Many studies have suggested that Arctic predators preying on lemmings, especially the Arctic fox, take advantage of the high prey densities during peak years, but have to switch to alternative prey during years of low lemming density, thereby forcing other prey species into the basic lemming cycle. Data on the number of breeding geese and waders in Arctic areas show three-year cycles, supporting this hypothesis. There are two alternative explanations for such a prey switch. The switch to the alternative prey could be due to a functional prey response, which assumes that the proportion of birds in fox diet should be related to lemming population phase due to a type III functional response between foxes and lemmings. The numerical switch explanation on the other hand, assumes that bird numbers should be related to lemming population phase because of a time lag in the numerical response between foxes and lemmings. With data on the diet and population numbers of Arctic foxes, Arctic lemmings and Arctic birds from eight different sites in northern Siberia, we tested these hypotheses. The use of birds in fox diet was not related to lemming population phase. Instead, we found a time lag in numerical response to lemmings, thus supporting the numerical switch explanation for the alternative prey hypothesis.

After the wider deglaciation of Northern Europe, pioneer reindeer populations started to move into southern Scandinavia; however, this process is poorly understood. In this paper we aim to reconstruct dispersal processes of reindeer into southern and western Sweden from the Late Palaeolithic through to the Early Mesolithic, when reindeer disappear from the record. Has presence of reindeer in southern Sweden changed over time, were there changes in habitat and was the hunt of reindeer a possible driving factor to their disappearance? We have assembled and analysed a dataset of 220 unburnt reindeer skeletal elements from wetlands, earthen finds and shell middens from southern and western Sweden. Additional 14C-analysis have been performed to set the chronological frame. The results demonstrate that reindeer were present in southern and western Sweden from 12,066 to 7079 cal BCE and that the number of reindeer was highest during the Early Holocene. Stable isotope analyses (δ13C and δ15N), provided information on changes in reindeer habitat. The marked variation in δ13C and δ15N values suggests that reindeer grazed in different habitats or that the habitat change over time. We suggest that the decrease and final disappearance of reindeer in the Late Palaeolithic/Early Mesolithic was caused by changes in climate and habitat rather than anthropogenically induced.

BACKGROUND: The Norwegian lemming (Lemmus lemmus) is a small rodent endemic to the Fennoscandian alpine and arctic tundra. The species is known for cyclic population outbreaks and mass movements during peak years. Previous research based on microsatellites revealed high genetic variation but a weak population structure in the Norwegian lemming. RESULTS: In this study, we revisit the population structure of the species using genome-wide data. To do this, we generated a high-quality de novo reference genome for Lemmus lemmus, and resequenced genomes to 2.5-5 × coverage, from 86 lemmings sampled across the species' entire geographic distribution. Our results reveal that the population is geographically structured into distinct subpopulations, with an overall pattern characterised by isolation-by-distance among subpopulations. Furthermore, our results are consistent with earlier work suggesting that the species survived the last ice age within a northern refugium. CONCLUSIONS: Together, these findings provide a genome-wide perspective on today's population structure of the Norwegian lemming. In addition, we provide a de novo reference genome, which we believe will be a valuable resource to the research community.

Ancient genomic studies have extensively explored human-microbial interactions, yet research on non-human animals remains limited. In this study, we analyzed ancient microbial DNA from 483 mammoth remains spanning over 1 million years, including 440 newly sequenced and unpublished samples from a 1.1-million-year-old steppe mammoth. Using metagenomic screening, contaminant filtering, damage pattern analysis, and phylogenetic inference, we identified 310 microbes associated with different mammoth tissues. While most microbes were environmental or post-mortem colonizers, we recovered genomic evidence of six host-associated microbial clades spanning Actinobacillus, Pasteurella, Streptococcus, and Erysipelothrix. Some of these clades contained putative virulence factors, including a Pasteurella-related bacterium that had previously been linked to the deaths of African elephants. Notably, we reconstructed partial genomes of Erysipelothrix from the oldest mammoth sample, representing the oldest authenticated host-associated microbial DNA to date. This work demonstrates the potential of obtaining ancient animal microbiomes, which can inform further paleoecological and evolutionary research.

