stockholm University, Faculty of Science, Department of Zoology.
    van der Bijl, Wouter
    Stockholm University, Faculty of Science, Department of Zoology.
    Buechel, Severine D.
    Stockholm University, Faculty of Science, Department of Zoology.
    Mank, Judith E.
    Kolm, Niclas
    Stockholm University, Faculty of Science, Department of Zoology.
    Female brain size affects the assessment of male attractiveness during mate choice2017In: Science Advances, ISSN 0036-8156, E-ISSN 2375-2548, Vol. 3, no 3, article id e1601990Article in journal (Refereed)
    Abstract [en]

    Mate choice decisions are central in sexual selection theory aimed to understand how sexual traits evolve and their role in evolutionary diversification. We test the hypothesis that brain size and cognitive ability are important for accurate assessment of partner quality and that variation in brain size and cognitive ability underlies variation in mate choice. We compared sexual preference in guppy female lines selected for divergence in relative brain size, which we have previously shown to have substantial differences in cognitive ability. In a dichotomous choice test, large-brained and wild-type females showed strong preference for males with color traits that predict attractiveness in this species. In contrast, small-brained females showed no preference for males with these traits. In-depth analysis of optomotor response to color cues and gene expression of key opsins in the eye revealed that the observed differences were not due to differences in visual perception of color, indicating that differences in the ability to process indicators of attractiveness are responsible. We thus provide the first experimental support that individual variation in brain size affects mate choice decisions and conclude that differences in cognitive ability may be an important underlying mechanism behind variation in female mate choice.

  • 6.
    Corral-López, Alberto
    et al.
    Stockholm University, Faculty of Science, Department of Zoology.
    Eckerström-Liedholm, Simon
    Stockholm University, Faculty of Science, Department of Zoology.
    Van der Bijl, Wouter
    Stockholm University, Faculty of Science, Department of Zoology.
    Kotrschal, Alexander
    Stockholm University, Faculty of Science, Department of Zoology.
    Kolm, Niclas
    Stockholm University, Faculty of Science, Department of Zoology.
    No association between brain size and male sexual behavior in the guppy2015In: Current Zoology, ISSN 1674-5507, Vol. 61, no 2, p. 265-273Article in journal (Refereed)
    Abstract [en]

    Animal behavior is remarkably variable at all taxonomic levels. Over the last decades, research on animal behavior has focused on understanding ultimate processes. Yet, it has progressively become more evident that to fully understand behavioral variation, ultimate explanations need to be complemented with proximate ones. In particular, the mechanisms generating variation in sexual behavior remain an open question. Variation in aspects of brain morphology has been suggested as a plausible mechanism underlying this variation. However, our knowledge of this potential association is based almost exclusively on comparative analyses. Experimental studies are needed to establish causality and bridge the gap between micro-and macroevolutionary mechanisms concerning the link between brain and sexual behavior. We used male guppies that had been artificially selected for large or small relative brain size to study this association. We paired males with females and scored the full known set of male and female sexual behaviors described in guppies. We found several previously demonstrated associations between male traits, male behavior and female behavior. Females responded more strongly towards males that courted more and males with more orange coloration. Also, larger males and males with less conspicuous coloration attempted more coerced copulations. However, courting, frequency of coerced copulation attempts, total intensity of sexual behavior, and female response did not differ between large-and small-brained males. Our data suggest that relative brain size is an unlikely mechanism underlying variation in sexual behavior of the male guppy. We discuss these findings in the context of the conditions under which relative brain size might affect male sexual behavior

  • 7.
    Corral-López, Alberto
    et al.
    Stockholm University, Faculty of Science, Department of Zoology.
    Garate-Olaizola, Maddi
    Stockholm University, Faculty of Science, Department of Zoology.
    Buechel, Severine D.
    Stockholm University, Faculty of Science, Department of Zoology.
    Kolm, Niclas
    Stockholm University, Faculty of Science, Department of Zoology.
    Kotrschal, Alexander
    Stockholm University, Faculty of Science, Department of Zoology.
    On the role of body size, brain size, and eye size in visual acuity2017In: Behavioral Ecology and Sociobiology, ISSN 0340-5443, E-ISSN 1432-0762, Vol. 71, no 12, article id UNSP 179Article in journal (Refereed)
    Abstract [en]

    The visual system is highly variable across species, and such variability is a key factor influencing animal behavior. Variation in the visual system, for instance, can influence the outcome of learning tasks when visual stimuli are used. We illustrate this issue in guppies (Poecilia reticulata) artificially selected for large and small relative brain size with pronounced behavioral differences in learning experiments and mate choice tests. We performed a study of the visual system by quantifying eye size and optomotor response of large-brained and small-brained guppies. This represents the first experimental test of the link between brain size evolution and visual acuity. We found that female guppies have larger eyes than male guppies, both in absolute terms and in relation to their body size. Likewise, individuals selected for larger brains had slightly larger eyes but not better visual acuity than small-brained guppies. However, body size was positively associated with visual acuity. We discuss our findings in relation to previous macroevolutionary studies on the evolution of brain morphology, eye morphology, visual acuity, and ecological variables, while stressing the importance of accounting for sensory abilities in behavioral studies.

  • 8.
    Corral-López, Alberto
    et al.
    Stockholm University, Faculty of Science, Department of Zoology.
    Garate-Olaizola, Maddi
    Buechel, Severine
    Stockholm University, Faculty of Science, Department of Zoology.
    Kolm, Niclas
    Stockholm University, Faculty of Science, Department of Zoology.
    Kotrschal, Alexander
    Stockholm University, Faculty of Science, Department of Zoology.
    On the role of body size, brain size and eye size in visual acuityManuscript (preprint) (Other academic)
  • 9.
    Corral-López, Alberto
    et al.
    Stockholm University, Faculty of Science, Department of Zoology.
    Kotrschal, Alexander
    Stockholm University, Faculty of Science, Department of Zoology.
    Kolm, Niclas
    Stockholm University, Faculty of Science, Department of Zoology.
    Brain size affects the judgment of female quality during male mate choiceManuscript (preprint) (Other academic)
  • 10.
    Corral-López, Alberto
    et al.
    Stockholm University, Faculty of Science, Department of Zoology.
    Kotrschal, Alexander
    Stockholm University, Faculty of Science, Department of Zoology.
    Kolm, Niclas
    Stockholm University, Faculty of Science, Department of Zoology.
    Selection for relative brain size affects context-dependent male preference for, but not discrimination of, female body size in guppies2018In: Journal of Experimental Biology, ISSN 0022-0949, E-ISSN 1477-9145, Vol. 221, no 12, article id jeb175240Article in journal (Refereed)
    Abstract [en]

    Understanding what drives animal decisions is fundamental in evolutionary biology, and mate choice decisions are arguably some of the most important in any individual's life. As cognitive ability can impact decision making, elucidating the link between mate choice and cognitive ability is necessary to fully understand mate choice. To experimentally study this link, we used guppies (Poecilia reticulata) artificially selected for divergence in relative brain size and with previously demonstrated differences in cognitive ability. A previous test in our female guppy selection lines demonstrated the impact of brain size and cognitive ability on information processing during female mate choice decisions. Here, we evaluated the effect of brain size and cognitive ability on male mate choice decisions. Specifically, we investigated the preference of large-brained, small-brained and non-selected guppy males for female body size, a key indicator of female fecundity in this species. For this, male preference was quantified in dichotomous choice tests when presented with dyads of females with small, medium and large body size differences. All types of males showed a preference for larger females but no effect of brain size was found in the ability to discriminate between differently sized females. However, we found that non-selected and large-brained males, but not small-brained males, showed a context-dependent preference for larger females depending on the difference in female size. Our results have two important implications. First, they provide further evidence that male mate choice also occurs in a species in which secondary sexual omamentation is present only in males. Second, they show that brain size and cognitive ability have important effects on individual variation in mating preference and sexually selected traits.

