Heatwaves negatively impact behaviour with associated cognitive impairment in humans. A growing body of literature also reports negative effects of heatwaves on cognition in other animals. A larger brain is known to generate enhanced cognitive abilities that may buffer against environmental changes and thereby potentially increase fitness in large-brained individuals. How a larger brain buffers against adverse effects on cognitive abilities induced by thermal stress, such as that experienced during heatwaves, remains unknown. We examined detour problem solving and working memory during an experimental heatwave in guppies artificially selected on brain size with matching differences in neuron number. Overall, detour problem-solving was impaired among guppies during the heatwave, while working memory was unaffected. Large-brained guppies outperformed small-brained guppies in detour problem-solving and working memory in both the heatwave and control temperature treatments. During the heatwave, large-brained guppies exhibited cognitive performance levels comparable to those of small-brained guppies under normal temperature conditions in the detour task. Our study thus suggests that small-brained individuals might have lower fitness also during heatwaves if increased temperature impair cognitive abilities required for survival and reproduction. Furthermore, our results open up the possibility that cognition-driven brain size evolution may have been influenced by abiotic factors.

Female multiple mating is widespread among animals. Consequently, competition among males can continue after mating in the form of sperm competition, the contest between ejaculates of different males to fertilize a single egg or set of eggs. Sperm competition is a powerful selective force acting on a wide range of male reproductive traits to maximize the probability of fertilization success in the presence of competitors. Here we summarize the logic underlying sperm competition theory and describe how sperm competition has shaped the evolution of male reproductive anatomy, physiology and behavior in animals.

Avoiding predation is essential for most animals. For group-living species, effective predator avoidance relies on making fast and accurate collective decisions. However, the mechanisms underlying the ability to make adaptive collective decisions and to coordinate movements under predation threat remains unclear. Here, we used guppies artificially selected for divergence in the size of the telencephalon, the main brain region for advanced decision-making in vertebrates, to test the influence of telencephalon size on collective decision-making under predation threat. We measured the latency and accuracy of collective decision-making to avoid a model predator in guppy shoals. In addition, we used high-resolution tracking analysis to assess shoaling dynamics under predator threat between the telencephalon size selection lines. We found that collective decision-making latency was shorter in large telencephalon guppy shoals, indicating that variation in telencephalon size can cause variation in the ability to avoid predation. This result is unlikely to be driven by differences in boldness, as several standard tests suggest that there is no difference in boldness between the telencephalon size selection lines. General aspects of shoaling dynamics did not differ between the telencephalon size selected lines. Our study highlights that rapid mosaic changes in brain region size may be an important mechanism behind social behavioural variation with strong fitness implications.

Animals can employ various spatial learning strategies to navigate through their environment, and the proficiency in specific strategies varies greatly both intraspecifically and interspecifically. Currently, the neural basis of this variation is poorly understood. Here, we tested whether variation in performance in egocentric and allocentric spatial learning strategies is related to differential investment in distinct brain regions. To do so, we used guppies (Poecilia reticulata) from artificial selection lines expressing differences in relative telencephalon size, and tested their ability to learn a spatial task, based on either egocentric (left–right) or allocentric (environmental) cues. Surprisingly, fish with larger telencephalons showed enhanced performance in both tasks, regardless of cue type, suggesting a more complicated role of the fish telencephalon in spatial learning than previously thought. Our study provides the first direct evidence that evolutionary changes in relative telencephalon size lead to corresponding shifts in spatial cognition at the within-species level. Furthermore, our results offer critical and novel insights regarding the function of the telencephalon and its role in the evolution of spatial cognition.

Animals often reproduce in complex environments, which should generate selection for both enhanced detectability in signaling traits and improved cognitive processing abilities. However, the extent to which signaling and cognitive traits have evolved to overcome the challenges of interacting in complex habitats remains understudied. We examined whether habitat complexity influences sexual selection in the pygmy halfbeak, Dermogenys collettei, a small livebearing freshwater fish. Using free-swimming arenas, we created low- and high-complexity environments and observed mating behaviors in mixed-sex groups. While the opportunity for sexual selection did not differ significantly between environments for either sex, we observed positive selection gradients for female brain size in open arenas, but not in complex habitats. Selection on morphological traits associated with visual signaling was also primarily detected in open environments, particularly in females. These results suggest that habitat complexity may reduce selection pressures on both cognitive traits, such as brain size, and signaling traits relevant to mate choice. Together, our findings highlight the importance of integrating cognitive traits into sexual selection theory and considering sex-specific selection across ecologically relevant contexts.