Neighbour-stranger discrimination, where territorial individuals distinguish between familiar neighbours and unfamiliar strangers, is a well-documented phenomenon in the animal kingdom. It often manifests as a ‘dear enemy’ strategy, where territory holders respond more aggressively towards strangers than neighbours, although a ‘nasty neighbour’ strategy with the opposite tendencies has also been described. A connection between neighbour-stranger discrimination and animal social systems has not previously been considered, but such an association might be expected if social species were more likely to show neighbour-stranger discrimination due to a potentially higher level of individual recognition in line with the social intelligence hypothesis. Here, such an association is explored through two different approaches. (1) We critically reviewed 63 studies, involving 48 different mammal species with varying degrees of sociality, across 23 families and 5 orders, that conclusively tested neighbour-stranger discrimination. No link between social system and neighbour-stranger discrimination or behaviour was found. (2) We investigated neighbour-stranger discrimination in the distinctly solitary American mink, Neogale vison, observed at three different research areas in Iceland, by analysing 50 unmanipulated direct encounters between feral American mink, with 32 distinct pair combinations. These observations revealed strong evidence for dear enemy behaviour. The findings uncover that neighbour-stranger discrimination is prevalent across territorial mammalian species, irrespective of differences in social systems. This emphasizes the adaptive value of individual recognition, neighbour-stranger discrimination and behavioural modifications towards territory intruders based on identity, highlighting the complexity of social interactions and territorial dynamics even in solitary species. In addition, adherence to a dear enemy or nasty neighbour strategy is variable, influenced by multiple factors, and underscores the significance of individual recognition and aggression modulation in broader contexts, including invasive species management and self-recognition research. Future research should focus on documenting discrimination in hitherto underrepresented taxa and elucidating behaviour patterns under varying conditions.

Long-term studies of cyclic rodent populations have contributed fundamentally to the development of population ecology. Pioneering rodent studies have shown macroecological patterns of population dynamics in relation to latitude and have inspired similar studies in several other taxa. Nevertheless, such studies have not been able to disentangle the role of different environmental variables in shaping the macroecological patterns. We collected rodent time-series from 26 locations spanning 10 latitudinal degrees in the tundra biome of Fennoscandia and assessed how population dynamics characteristics of the most prevalent species varied with latitude and environmental variables. While we found no relationship between latitude and population cycle peak interval, other characteristics of population dynamics showed latitudinal patterns. The environmental predictor variables provided insight into causes of these patterns, as 1) increased proportion of optimal habitat in the landscape led to higher density amplitudes in all species and 2) mid-winter climate variability lowered the amplitude in Norwegian lemmings and grey-sided voles. These results indicate that biome-scale climate and landscape change can be expected to have profound impacts on rodent population cycles and that the macro-ecology of such functionally important tundra ecosystem characteristics is likely to be subjected to transient dynamics.

Demographic declines have important consequences for population viability, since they can lead to losses in genome diversity, as well as increased inbreeding and expression of deleterious mutations. Scandinavia was colonized by the Arctic fox (Vulpes lagopus) at the Pleistocene/Holocene transition, and the population has since been on the periphery of the global distribution. The Scandinavian population became even more fragmented in the early 1900s due to human persecution, and experienced an additional decline in the 1980s. We generated high-coverage genomes from pre-bottleneck, as well as modern Scandinavian and Russian specimens, and found that genome-wide diversity was lower and inbreeding higher in Scandinavia compared to the Siberian population, even prior to the historical bottleneck, most likely reflecting the long-term partial isolation and recent postglacial origin of the Scandinavian population. The southern subpopulation has the highest inbreeding levels, likely due to having been recently founded and highly isolated. Our results also show that although inbreeding increased substantially over the past century, the amount of total genetic load did not change. Overall, these findings illustrate the utility of a temporal approach to disentangle the genomic consequences of recent declines from ancient biogeographic processes.