  • 11.
    Corral-López, Alberto
    et al.
    Stockholm University, Faculty of Science, Department of Zoology. University College of London, UK.
    Romensky, Maksym
    Kotrschal, Alexander
    Stockholm University, Faculty of Science, Department of Zoology. Wageningen University, The Netherlands.
    Buechel, Severine D.
    Stockholm University, Faculty of Science, Department of Zoology.
    Kolm, Niclas
    Stockholm University, Faculty of Science, Department of Zoology.
    Brain size affects responsiveness in mating behaviour to variation in predation pressure and sex ratio2020In: Journal of Evolutionary Biology, ISSN 1010-061X, E-ISSN 1420-9101, Vol. 33, no 2, p. 165-177Article in journal (Refereed)
    Abstract [en]

    Despite ongoing advances in sexual selection theory, the evolution of mating decisions remains enigmatic. Cognitive processes often require simultaneous processing of multiple sources of information from environmental and social cues. However, little experimental data exist on how cognitive ability affects such fitness-associated aspects of behaviour. Using advanced tracking techniques, we studied mating behaviours of guppies artificially selected for divergence in relative brain size, with known differences in cognitive ability, when predation threat and sex ratio was varied. In females, we found a general increase in copulation behaviour in when the sex ratio was female biased, but only large-brained females responded with greater willingness to copulate under a low predation threat. In males, we found that small-brained individuals courted more intensively and displayed more aggressive behaviours than large-brained individuals. However, there were no differences in female response to males with different brain size. These results provide further evidence of a role for female brain size in optimal decision-making in a mating context. In addition, our results indicate that brain size may affect mating display skill in male guppies. We suggest that it is important to consider the association between brain size, cognitive ability and sexual behaviour when studying how morphological and behavioural traits evolve in wild populations.

  • 12.
    Corral-López, Alberto
    et al.
    Stockholm University, Faculty of Science, Department of Zoology.
    Romensky, Maksym
    Kotrschal, Alexander
    Stockholm University, Faculty of Science, Department of Zoology.
    Buechel, Severine
    Stockholm University, Faculty of Science, Department of Zoology.
    Kolm, Niclas
    Stockholm University, Faculty of Science, Department of Zoology.
    Brain size, environmental complexity and mating behaviourManuscript (preprint) (Other academic)
  • 13. Fischer, Stefan
    et al.
    Bessert-Nettelbeck, Mathilde
    Kotrschal, Alexander
    Stockholm University, Faculty of Science, Department of Zoology, Ethology.
    Taborsky, Barbara
    Rearing-Group Size Determines Social Competence and Brain Structure in a Cooperatively Breeding Cichlid2015In: American Naturalist, ISSN 0003-0147, E-ISSN 1537-5323, Vol. 186, no 1, p. 123-140Article in journal (Refereed)
    Abstract [en]

    Social animals can greatly benefit from well-developed social skills. Because the frequency and diversity of social interactions often increase with the size of social groups, the benefits of advanced social skills can be expected to increase with group size. Variation in social skills often arises during ontogeny, depending on early social experience. Whether variation of social-group sizes affects development of social skills and related changes in brain structures remains unexplored. We investigated whether, in a cooperatively breeding cichlid, early group size (1) shapes social behavior and social skills and (2) induces lasting plastic changes in gross brain structures and (3) whether the development of social skills is confined to a sensitive ontogenetic period. Rearing-group size and the time juveniles spent in these groups interactively influenced the development of social skills and the relative sizes of four main brain regions. We did not detect a sensitive developmental period for the shaping of social behavior within the 2-month experience phase. Instead, our results suggest continuous plastic behavioral changes over time. We discuss how developmental effects on social behavior and brain architecture may adaptively tune phenotypes to their current or future environments.

  • 14.
    Fong, Stephanie
    et al.
    Stockholm University, Faculty of Science, Department of Zoology.
    Buechel, Séverine D.
    Stockholm University, Faculty of Science, Department of Zoology.
    Boussard, Annika
    Stockholm University, Faculty of Science, Department of Zoology.
    Kotrschal, Alexander
    Stockholm University, Faculty of Science, Department of Zoology.
    Kolm, Niclas
    Stockholm University, Faculty of Science, Department of Zoology.
    Plastic changes in brain morphology in relation to learning and environmental enrichment in the guppy (Poecilia reticulata)2019In: Journal of Experimental Biology, ISSN 0022-0949, E-ISSN 1477-9145, Vol. 222, no 10, article id UNSP jeb200402Article in journal (Refereed)
    Abstract [en]

    Despite the common assumption that the brain is malleable to surrounding conditions mainly during ontogeny, plastic neural changes can occur also in adulthood. One of the driving forces responsible for alterations in brain morphology is increasing environmental complexity that may demand enhanced cognitive abilities (e.g. attention, memory and learning). However, studies looking at the relationship between brain morphology and learning are scarce. Here, we tested the effects of both learning and environmental enrichment on neural plasticity in guppies (Poecilia reticulata), by means of either a reversal-learning test or a spatial-learning test. Given considerable evidence supporting environmentally induced plastic alterations, two separate control groups that were not subjected to any cognitive test were included to account for potential changes induced by the experimental setup alone. We did not find any effect of learning on any of our brain measurements. However, we found strong evidence for an environmental effect, where fish given access to the spatial-learning environment had larger relative brain size and optic tectum size in relation to those exposed to the reversal-learning environment. Our results demonstrate the plasticity of the adult brain to respond adaptively mainly to environmental conditions, providing support for the environmental enhancement theory.

  • 15. Hayward, A.
    et al.
    Tsuboi, M.
    Owusu, C.
    Kotrschal, Alexander
    Stockholm University, Faculty of Science, Department of Zoology.
    Buechel, Severine D.
    Stockholm University, Faculty of Science, Department of Zoology.
    Zidar, J.
    Cornwallis, C. K.
    Lovlie, H.
    Kolm, Niclas
    Stockholm University, Faculty of Science, Department of Zoology.
    Evolutionary associations between host traits and parasite load: insights from Lake Tanganyika cichlids2017In: Journal of Evolutionary Biology, ISSN 1010-061X, E-ISSN 1420-9101, Vol. 30, no 6, p. 1056-1067Article in journal (Refereed)
    Abstract [en]

    Parasite diversity and abundance (parasite load) vary greatly among host species. However, the influence of host traits on variation in parasitism remains poorly understood. Comparative studies of parasite load have largely examined measures of parasite species richness and are predominantly based on records obtained from published data. Consequently, little is known about the relationships between host traits and other aspects of parasite load, such as parasite abundance, prevalence and aggregation. Meanwhile, understanding of parasite species richness may be clouded by limitations associated with data collation from multiple independent sources. We conducted a field study of Lake Tanganyika cichlid fishes and their helminth parasites. Using a Bayesian phylogenetic comparative framework, we tested evolutionary associations between five key host traits (body size, gut length, diet breadth, habitat complexity and number of sympatric hosts) predicted to influence parasitism, together with multiple measures of parasite load. We find that the number of host species that a particular host may encounter due to its habitat preferences emerges as a factor of general importance for parasite diversity, abundance and prevalence, but not parasite aggregation. In contrast, body size and gut size are positively related to aspects of parasite load within, but not between species. The influence of host phylogeny varies considerably among measures of parasite load, with the greatest influence exerted on parasite diversity. These results reveal that both host morphology and biotic interactions are key determinants of host-parasite associations and that consideration of multiple aspects of parasite load is required to fully understand patterns in parasitism.

  • 16.
    Herbert-Read, James E.
    et al.
    Stockholm University, Faculty of Science, Department of Zoology. Uppsala University, Sweden.
    Rosén, Emil
    Szorkovszky, Alex
    Ioannou, Christos C.
    Rogell, Björn
    Stockholm University, Faculty of Science, Department of Zoology.
    Perna, Andrea
    Ramnarine, Indar W.
    Kotrschal, Alexander
    Stockholm University, Faculty of Science, Department of Zoology.
    Kolm, Niclas
    Stockholm University, Faculty of Science, Department of Zoology.
    Krause, Jens
    Sumpter, David J. T.
    How predation shapes the social interaction rules of shoaling fish2017In: Proceedings of the Royal Society of London. Biological Sciences, ISSN 0962-8452, E-ISSN 1471-2954, Vol. 284, no 1861, article id 20171126Article in journal (Refereed)
    Abstract [en]

    Predation is thought to shape the macroscopic properties of animal groups, making moving groups more cohesive and coordinated. Precisely how predation has shaped individuals' fine-scale social interactions in natural populations, however, is unknown. Using high-resolution tracking data of shoaling fish (Poecilia reticulata) from populations differing in natural predation pressure, we show how predation adapts individuals' social interaction rules. Fish originating from high predation environments formed larger, more cohesive, but not more polarized groups than fish from low predation environments. Using a new approach to detect the discrete points in time when individuals decide to update their movements based on the available social cues, we determine how these collective properties emerge from individuals' microscopic social interactions. We first confirm predictions that predation shapes the attraction-repulsion dynamic of these fish, reducing the critical distance at which neighbours move apart, or come back together. While we find strong evidence that fish align with their near neighbours, we do not find that predation shapes the strength or likelihood of these alignment tendencies. We also find that predation sharpens individuals' acceleration and deceleration responses, implying key perceptual and energetic differences associated with how individuals move in different predation regimes. Our results reveal how predation can shape the social interactions of individuals in groups, ultimately driving differences in groups' collective behaviour.