Trade-offs should be ubiquitous in nature. Yet, direct trade-offs between traits essential for fitness are challenging to detect. Recent theory suggests that population-level variation in resource acquisition could play an important role in our ability to detect trade-offs. Here, we test experimentally the hypothesis that the detection of trade-offs depends on the underlying distribution of individuals with different resource acquisition in a population. Specifically, we resampled ecologically and experimentally relevant resource acquisition distributions from a population of male Australian field crickets (Teleogryllus oceanicus) subjected to a continuous range of diet manipulation. While we found evidence for trade-offs between different male fitness traits, the distribution of resource acquisition in the population had no systematic effect on the strength of these trade-offs. Interestingly, trade-offs were most pronounced between postcopulatory traits and immune function, but trade-offs involving precopulatory traits were relatively weak. Overall, our findings question the hypothesis that resource acquisition may influence our ability to detect trade-offs and instead suggest that other factors, like the hierarchical complexity of resource allocation, make detecting trade-offs so elusive.

Sperm length is highly variable within ejaculates, between males, among populations, and across species. While theory makes strong predictions about expected mean sperm size, there is less clarity on variation in sperm, although studies have reported sperm-length variation consistent with some theoretical expectations. Typically, the coefficient of variation (CV) is used in these investigations to control for mean–variance scaling. However, a key assumption for this metric to be appropriate in controlling for mean sperm size is that the standard deviation in size scales linearly with the mean. Unfortunately, sperm-length mean–variation relationships are rarely reported making it hard to assess the validity of using CV as a way to compare mean-corrected sperm variation. Here, we investigate mean–variation relationships using 19,873 sperm length measures from 54 species and find little evidence of a consistent relationship between mean sperm-length and sperm-length variation among males within species, meaning CV is not appropriate for comparing relative (mean corrected) variation in sperm size at this level. We also find significant scaling of sperm-length variation with mean sperm-length across species, but the scaling exponent is consistently less than one, the exponent required by analyses using CV to control for sperm size. Our assessment shows that sperm mean–variation scaling relationships are rare within species and strong across species, but that neither supports the uncritical use of CV in studies of relative variation in sperm length.

Extensive research indicates that fertilization outcomes are shaped by individual female and male traits that reflect their intrinsic quality. Yet, surprisingly little is known about the influence of interactions between the sexes and their adaptive significance in either externally or internally fertilizing species. Here, we review empirical evidence on how female–male interactions influence each stage of the fertilization process, including sperm transfer, transport, storage, chemoattraction and fertilization. We also address the challenges of examining female–male interaction effects within a realistic biological context and why research in this area lags behind understanding the role of individual sex-specific traits. While relatively little data are currently available to address the interactive effects between the sexes and their impact on the fertilization process, what is presently known suggests that these effects are likely to be more common across the animal tree of life than appreciated. Future research will help identify these interactions, and their understanding can also help to explain the maintenance of genetic variation and inform applied studies of fertility.

Reproductive accessory glands are organs involved in reproduction that do not directly produce or release gametes but can play crucial roles in securing reproductive success. In fishes, the 2 leading hypotheses about why accessory glands evolved are (1) in response to sperm competition, or (2) to facilitate parental care activities. Here, we investigate the evolutionary history of accessory glands and test these hypotheses by estimating quantitative differences in evolutionary rates. We found that accessory glands are present in 116 of the 607 sampled species of ray-finned fishes, representing 26/267 families. We estimated that accessory glands have arisen independently ~20 times and that these glands were gained 5.8 times faster in lineages with male parental care, compared to those without male care, supporting the hypothesis that they evolved to facilitate care. In contrast, group spawning, used as a proxy for sperm competition risk, seemed to select against the evolution of accessory glands, as lineages exhibiting group spawning gained accessory glands 3.9 times slower than those with pair spawning (though this failed to reach statistical significance). This study provides new insights into the evolutionary history of accessory glands in fishes and highlights the importance of parental care in shaping reproductive anatomy.

Collective motion is common across all animal taxa, from swarming insects to schools of fish. The collective motion requires intricate behavioral integration among individuals, yet little is known about how evolutionary changes in brain morphology influence the ability for individuals to coordinate behavior in groups. In this study, we utilized guppies that were selectively bred for relative telencephalon size, an aspect of brain morphology that is normally associated with advanced cognitive functions, to examine its role in collective motion using an open-field assay. We analyzed high-resolution tracking data of same-sex shoals consisting of 8 individuals to assess different aspects of collective motion, such as alignment, attraction to nearby shoal members, and swimming speed. Our findings indicate that variation in collective motion in guppy shoals might not be strongly affected by variation in relative telencephalon size. Our study suggests that group dynamics in collectively moving animals are likely not driven by advanced cognitive functions but rather by fundamental cognitive processes stemming from relatively simple rules among neighboring individuals.