Humans have relied on animal fur for centuries, yet fur farming only began recently during the mid-19th Century. Little is known about this incipient domestication or the genomic processes involved. Domestication may involve founder effects, population bottlenecks and low population size, which, when combined with intense artificial selection, lead to inbreeding, a limited gene pool and reduced fitness. The arctic fox (Vulpes lagopus) has been farmed intensively since the early 1900s and has been artificially selected for economic phenotypes. We investigated the origin of these lineages and the genomic consequences of intensive farming by comparing the genomes of farmed and wild arctic foxes from across their range. Our research indicates recent inbreeding through long Runs of Homozygosity and reduced genomic variation in farmed foxes relative to their respective wild populations. We identified a coastal ecotype origin for all Fennoscandian farmed arctic foxes, aligning them phylogenetically with the wild Icelandic population, a geographically isolated and phenotypically distinct coastal lineage. The depleted genome-wide heterozygosity and increased recent inbreeding in farmed fox lineages is consistent with a heavy consequence of domestication, shedding light on the demographic history and genomic consequences of human manipulation. We highlight the need for increased genomic investigations into fur farm populations to understand the incipient domestication process and uncover the cost of intense farming. The genomic consequences of domestication must be considered in the management of fur farms, with actionable steps needed to prevent descendants of escaped farmed foxes from polluting the gene pool in the wild through introgression.

Diseases and parasites are important drivers of population dynamics in wild mammal populations. Small and endangered populations that overlap with larger, reservoir populations are particularly vulnerable to diseases and parasites, especially in ecosystems highly influenced by climate change. Sarcoptic mange, caused by a parasitic mite Sarcoptes scabiei, constitutes a severe threat to many wildlife populations and is today considered a panzootic. The Scandinavian arctic fox Vulpes lagopus is endangered with a fragmented distribution and is threatened by e.g. red fox Vulpes vulpes expansion, prey scarcity and inbreeding depression. Moreover, one of the subpopulations in Scandinavia has suffered from repeated outbreaks of sarcoptic mange during the past decade, most likely spread by red foxes. This was first documented in 2013 and then again 2014, 2017, 2019, 2020 and 2021. We used field inventories and wildlife cameras to follow the development of sarcoptic mange outbreaks in this arctic fox subpopulation with specific focus on disease transmission and consequences for reproductive output. In 2013–2014, we documented visual symptoms of sarcoptic mange in about 30% of the total population. Despite medical treatment, we demonstrate demographic consequences where the number of arctic fox litters plateaued and litter size was reduced after the introduction of S. scabiei. Furthermore, we found indications that mange likely was transmitted by a few arctic foxes travelling between several dens, i.e. ‘super-spreaders'. This study highlights sarcoptic mange as a severe threat to small populations and can put the persistence of the entire Scandinavian arctic fox population at risk.

The trophic structures of tundra ecosystems are often viewed as a result of local terrestrial primary productivity. However, other resources can be brought in through long-distant migrants or be directly accessible in coastal areas. Hence, trophic structures may deviate from predictions based on local terrestrial resources. The Arctic fox (Vulpes lagopus) is a small canid that may use marine resources when available. We used stable isotope values in Arctic fox fur and literature data on potential prey to evaluate Arctic fox summer resource use in a mountain tundra without coastal access. The dietary contribution of local prey, presumably mostly rodents, declined with declining rodent abundance, with a subsequent increased contribution of migratory prey relying on marine resources. Stable isotope values did not differ between this terrestrial area and an area with direct coastal access during years of high rodent abundance, but isotope values during low rodent abundances suggested less marine input than in a coastal population feeding primarily on marine prey. Our study shows that marine resources may be used by animals in areas without any coastal access, and we highlight that such partial coupling of ecosystems must be included in the modeling and assessments of tundra environments.