  • 17. Herczeg, Gábor
    et al.
    Urszán, Tamás J.
    Orf, Stephanie
    Nagy, Gergely
    Kotrschal, Alexander
    Stockholm University, Faculty of Science, Department of Zoology.
    Kolm, Niclas
    Stockholm University, Faculty of Science, Department of Zoology.
    Brain size predicts behavioural plasticity in guppies (Poecilia reticulata): An experiment2019In: Journal of Evolutionary Biology, ISSN 1010-061X, E-ISSN 1420-9101, Vol. 32, no 3, p. 218-226Article in journal (Refereed)
    Abstract [en]

    Understanding how animal personality (consistent between-individual behavioural differences) arises has become a central topic in behavioural sciences. This endeavour is complicated by the fact that not only the mean behaviour of individuals (behavioural type) but also the strength of their reaction to environmental change (behavioural plasticity) varies consistently. Personality and cognitive abilities are linked, and we suggest that behavioural plasticity could also be explained by differences in brain size (a proxy for cognitive abilities), since accurate decisions are likely essential to make behavioural plasticity beneficial. We test this idea in guppies (Poecilia reticulata), artificially selected for large and small brain size, which show clear cognitive differences between selection lines. To test whether those lines differed in behavioural plasticity, we reared them in groups in structurally enriched environments and then placed adults individually into empty tanks, where we presented them daily with visual predator cues and monitored their behaviour for 20 days with video-aided motion tracking. We found that individuals differed consistently in activity and risk-taking, as well as in behavioural plasticity. In activity, only the large-brained lines demonstrated habituation (increased activity) to the new environment, whereas in risk-taking, we found sensitization (decreased risk-taking) in both brain size lines. We conclude that brain size, potentially via increasing cognitive abilities, may increase behavioural plasticity, which in turn can improve habituation to novel environments. However, the effects seem to be behaviour-specific. Our results suggest that brain size likely explains some of the variation in behavioural plasticity found at the intraspecific level.

  • 18. Herczeg, Gábor
    et al.
    Urszán, Tamás János
    Orf, Stephanie
    Nagy, Gergely
    Kotrschal, Alexander
    Stockholm University, Faculty of Science, Department of Zoology.
    Kolm, Niclas
    Stockholm University, Faculty of Science, Department of Zoology.
    Yes, correct context is indeed the key: An answer to Haave-Audet et al. 20192019In: Journal of Evolutionary Biology, ISSN 1010-061X, E-ISSN 1420-9101, Vol. 32, no 12, p. 1450-1455Article in journal (Refereed)
    Abstract [en]

    We published a study recently testing the link between brain size and behavioural plasticity using brain size selected guppy (Poecilia reticulata) lines (2019, Journal of Evolutionary Biology, 32, 218-226). Only large-brained fish showed habituation to a new, but actually harmless environment perceived as risky, by increasing movement activity over the 20-day observation period. We concluded that Our results suggest that brain size likely explains some of the variation in behavioural plasticity found at the intraspecific level. In a commentary published in the same journal, Haave-Audet et al. challenged the main message of our study, stating that (a) relative brain size is not a suitable proxy for cognitive ability and (b) habituation measured by us is likely not adaptive and costly. In our response, we first show that a decade's work has proven repeatedly that relative brain size is indeed positively linked to cognitive performance in our model system. Second, we discuss how switching from stressed to unstressed behaviour in stressful situations without real risk is likely adaptive. Finally, we point out that the main cost of behavioural plasticity in our case is the development and maintenance of the neural system needed for information processing, and not the expression of plasticity. We hope that our discussion with Haave-Audet et al. helps clarifying some central issues in this emerging research field.

  • 19. Huang, Chun Hua
    et al.
    Zhong, Mao Jun
    Liao, Wen Bo
    Kotrschal, Alexander
    Stockholm University, Faculty of Science, Department of Zoology.
    Investigating the role of body size, ecology, and behavior in anuran eye size evolution2019In: Evolutionary Ecology, ISSN 0269-7653, E-ISSN 1573-8477, Vol. 33, no 4, p. 585-598Article in journal (Refereed)
    Abstract [en]

    Vertebrate eye size typically scales hypoallemetrically with body size-as animals grow larger their eyes get relatively smaller. Additionally, eye size is highly variable across species, and such variability often reflects functional adaptations to differences in behavior and/or ecology. The selective pressures underlying the evolution of eye size are especially well studied in birds, mammals, and fishes. However, whether similar scaling rules and selective pressures also underlie the evolution of eye size in amphibians remains enigmatic. Variation in eye size is intimately linked with variation in brain anatomy, as the retina is ontogenetically part of the brain. Eye size may therefore coevolve with brain size. Here we use phylogenetic comparative methods to study interspecific variation in eye volume across 44 species of anurans from 8 families from the Hengduan Mountains, China. We relate this variation to key factors known to impact eye size evolution in other vertebrate taxa such as body mass, habitat use, defense strategy and foraging mobility. We found that also in anurans eyes size scaled hypoallometrically with body mass. However, neither of the behavioral or ecological factors explained any variation in relative eye size in our sample. Whether this is representative for other frog species needs to be clarified. We therefore conclude that eye size in frogs is tightly linked to body mass evolution but that, at least in the species investigated here, none of our tested ecological and behavioral factors have a strong influence on eye size evolution.

  • 20.
    Kotrschal, Alexander
    et al.
    Stockholm University, Faculty of Science, Department of Zoology, Ethology. University of Veterinary Medicine, Austria.
    Buechel, Séverine D.
    Stockholm University, Faculty of Science, Department of Zoology, Ethology. University of Veterinary Medicine, Austria.
    Zala, Sarah M.
    Corral Lopez, Alberto
    Stockholm University, Faculty of Science, Department of Zoology, Ethology.
    Penn, Dustin J.
    Kolm, Niclas
    Stockholm University, Faculty of Science, Department of Zoology, Ethology.
    Brain size affects female but not male survival under predation threat2015In: Ecology Letters, ISSN 1461-023X, E-ISSN 1461-0248, Vol. 18, no 7, p. 646-652Article in journal (Refereed)
    Abstract [en]

    There is remarkable diversity in brain size among vertebrates, but surprisingly little is known about how ecological species interactions impact the evolution of brain size. Using guppies, artificially selected for large and small brains, we determined how brain size affects survival under predation threat in a naturalistic environment. We cohoused mixed groups of small- and large-brained individuals in six semi-natural streams with their natural predator, the pike cichlid, and monitored survival in weekly censuses over 5 months. We found that large-brained females had 13.5% higher survival compared to small-brained females, whereas the brain size had no discernible effect on male survival. We suggest that large-brained females have a cognitive advantage that allows them to better evade predation, whereas large-brained males are more colourful, which may counteract any potential benefits of brain size. Our study provides the first experimental evidence that trophic interactions can affect the evolution of brain size.

  • 21.
    Kotrschal, Alexander
    et al.
    Stockholm University, Faculty of Science, Department of Zoology, Ethology. Uppsala University, Sweden.
    Corral-Lopez, Alberto
    Stockholm University, Faculty of Science, Department of Zoology, Ethology.
    Amcoff, Mirjam
    Kolm, Niclas
    Stockholm University, Faculty of Science, Department of Zoology, Ethology. Uppsala University, Sweden.
    A larger brain confers a benefit in a spatial mate search learning task in male guppies2015In: Behavioral Ecology, ISSN 1045-2249, E-ISSN 1465-7279, Vol. 26, no 2, p. 527-532Article in journal (Refereed)
    Abstract [en]

    Brain size varies dramatically among vertebrates, and selection for increased cognitive abilities is thought to be the key force underlying the evolution of a large brain. Indeed, numerous comparative studies suggest positive relationships between cognitively demanding aspects of behavior and brain size controlled for body size. However, experimental evidence for the link between relative brain size and cognitive ability is surprisingly scarce and to date stems from a single study on brain size selected guppies (Poecilia reticulata), where large-brained females were shown to outperform small-brained females in a numerical learning assay. Because the results were inconclusive for males in that study, we here use a more ecologically relevant test of male cognitive ability to investigate whether or not a relatively larger brain increases cognitive ability also in males. We compared mate search ability of these artificially selected large-and small-brained males in a maze and found that large-brained males were faster at learning to find a female in a maze. Large-brained males decreased the time spent navigating the maze faster than small-brained males and were nearly twice as fast through the maze after 2 weeks of training. Our results support that relatively larger brains are better also for males in some contexts, which further substantiates that variation in vertebrate brain size is generated through the balance between energetic costs and cognitive benefits.

  • 22.
    Kotrschal, Alexander
    et al.
    Stockholm University, Faculty of Science, Department of Zoology.
    Corral-Lopez, Alberto
    Stockholm University, Faculty of Science, Department of Zoology.
    Kolm, Niclas
    Stockholm University, Faculty of Science, Department of Zoology.
    Large brains, short life: selection on brain size impacts intrinsic lifespan2019In: Biology Letters, ISSN 1744-9561, E-ISSN 1744-957X, Vol. 15, no 5, article id 20190137Article in journal (Refereed)
    Abstract [en]

    The relationship between brain size and ageing is a paradox. The cognitive benefits of large brains should protect from extrinsic mortality and thus indirectly select for slower ageing. However, the substantial energetic cost of neural tissue may also impact the energetic budget of large-brained organisms, causing less investment in somatic maintenance and thereby faster ageing. While the positive association between brain size and survival in the wild is well established, no studies exist on the direct effects of brain size on ageing. Here we test how brain size influences intrinsic ageing in guppy (Poecilia reticulata) brain size selection lines with 12% difference in relative brain size. Measuring survival under benign conditions, we find that large-brained animals live 22% shorter than small-brained animals and the effect is similar in both males and females. Our results suggest a trade-off between investment into brain size and somatic maintenance. This implies that the link between brain size and ageing is contingent on the mechanism of mortality, and selection for positive correlations between brain size and ageing should occur mainly under cognition-driven survival benefits from increased brain size. We show that accelerated ageing can be a cost of evolving a larger brain.

  • 23.
    Kotrschal, Alexander
    et al.
    Stockholm University, Faculty of Science, Department of Zoology, Ethology.
    Corral-Lopez, Alberto
    Stockholm University, Faculty of Science, Department of Zoology, Ethology.
    Szidat, Soenke
    Kolm, Niclas
    Stockholm University, Faculty of Science, Department of Zoology, Ethology.
    The effect of brain size evolution on feeding propensity, digestive efficiency, and juvenile growth2015In: Evolution, ISSN 0014-3820, E-ISSN 1558-5646, Vol. 69, no 11, p. 3013-3020Article in journal (Refereed)
    Abstract [en]

    One key hypothesis in the study of brain size evolution is the expensive tissue hypothesis; the idea that increased investment into the brain should be compensated by decreased investment into other costly organs, for instance the gut. Although the hypothesis is supported by both comparative and experimental evidence, little is known about the potential changes in energetic requirements or digestive traits following such evolutionary shifts in brain and gut size. Organisms may meet the greater metabolic requirements of larger brains despite smaller guts via increased food intake or better digestion. But increased investment in the brain may also hamper somatic growth. To test these hypotheses we here used guppy (Poecilia reticulata) brain size selection lines with a pronounced negative association between brain and gut size and investigated feeding propensity, digestive efficiency (DE), and juvenile growth rate. We did not find any difference in feeding propensity or DE between large-and small-brained individuals. Instead, we found that large-brained females had slower growth during the first 10 weeks after birth. Our study provides experimental support that investment into larger brains at the expense of gut tissue carries costs that are not necessarily compensated by a more efficient digestive system.

  • 24.
    Kotrschal, Alexander
    et al.
    Stockholm University, Faculty of Science, Department of Zoology. Uppsala University, Sweden.
    Corral-Lopez, Alberto
    Stockholm University, Faculty of Science, Department of Zoology.
    Zajitschek, S.
    Immler, S.
    Maklakov, A. A.
    Kolm, Niclas
    Stockholm University, Faculty of Science, Department of Zoology.
    Positive genetic correlation between brain size and sexual traits in male guppies artificially selected for brain size2015In: Journal of Evolutionary Biology, ISSN 1010-061X, E-ISSN 1420-9101, Vol. 28, no 4, p. 841-850Article in journal (Refereed)
    Abstract [en]

    Brain size is an energetically costly trait to develop and maintain. Investments into other costly aspects of an organism's biology may therefore place important constraints on brain size evolution. Sexual traits are often costly and could therefore be traded off against neural investment. However, brain size may itself be under sexual selection through mate choice on cognitive ability. Here, we use guppy (Poecilia reticulata) lines selected for large and small brain size relative to body size to investigate the relationship between brain size, a large suite of male primary and secondary sexual traits, and body condition index. We found no evidence for trade-offs between brain size and sexual traits. Instead, larger-brained males had higher expression of several primary and precopulatory sexual traits - they had longer genitalia, were more colourful and developed longer tails than smaller-brained males. Larger-brained males were also in better body condition when housed in single-sex groups. There was no difference in post-copulatory sexual traits between males from the large- and small-brained lines. Our data do not support the hypothesis that investment into sexual traits is an important limiting factor to brain size evolution, but instead suggest that brain size and several sexual traits are positively genetically correlated.

  • 25.
    Kotrschal, Alexander
    et al.
    Stockholm University, Faculty of Science, Department of Zoology.
    Deacon, Amy E.
    Magurran, Anne E.
    Kolm, Niclas
    Stockholm University, Faculty of Science, Department of Zoology.
    Predation pressure shapes brain anatomy in the wild2017In: Evolutionary Ecology, ISSN 0269-7653, E-ISSN 1573-8477, Vol. 31, no 5, p. 619-633Article in journal (Refereed)
    Abstract [en]

    There is remarkable diversity in brain anatomy among vertebrates and evidence is accumulating that predatory interactions are crucially important for this diversity. To test this hypothesis, we collected female guppies (Poecilia reticulata) from 16 wild populations and related their brain anatomy to several aspects of predation pressure in this ecosystem, such as the biomass of the four major predators of guppies (one prawn and three fish species), and predator diversity (number of predatory fish species in each site). We found that populations from localities with higher prawn biomass had relatively larger telencephalon size as well as larger brains. Optic tectum size was positively associated with one of the fish predator's biomass and with overall predator diversity. However, both olfactory bulb and hypothalamus size were negatively associated with the biomass of another of the fish predators. Hence, while fish predator occurrence is associated with variation in brain anatomy, prawn occurrence is associated with variation in brain size. Our results suggest that cognitive challenges posed by local differences in predator communities may lead to changes in prey brain anatomy in the wild.

  • 26.
    Kotrschal, Alexander
    et al.
    Stockholm University, Faculty of Science, Department of Zoology. University of Veterinary Medicine, Austria.
    Kolm, Niclas
    Stockholm University, Faculty of Science, Department of Zoology.
    Penn, Dustin J.
    Selection for brain size impairs innate, but not adaptive immune responses2016In: Proceedings of the Royal Society of London. Biological Sciences, ISSN 0962-8452, E-ISSN 1471-2954, Vol. 283, no 1826, article id 20152857Article in journal (Refereed)
    Abstract [en]

    Both the brain and the imnume system are energetically demanding organs, and when natural selection favours increased investment into one, then the size or performance of the other should be reduced. While comparative analyses have attempted to test this potential evolutionary trade-off, the results remain inconclusive. To test this hypothesis, we compared the tissue graft rejection (an assay for measuring innate and acquired immune responses) in guppies (Poecilia reticulata) artificially selected for large and small relative brain size. Individual scales were transplanted between pairs of fish, creating reciprocal allografts, and the rejection reaction was scored over 8 days (before acquired immunity develops). Acquired immune responses were tested two weeks later, when the same pairs of fish received a second set of allografts and were scored again. Compared with large-brained animals, small-brained animals of both sexes mounted a significantly stronger rejection response to the first allograft. The rejection response to the second set of allografts did not differ between large- and small-brained fish. Our results show that selection for large brain size reduced innate immune responses to an allograft, which supports the hypothesis that there is a selective trade-off between investing into brain size and innate immunity.

  • 27.
    Kotrschal, Alexander
    et al.
    Department of Ecology & Genetics/Animal Ecology, Uppsala University, Sweden.
    Lievens, Eva J. P.
    Dahlbom, Josefin
    Bundsen, Andreas
    Semenova, Svetlana
    Sundvik, Maria
    Maklakov, Alexei A.
    Winberg, Svante
    Panula, Pertti
    Kolm, Niclas
    Stockholm University, Faculty of Science, Department of Zoology, Ethology.
    Artificial selection on relative brain size reveals a positive genetic correlation between brain size and proactive personality in the guppy2014In: Evolution, ISSN 0014-3820, E-ISSN 1558-5646, Vol. 68, no 4, p. 1139-1149Article in journal (Refereed)
    Abstract [en]

    Animal personalities range from individuals that are shy, cautious, and easily stressed (a reactive personality type) to individuals that are bold, innovative, and quick to learn novel tasks, but also prone to routine formation (a proactive personality type). Although personality differences should have important consequences for fitness, their underlying mechanisms remain poorly understood. Here, we investigated how genetic variation in brain size affects personality. We put selection lines of large- and small-brained guppies (Poecilia reticulata), with known differences in cognitive ability, through three standard personality assays. First, we found that large-brained animals were faster to habituate to, and more exploratory in, open field tests. Large-brained females were also bolder. Second, large-brained animals excreted less cortisol in a stressful situation (confinement). Third, large-brained animals were slower to feed from a novel food source, which we interpret as being caused by reduced behavioral flexibility rather than lack of innovation in the large-brained lines. Overall, the results point toward a more proactive personality type in large-brained animals. Thus, this study provides the first experimental evidence linking brain size and personality, an interaction that may affect important fitness-related aspects of ecology such as dispersal and niche exploration.

  • 28.
    Kotrschal, Alexander
    et al.
    Uppsala universitet, Zooekologi.
    Rasanen, Katja
    Kristjansson, Bjarni K.
    Senn, Mike
    Kolm, Niclas
    Uppsala universitet, Zooekologi.
    Extreme Sexual Brain Size Dimorphism in Sticklebacks: A Consequence of the Cognitive Challenges of Sex and Parenting?2012In: PLoS ONE, ISSN 1932-6203, E-ISSN 1932-6203, Vol. 7, no 1, article id e30055Article in journal (Refereed)
    Abstract [en]

    Selection pressures that act differently on males and females produce numerous differences between the sexes in morphology and behaviour. However, apart from the controversial report that males have slightly heavier brains than females in humans, evidence for substantial sexual dimorphism in brain size is scarce. This apparent sexual uniformity is surprising given that sexually distinct selection pressures are ubiquitous and that brains are one of the most plastic vertebrate organs. Here we demonstrate the highest level of sexual brain size dimorphism ever reported in any vertebrate: male three-spined stickleback of two morphs in an Icelandic lake have 23% heavier brains than females. We suggest that this dramatic sexual size dimorphism is generated by the many cognitively demanding challenges that males are faced in this species, such as an elaborate courtship display, the construction of an ornate nest and a male-only parental care system. However, we consider also alternative explanations for smaller brains in females, such as life-history trade-offs. Our demonstration of unprecedented levels of sexual dimorphism in brain size in the three-spined stickleback implies that behavioural and life-history differences among the sexes can have strong effects also on neural development and proposes new fields of research for understanding brain evolution.

  • 29. Kotrschal, Alexander
    et al.
    Rogell, Björn
    Uppsala universitet, Zooekologi.
    Bundsen, Andreas
    Svensson, Beatrice
    Zajitschek, Susanne
    Brännström, Ioana
    Immler, Simone
    Maklakov, Alexei A.
    Kolm, Niclas
    Uppsala universitet, Zooekologi.
    The benefit of evolving a larger brain: big-brained guppies perform better in a cognitive task2013In: Animal Behaviour, ISSN 0003-3472, E-ISSN 1095-8282, Vol. 86, no 4, p. e4-e6Article in journal (Refereed)
    Abstract [en]

    We previously selected for large and small brain size in guppies. Large-brained females outperformed small-brained females in a learning task. Healy and Rowe challenged our interpretations of larger brains = better learning. Here we argue why we think they are mistaken.

  • 30.
    Kotrschal, Alexander
    et al.
    Uppsala universitet, Zooekologi.
    Rogell, Björn
    Uppsala universitet, Zooekologi.
    Bundsen, Andreas
    Svensson, Beatrice
    Zajitschek, Susanne
    Uppsala universitet, Evolutionsbiologi.
    Brännström, Ioana Onut
    Uppsala universitet, Evolutionsbiologi.
    Immler, Simone
    Maklakov, Alexei A.
    Kolm, Niclas
    Uppsala universitet, Zooekologi.
    Artificial selection on relative brain size in the guppy reveals costs and benefits of evolving a larger brain2013In: Current Biology, ISSN 0960-9822, E-ISSN 1879-0445, Vol. 23, no 2, p. 168-171Article in journal (Refereed)
    Abstract [en]

    The large variation in brain size that exists in the animal kingdom has been suggested to have evolved through the balance between selective advantages of greater cognitive ability and the prohibitively high energy demands of a larger brain (the "expensive-tissue hypothesis" [1]). Despite over a century of research on the evolution of brain size, empirical support for the trade-off between cognitive ability and energetic costs is based exclusively on correlative evidence [2], and the theory remains controversial [3, 4]. Here we provide experimental evidence for costs and benefits of increased brain size. We used artificial selection for large and small brain size relative to body size in a live-bearing fish, the guppy (Poecilia reticulata), and found that relative brain size evolved rapidly in response to divergent selection in both sexes. Large-brained females outperformed small-brained females in a numerical learning assay designed to test cognitive ability. Moreover, large-brained lines, especially males, developed smaller guts, as predicted by the expensive-tissue hypothesis [1], and produced fewer offspring. We propose that the evolution of brain size is mediated by a functional trade-off between increased cognitive ability and reproductive performance and discuss the implications of these findings for vertebrate brain evolution.

  • 31.
    Kotrschal, Alexander
    et al.
    Uppsala universitet, Zooekologi.
    Rogell, Björn
    Uppsala universitet, Zooekologi.
    Maklakov, Alexei A.
    Uppsala universitet, Zooekologi.
    Kolm, Nichlas
    Uppsala universitet, Zooekologi.
    Sex-specific plasticity in brain morphology depends on social environment of the guppy, Poecilia reticulata2012In: Behavioral Ecology and Sociobiology, ISSN 0340-5443, E-ISSN 1432-0762, Vol. 66, no 11, p. 1485-1492Article in journal (Refereed)
    Abstract [en]

    The vertebrate brain is a remarkably plastic organ, which responds quickly to environmental changes. However, to date, studies investigating plasticity in brain morphology have focused mostly on the physical properties of the surrounding environment, and little is known about brain plasticity in response to the social environment. Moreover, sex differences in brain plasticity remain virtually unexplored. Here, we tested how the social environment influenced brain morphology in adult males and females using experimental manipulation of the sex composition of social pairs (same sex vs. mixed sex) in the guppy (Poecilia reticulata). We detected substantial sex-specific plasticity in both the overall brain size (controlling for body size) and separate brain structures. The brain size was larger in males that interacted with females, and female optic tectum was larger in female-only groups. Overall, females had larger olfactory bulbs and cerebellum in comparison to males. While net sexual dimorphism in the brain structure can be explained in light of the known differences in boldness and foraging behaviour between the sexes, our results also support that cognitive demands associated with courtship behaviour can lead to plastic changes in the brain size. Our findings demonstrate that not only social environment can generate rapid, plastic responses in the vertebrate brain but also that such responses can depend strongly on sex.

  • 32.
    Kotrschal, Alexander
    et al.
    Uppsala universitet, Zooekologi.
    Sundström, L. Fredrik
    Brelin, D.
    Devlin, R. H.
    Kolm, Niclas
    Uppsala universitet, Zooekologi.
    Inside the heads of David and Goliath: environmental effects on brain morphology among wild and growth-enhanced coho salmon Oncorhynchus kisutch2012In: Journal of Fish Biology, ISSN 0022-1112, E-ISSN 1095-8649, Vol. 81, no 3, p. 987-1002Article in journal (Refereed)
    Abstract [en]

    Transgenic and wild-type individual coho salmon Oncorhynchus kisutch were reared in hatchery and near-natural stream conditions and their brain and structure sizes were determined. Animals reared in the hatchery grew larger and developed larger brains, both absolutely and when controlling for body size. In both environments, transgenics developed relatively smaller brains than wild types. Further, the volume of the optic tectum of both genotypes was larger in the hatchery animals and the cerebellum of transgenics was smaller when reared in near-natural streams. Finally, wild types developed a markedly smaller telencephalon under hatchery conditions. It is concluded that, apart from the environment, genetic factors that modulate somatic growth rate also have a strong influence on brain size and structure.

  • 33.
    Kotrschal, Alexander
    et al.
    Stockholm University, Faculty of Science, Department of Zoology.
    Szorkovszky, Alexander
    Romenskyy, Maksym
    Perna, Andrea
    Buechel, Severine D.
    Stockholm University, Faculty of Science, Department of Zoology.
    Zeng, Hong-Li
    Pelckmans, Kristiaan
    Sumpter, David
    Kolm, Niclas
    Stockholm University, Faculty of Science, Department of Zoology.
    Brain size does not impact shoaling dynamics in unfamiliar groups of guppies (Poecilia reticulata)2018In: Behavioural Processes, ISSN 0376-6357, E-ISSN 1872-8308, Vol. 147, p. 13-20Article in journal (Refereed)
    Abstract [en]

    Collective movement is achieved when individuals adopt local rules to interact with their neighbours. How the brain processes information about neighbours' positions and movements may affect how individuals interact in groups. As brain size can determine such information processing it should impact collective animal movement. Here we investigate whether brain size affects the structure and organisation of newly forming fish shoals by quantifying the collective movement of guppies (Poecilia reticulata) from large- and small-brained selection lines, with known differences in learning and memory. We used automated tracking software to determine shoaling behaviour of single-sex groups of eight or two fish and found no evidence that brain size affected the speed, group size, or spatial and directional organisation of fish shoals. Our results suggest that brain size does not play an important role in how fish interact with each other in these types of moving groups of unfamiliar individuals. Based on these results, we propose that shoal dynamics are likely to be governed by relatively basic cognitive processes that do not differ in these brain size selected lines of guppies.

  • 34.
    Kotrschal, Alexander
    et al.
    Stockholm University, Faculty of Science, Department of Zoology.
    Zeng, Hong-Li
    van der Bijl, Wouter
    Stockholm University, Faculty of Science, Department of Zoology.
    Öhman-Mägi, Caroline
    Kotrschal, Kurt
    Pelckmans, Kristiaan
    Kolm, Niclas
    Stockholm University, Faculty of Science, Department of Zoology.
    Evolution of brain region volumes during artificial selection for relative brain size2017In: Evolution, ISSN 0014-3820, E-ISSN 1558-5646, Vol. 71, no 12, p. 2942-2951Article in journal (Refereed)
    Abstract [en]

    The vertebrate brain shows an extremely conserved layout across taxa. Still, the relative sizes of separate brain regions vary markedly between species. One interesting pattern is that larger brains seem associated with increased relative sizes only of certain brain regions, for instance telencephalon and cerebellum. Till now, the evolutionary association between separate brain regions and overall brain size is based on comparative evidence and remains experimentally untested. Here, we test the evolutionary response of brain regions to directional selection on brain size in guppies (Poecilia reticulata) selected for large and small relative brain size. In these animals, artificial selection led to a fast response in relative brain size, while body size remained unchanged. We use microcomputer tomography to investigate how the volumes of 11 main brain regions respond to selection for larger versus smaller brains. We found no differences in relative brain region volumes between large- and small-brained animals and only minor sex-specific variation. Also, selection did not change allometric scaling between brain and brain region sizes. Our results suggest that brain regions respond similarly to strong directional selection on relative brain size, which indicates that brain anatomy variation in contemporary species most likely stem from direct selection on key regions.

  • 35. Liao, Wen Bo
    et al.
    Lou, Shang Ling
    Zeng, Yu
    Kotrschal, Alexander
    Stockholm University, Faculty of Science, Department of Zoology.
    Large Brains, Small Guts: The Expensive Tissue Hypothesis Supported within Anurans2016In: American Naturalist, ISSN 0003-0147, E-ISSN 1537-5323, Vol. 188, no 6, p. 693-700Article in journal (Refereed)
    Abstract [en]

    Brain size differs substantially among species, and several hypotheses have been proposed to explain the evolution of brain size. Because the brain is among the most energetically expensive organs in the vertebrate body, trade-offs have been hypothesized to exert constraints on brain size evolution. Prominently, the expensive tissue hypothesis (ETH) proposes that reducing the size of another expensive organ, such as the gut, should compensate for the cost of a large brain. But energetic constraints may also drive covariation between the brain and other costly traitssuch as body maintenance, locomotion, or reproductionas formulated in the energy trade-off hypothesis. To date, these hypotheses have mainly been tested in homeothermic animals and within the ectothermic animals, primarily in fishes. Here, we undertake a comparative test of the interplay between energetic limitations and brain size evolution within amphibians. After controlling for phylogenetic relationships and body size, we find a negative correlation between brain mass and the length of the digestive tract within 30 species of anurans. We further find that the evolution of large brain size is accompanied by an increase in female reproductive investment into egg size. Our results suggest that the evolution of brain size follows general patterns across vertebrate clades.

  • 36. Luo, Yi
    et al.
    Zhong, Mao Jun
    Huang, Yan
    Li, Feng
    Liao, Wen Bo
    Kotrschal, Alexander
    Stockholm University, Faculty of Science, Department of Zoology.
    Seasonality and brain size are negatively associated in frogs: evidence for the expensive brain framework2017In: Scientific Reports, ISSN 2045-2322, E-ISSN 2045-2322, Vol. 7, article id 16629Article in journal (Refereed)
    Abstract [en]

    The challenges of seasonal environments are thought to contribute to brain evolution, but in which way is debated. According to the Cognitive Buffer Hypothesis (CBH) brain size should increase with seasonality, as the cognitive benefits of a larger brain should help overcoming periods of food scarcity via, for instance, increased behavioral flexibility. However, in line with the Expensive Brain Framework (EBF) brain size should decrease with seasonality because a smaller brain confers energetic benefits in periods of food scarcity. Empirical evidence is inconclusive and mostly limited to homoeothermic animals. Here we used phylogenetic comparative analyses to test the impact of seasonality on brain evolution across 30 species of anurans (frogs) experiencing a wide range of temperature and precipitation. Our results support the EBF because relative brain size and the size of the optic tectum were negatively correlated with variability in temperature. In contrast, we found no association between the variability in precipitation and the length of the dry season with either brain size or the sizes of other major brain regions. We suggest that seasonality-induced food scarcity resulting from higher variability in temperature constrains brain size evolution in anurans. Less seasonal environments may therefore facilitate the evolution of larger brains in poikilothermic animals.

  • 37. Marhounová, Lucie
    et al.
    Kotrschal, Alexander
    Stockholm University, Faculty of Science, Department of Zoology. Wageningen University & Research, Netherlands.
    Kverková, Kristina
    Kolm, Niclas
    Stockholm University, Faculty of Science, Department of Zoology.
    Nemec, Pavel
    Artificial selection on brain size leads to matching changes in overall number of neurons2019In: Evolution, ISSN 0014-3820, E-ISSN 1558-5646, Vol. 73, no 9, p. 2003-2012Article in journal (Refereed)
    Abstract [en]

    Neurons are the basic computational units of the brain, but brain size is the predominant surrogate measure of brain functional capacity in comparative and cognitive neuroscience. This approach is based on the assumption that larger brains harbor higher numbers of neurons and their connections, and therefore have a higher information-processing capacity. However, recent studies have shown that brain mass may be less strongly correlated with neuron counts than previously thought. Till now, no experimental test has been conducted to examine the relationship between evolutionary changes in brain size and the number of brain neurons. Here, we provide such a test by comparing neuron number in artificial selection lines of female guppies (Poecilia reticulata) with >15% difference in relative brain mass and numerous previously demonstrated cognitive differences. Using the isotropic fractionator, we demonstrate that large-brained females have a higher overall number of neurons than small-brained females, but similar neuronal densities. Importantly, this difference holds also for the telencephalon, a key region for cognition. Our study provides the first direct experimental evidence that selection for brain mass leads to matching changes in number of neurons and shows that brain size evolution is intimately linked to the evolution of neuron number and cognition.

  • 38. Masahito, Tsuboi
    et al.
    van der Bijl, Wouter
    Stockholm University, Faculty of Science, Department of Zoology.
    Kopperud, Bjørn Tore
    Erritzøe, Johannes
    Voje, Kjetil L.
    Kotrschal, Alexander
    Stockholm University, Faculty of Science, Department of Zoology.
    Yopak, Kara E.
    Collin, Shaun P.
    Hansen, Thomas F.
    Iwaniuk, Andrew
    Kolm, Niclas
    Stockholm University, Faculty of Science, Department of Zoology.
    Breakdown of brain-body allometry and the exceptional encephalization of mammals and birdsManuscript (preprint) (Other academic)
  • 39. Sumpter, David J. T.
    et al.
    Szorkovszky, Alex
    Kotrschal, Alexander
    Stockholm University, Faculty of Science, Department of Zoology.
    Kolm, Niclas
    Stockholm University, Faculty of Science, Department of Zoology.
    Herbert-Read, James E.
    Stockholm University, Faculty of Science, Department of Zoology.
    Using activity and sociability to characterize collective motion2018In: Philosophical Transactions of the Royal Society of London. Biological Sciences, ISSN 0962-8436, E-ISSN 1471-2970, Vol. 373, no 1746, article id 20170015Article, review/survey (Refereed)
    Abstract [en]

    A wide range of measurements can be made on the collective motion of groups, and the movement of individuals within them. These include, but are not limited to: group size, polarization, speed, turning speed, speed or directional correlations, and distances to near neighbours. From an ecological and evolutionary perspective, we would like to know which of these measurements capture biologically meaningful aspects of an animal's behaviour and contribute to its survival chances. Previous simulation studies have emphasized two main factors shaping individuals' behaviour in groups; attraction and alignment. Alignment responses appear to be important in transferring information between group members and providing synergistic benefits to group members. Likewise, attraction to conspecifics is thought to provide benefits through, for example, selfish herding. Here, we use a factor analysis on a wide range of simple measurements to identify two main axes of collective motion in guppies (Poecilia reticulata): (i) sociability, which corresponds to attraction (and to a lesser degree alignment) to neighbours, and (ii) activity, which combines alignment with directed movement. We show that for guppies, predation in a natural environment produces higher degrees of sociability and (in females) lower degrees of activity, while female guppies sorted for higher degrees of collective alignment have higher degrees of both sociability and activity. We suggest that the activity and sociability axes provide a useful framework for measuring the behaviour of animals in groups, allowing the comparison of individual and collective behaviours within and between species.

  • 40. Szorkovszky, Alex
    et al.
    Kotrschal, Alexander
    Stockholm University, Faculty of Science, Department of Zoology. Stockholm Univ, Zool Dept, Stockholm, Sweden.
    Herbert-Read, James E.
    Stockholm University, Faculty of Science, Department of Zoology. University of Bristol, U.K..
    Buechel, Severine D.
    Stockholm University, Faculty of Science, Department of Zoology.
    Romenskyy, Maksym
    Rosen, Emil
    van der Bijl, Wouter
    Stockholm University, Faculty of Science, Department of Zoology.
    Pelckmans, Kristiaan
    Kolm, Niclas
    Stockholm University, Faculty of Science, Department of Zoology.
    Sumpter, David J. T.
    Assortative interactions revealed by sorting of animal groups2018In: Animal Behaviour, ISSN 0003-3472, E-ISSN 1095-8282, Vol. 142, p. 165-179Article in journal (Refereed)
    Abstract [en]

    Animals living in groups can show substantial variation in social traits and this affects their social organization. However, as the specific mechanisms driving this organization are difficult to identify in already organized groups typically found in the wild, the contribution of interindividual variation to group level behaviour remains enigmatic. Here, we present results of an experiment to create and compare groups that vary in social organization, and study how individual behaviour varies between these groups. We iteratively sorted individuals between groups of guppies, Poecilia reticulata, by ranking the groups according to their directional alignment and then mixing similar groups. Over the rounds of sorting the consistency of the group rankings increased, producing groups that varied significantly in key social behaviours such as collective activity and group cohesion. The repeatability of the underlying individual behaviour was then estimated by comparing the experimental data to simulations. At the level of basic locomotion, individuals in more coordinated groups displayed stronger interactions with the centre of the group, and weaker interactions with their nearest neighbours. We propose that this provides the basis for a passive phenotypic assortment mechanism that may explain the structures of social networks in the wild.

  • 41. Szorkovszky, Alex
    et al.
    Kotrschal, Alexander
    Stockholm University, Faculty of Science, Department of Zoology.
    Read, James E. Herbert
    Stockholm University, Faculty of Science, Department of Zoology.
    Sumpter, David J. T.
    Kolm, Niclas
    Stockholm University, Faculty of Science, Department of Zoology.
    Pelckmans, Kristiaan
    An efficient method for sorting and quantifying individual social traits based on group-level behaviour2017In: Methods in Ecology and Evolution, ISSN 2041-210X, E-ISSN 2041-210X, Vol. 8, no 12, p. 1735-1744Article in journal (Refereed)
    Abstract [en]

    1. In social contexts, animal behaviour is often studied in terms of group-level characteristics. One clear example of this is the collective motion of animals in decentralized structures, such as bird flocks and fish schools. A major goal of research is to identify how group-level behaviours are shaped by the traits of individuals within them. Few methods exist to make these connections. Individual assessment is often limited, forcing alternatives such as fitting agent-based models to experimental data. 2. We provide a systematic experimental method for sorting animals according to socially relevant traits, without assaying them or even tagging them individually. Instead, they are repeatedly subjected to behavioural assays in groups, between which the group memberships are rearranged, in order to test the effect of many different combinations of individuals on a group-level property or feature. We analyse this method using a general model for the group feature, and simulate a variety of specific cases to track how individuals are sorted in each case. 3. We find that in the case where the members of a group contribute equally to the group feature, the sorting procedure increases the between-group behavioural variation well above what is expected for groups randomly sampled from a population. For a wide class of group feature models, the individual phenotypes are efficiently sorted across the groups and thus become available for further analysis on how individual properties affect group behaviour. We also show that the experimental data can be used to estimate the individual-level repeatability of the underlying traits. 4. Our method allows experimenters to find repeatable variation in social behaviours that cannot be assessed in solitary individuals. Furthermore, experiments in animal behaviour often focus on comparisons between groups randomly sampled from a population. Increasing the behavioural variation between groups increases statistical power for testing whether a group feature is related to other properties of groups or to their phenotypic composition. Sorting according to socially relevant traits is also beneficial in artificial selection experiments, and for testing correlations with other traits. Overall, the method provides a useful tool to study how individual properties influence social behaviour.

  • 42. Tsuboi, Masahito
    et al.
    Husby, Arild
    Kotrschal, Alexander
    Stockholm University, Faculty of Science, Department of Zoology, Ethology.
    Hayward, Alexander
    Stockholm University, Faculty of Science, Department of Zoology, Ethology.
    Buechel, Severine D.
    ETH Zürich Institute of Integrative Biology (IBZ), Switzerland.
    Zidar, Josefina
    Lovlie, Hanne
    Kolm, Niclas
    Stockholm University, Faculty of Science, Department of Zoology, Ethology.
    Comparative support for the expensive tissue hypothesis: Big brains are correlated with smaller gut and greater parental investment in Lake Tanganyika cichlids2015In: Evolution, ISSN 0014-3820, E-ISSN 1558-5646, Vol. 69, no 1, p. 190-200Article in journal (Refereed)
    Abstract [en]

    The brain is one of the most energetically expensive organs in the vertebrate body. Consequently, the energetic requirements of encephalization are suggested to impose considerable constraints on brain size evolution. Three main hypotheses concerning how energetic constraints might affect brain evolution predict covariation between brain investment and (1) investment into other costly tissues, (2) overall metabolic rate, and (3) reproductive investment. To date, these hypotheses have mainly been tested in homeothermic animals and the existing data are inconclusive. However, there are good reasons to believe that energetic limitations might play a role in large-scale patterns of brain size evolution also in ectothermic vertebrates. Here, we test these hypotheses in a group of ectothermic vertebrates, the Lake Tanganyika cichlid fishes. After controlling for the effect of shared ancestry and confounding ecological variables, we find a negative association between brain size and gut size. Furthermore, we find that the evolution of a larger brain is accompanied by increased reproductive investment into egg size and parental care. Our results indicate that the energetic costs of encephalization may be an important general factor involved in the evolution of brain size also in ectothermic vertebrates.

  • 43. Tsuboi, Masahito
    et al.
    Kotrschal, Alexander
    Stockholm University, Faculty of Science, Department of Zoology.
    Hayward, Alexander
    Stockholm University, Faculty of Science, Department of Zoology.
    Buechel, Severine Denise
    Stockholm University, Faculty of Science, Department of Zoology.
    Zidar, Josefina
    Lovlie, Hanne
    Kolm, Niclas
    Stockholm University, Faculty of Science, Department of Zoology.
    Evolution of brain-body allometry in Lake Tanganyika cichlids2016In: Evolution, ISSN 0014-3820, E-ISSN 1558-5646, Vol. 70, no 7, p. 1559-1568Article in journal (Refereed)
    Abstract [en]

    Brain size is strongly associated with body size in all vertebrates. This relationship has been hypothesized to be an important constraint on adaptive brain size evolution. The essential assumption behind this idea is that static (i.e., within species) brain-body allometry has low ability to evolve. However, recent studies have reported mixed support for this view. Here, we examine brain-body static allometry in Lake Tanganyika cichlids using a phylogenetic comparative framework. We found considerable variation in the static allometric intercept, which explained the majority of variation in absolute and relative brain size. In contrast, the slope of the brain-body static allometry had relatively low variation, which explained less variation in absolute and relative brain size compared to the intercept and body size. Further examination of the tempo and mode of evolution of static allometric parameters confirmed these observations. Moreover, the estimated evolutionary parameters indicate that the limited observed variation in the static allometric slope could be a result of strong stabilizing selection. Overall, our findings suggest that the brain-body static allometric slope may represent an evolutionary constraint in Lake Tanganyika cichlids.

  • 44.
    Tsuboi, Masahito
    et al.
    Stockholm University, Faculty of Science, Department of Zoology. University of Oslo, Norway; The Graduate University of Advanced Studies, Japan.
    van der Bijl, Wouter
    Stockholm University, Faculty of Science, Department of Zoology.
    Kopperud, Bjørn Tore
    Erritzøe, Johannes
    Voje, Kjetil L.
    Kotrschal, Alexander
    Stockholm University, Faculty of Science, Department of Zoology.
    Yopak, Kara E.
    Collin, Shaun P.
    Iwaniuk, Andrew N.
    Kolm, Niclas
    Stockholm University, Faculty of Science, Department of Zoology.
    Breakdown of brain-body allometry and the encephalization of birds and mammals2018In: Nature Ecology & Evolution, E-ISSN 2397-334X, Vol. 2, no 9, p. 1492-1500Article in journal (Refereed)
    Abstract [en]

    The allometric relationship between brain and body size among vertebrates is often considered a manifestation of evolutionary constraints. However, birds and mammals have undergone remarkable encephalization, in which brain size has increased without corresponding changes in body size. Here, we explore the hypothesis that a reduction of phenotypic integration between brain and body size has facilitated encephalization in birds and mammals. Using a large dataset comprising 20,213 specimens across 4,587 species of jawed vertebrates, we show that the among-species (evolutionary) brain-body allometries are remarkably constant, both across vertebrate classes and across taxonomic levels. Birds and mammals, however, are exceptional in that their within-species (static) allometries are shallower and more variable than in other vertebrates. These patterns are consistent with the idea that birds and mammals have reduced allometric constraints that are otherwise ubiquitous across jawed vertebrates. Further exploration of ontogenetic allometries in selected taxa of birds, fishes and mammals reveals that birds and mammals have extended the period of fetal brain growth compared to fishes. Based on these findings, we propose that avian and mammalian encephalization has been contingent on increased variability in brain growth patterns.

  • 45.
    van der Bijl, Wouter
    et al.
    Stockholm University, Faculty of Science, Department of Zoology.
    Kotrschal, Alexander
    Stockholm University, Faculty of Science, Department of Zoology.
    Buechel, Séverine D.
    Stockholm University, Faculty of Science, Department of Zoology.
    Kolm, Niclas
    Stockholm University, Faculty of Science, Department of Zoology.
    Cognitive ability and antagonistic social competence: contest duration depends on loser’s brain sizeManuscript (preprint) (Other academic)
  • 46.
    van der Bijl, Wouter
    et al.
    Stockholm University, Faculty of Science, Department of Zoology.
    Thyselius, Malin
    Stockholm University, Faculty of Science, Department of Zoology.
    Kotrschal, Alexander
    Stockholm University, Faculty of Science, Department of Zoology.
    Kolm, Niclas
    Stockholm University, Faculty of Science, Department of Zoology.
    Brain size affects the behavioural response to predators in female guppies (Poecilia reticulata)2015In: Proceedings of the Royal Society of London. Biological Sciences, ISSN 0962-8452, E-ISSN 1471-2954, Vol. 282, no 1812, p. 116-124, article id 20151132Article in journal (Refereed)
    Abstract [en]

    Large brains are thought to result from selection for cognitive benefits, but how enhanced cognition leads to increased fitness remains poorly understood. One explanation is that increased cognitive ability results in improved monitoring and assessment of predator threats. Here, we use male and female guppies (Poecilia reticulata), artificially selected for large and small brain size, to provide an experimental evaluation of this hypothesis. We examined their behavioural response as singletons, pairs or shoals of four towards a model predator. Large-brained females, but not males, spent less time performing predator inspections, an inherently risky behaviour. Video analysis revealed that large-brained females were further away from the model predator when in pairs but that they habituated quickly towards the model when in shoals of four. Males stayed further away from the predator model than females but again we found no brain size effect in males. We conclude that differences in brain size affect the female predator response. Large-brained females might be able to assess risk better or need less sensory information to reach an accurate conclusion. Our results provide experimental support for the general idea that predation pressure is likely to be important for the evolution of brain size in prey species.

  • 47. Yu, Xin
    et al.
    Zhong, Mao Jun
    Li, Da Yong
    Jin, Long
    Liao, Wen Bo
    Kotrschal, Alexander
    Stockholm University, Faculty of Science, Department of Zoology.
    Large-brained frogs mature later and live longer2018In: Evolution, ISSN 0014-3820, E-ISSN 1558-5646, Vol. 72, no 5, p. 1174-1183Article in journal (Refereed)
    Abstract [en]

    Brain sizes vary substantially across vertebrate taxa, yet, the evolution of brain size appears tightly linked to the evolution of life histories. For example, larger brained species generally live longer than smaller brained species. A larger brain requires more time to grow and develop at a cost of exceeded gestation period and delayed weaning age. The cost of slower development may be compensated by better homeostasis control and increased cognitive abilities, both of which should increase survival probabilities and hence life span. To date, this relationship between life span and brain size seems well established in homoeothermic animals, especially in mammals. Whether this pattern occurs also in other clades of vertebrates remains enigmatic. Here, we undertake the first comparative test of the relationship between life span and brain size in an ectothermic vertebrate group, the anuran amphibians. After controlling for the effects of shared ancestry and body size, we find a positive correlation between brain size, age at sexual maturation, and life span across 40 species of frogs. Moreover, we also find that the ventral brain regions, including the olfactory bulbs, are larger in long-lived species. Our results indicate that the relationship between life history and brain evolution follows a general pattern across vertebrate clades.

  • 48. Zeng, Yu
    et al.
    Lou, Shang Ling
    Liao, Wen Bo
    Jehle, Robert
    Kotrschal, Alexander
    Stockholm University, Faculty of Science, Department of Zoology.
    Sexual selection impacts brain anatomy in frogs and toads2016In: Ecology and Evolution, ISSN 2045-7758, E-ISSN 2045-7758, Vol. 6, no 19, p. 7070-7079Article in journal (Refereed)
    Abstract [en]

    Natural selection is a major force in the evolution of vertebrate brain size, but the role of sexual selection in brain size evolution remains enigmatic. At least two opposing schools of thought predict a relationship between sexual selection and brain size. Sexual selection should facilitate the evolution of larger brains because better cognitive abilities may aid the competition for mates. However, it may also restrict brain size evolution due to energetic trade-offs between brain tissue and sexually selected traits. Here, we examined the patterns of selection on brain size and brain anatomy in male anurans (frogs and toads), a group where the strength of sexual selection differs markedly among species, using a phylogenetically controlled generalized least-squared (PGLS) regression analyses. The analysis revealed that in 43 Chinese anuran species, neither mating system, nor type of courtship, or testes mass was significantly associated with relative brain size. While none of those factors related to the relative size of olfactory nerves, optic tecta, telencephalon, and cerebellum, the olfactory bulbs were relatively larger in monogamous species and those using calls during courtship. Our findings support the mosaic model of brain evolution and suggest that while the investigated aspects of sexual selection do not seem to play a prominent role in the evolution of brain size of anurans, they do impact their brain anatomy.

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