Comparative molecular analyses of Eimeria Schneider (Apicomplexa: Eimeriidae) species from rock ptarmigan in Iceland, Svalbard-Norway, and Japan.

The rock ptarmigan (Lagopus muta) has a Holarctic breeding distribution and is found in arctic and sub-arctic regions. Isolated populations and glacial relicts occur in alpine areas south of the main range, like the Pyrenees in Europe, the Pamir mountains in Central Asia, and the Japanese Alps. In recent decades considerable effort has been made to clarify parasite infections in the rock ptarmigan. Seven Eimeria spp. have been reported parasitizing rock ptarmigan. Two of those species, E. uekii and E. raichoi parasitizing rock ptarmigan (L. m. japonica) in Japan, have been identified genetically. Here we compare partial sequences of nuclear (18S rRNA) and mitochondrial (COI) genes and we detail the morphology of sporulated oocysts of E. uekii and E. raichoi from Japan, E. muta and E. rjupa, from the rock ptarmigan (L. m. islandorum) in Iceland, and two undescribed eimerian morphotypes, Eimeria sp. A, and Eimeria sp. B, from rock ptarmigan (L. m. hyperborea) in Norway (Svalbard in the Norwegian Archipelago). Two morphotypes, ellipsoidal and spheroidal, are recognized for each of the three host subspecies. Our phylogenetic analysis suggests that the ellipsoidal oocyst types, E. uekii, E. muta, and Eimeria sp. A (Svalbard-Norway) are identical and infects rock ptarmigan in Japan, Iceland, and Svalbard-Norway, respectively. Eimeria uekii was first described in Japan in 1981 so that E. muta, described in Iceland in 2007, and Eimeria sp. A in Svalbard-Norway are junior synonyms of E. uekii. Also, phylogenetic analysis shows that the spheroidal oocyst types, E. rjupa and Eimeria sp. B (Svalbard-Norway), are identical, indicating that rock ptarmigan in Iceland and Svalbard-Norway are infected by the same Eimeria species and differ from E. raichoi in Japan.

Mitochondrial genetic diversity and phylogenetic relationships of Echinococcus multilocularis in Europe.

The cestode Echinococcus multilocularis is the causative agent of alveolar echinococcosis, a fatal zoonotic parasitic disease of the northern hemisphere. Red foxes are the main reservoir hosts and, likely, the main drivers of the geographic spread of the disease in Europe. Knowledge of genetic relationships among E. multilocularis isolates at a European scale is key to understanding the dispersal characteristics of E. multilocularis. Hence, the present study aimed to describe the genetic diversity of E. multilocularis isolates obtained from different host species in 19 European countries. Based on the analysis of complete nucleotide sequences of the cob, atp6, nad2, nad1 and cox1 mitochondrial genes (4,968 bp), 43 haplotypes were inferred. Four haplotypes represented 62.56 % of the examined isolates (142/227), and one of these four haplotypes was found in each country investigated, except Svalbard, Norway. While the haplotypes from Svalbard were markedly different from all the others, mainland Europe appeared to be dominated by two main clusters, represented by most western, central and eastern European countries, and the Baltic countries and northeastern Poland, respectively. Moreover, one Asian-like haplotype was identified in Latvia and northeastern Poland. To better elucidate the presence of Asian genetic variants of E. multilocularis in Europe, and to obtain a more comprehensive Europe-wide coverage, further studies, including samples from endemic regions not investigated in the present study, especially some eastern European countries, are needed. Further, the present work proposes historical causes that may have contributed to shaping the current genetic variability of E. multilocularis in Europe.

Extreme events, trophic chain reactions, and shifts in phenotypic selection.

Demographic consequences of rapid environmental change and extreme climatic events (ECEs) can cascade across trophic levels with evolutionary implications that have rarely been explored. Here, we show how an ECE in high Arctic Svalbard triggered a trophic chain reaction, directly or indirectly affecting the demography of both overwintering and migratory vertebrates, ultimately inducing a shift in density-dependent phenotypic selection in migratory geese. A record-breaking rain-on-snow event and ice-locked pastures led to reindeer mass starvation and a population crash, followed by a period of low mortality and population recovery. This caused lagged, long-lasting reductions in reindeer carrion numbers and resultant low abundances of Arctic foxes, a scavenger on reindeer and predator of migratory birds. The associated decrease in Arctic fox predation of goose offspring allowed for a rapid increase in barnacle goose densities. As expected according to r- and K-selection theory, the goose body condition (affecting reproduction and post-fledging survival) maximising Malthusian fitness increased with this shift in population density. Thus, the winter ECE acting on reindeer and their scavenger, the Arctic fox, indirectly selected for higher body condition in migratory geese. This high Arctic study provides rare empirical evidence of links between ECEs, community dynamics and evolution, with implications for our understanding of indirect eco-evolutionary impacts of global change.

A circumpolar parasite: Evidence of a cryptic undescribed species of sucking louse, Linognathus sp., collected from Arctic foxes, Vulpes lagopus, in Nunavut (Canada) and Svalbard (Norway).

The North has experienced unprecedented rates of warming over the past few decades, impacting the survival and development of insects and the pathogens that they carry. Since 2019, Arctic foxes from Canada (Nunavut) have been observed with fur loss inconsistent with natural shedding of fur. Adult lice were collected from Arctic foxes from Nunavut (n = 1) and Svalbard (n = 2; Norway) and were identified as sucking lice (suborder Anoplura). Using conventional PCR targeting the mitochondrial cytochrome c oxidase subunit 1 gene (cox1), lice from Canada and Svalbard were 100% similar (8 pooled samples from Nunavut and 3 pooled samples from Svalbard), indicating that there is potential gene flow between ectoparasites on Scandinavian and North American Arctic fox populations. The cox1 sequences of Arctic fox lice and dog sucking lice (Linognathus setosus) had significant differences (87% identity), suggesting that foxes may harbour a cryptic species that has not previously been recognised. Conventional PCR targeting the gltA gene for Bartonella bacteria amplified DNA from an unknown gammaproteobacteria from two pooled louse samples collected from Svalbard foxes. The amplified sequences were 100% identical to each other but were only 78% like Proteus mirabilis reported in GenBank (CP053614), suggesting that lice on Arctic foxes may carry unique microorganisms that have yet to be described.

Occurrence of ingested human litter in winter arctic foxes (Vulpes lagopus) from Svalbard, Norway.

The aim of this study is to assess the occurrence of human litter ingested by arctic foxes (Vulpes lagopus) caught in Svalbard, Norway, in winter when scavenging is at its highest. Twenty arctic fox stomachs and intestines were examined for human litter and plastic using the protocol from the Oslo-Paris Convention (OSPAR) for monitoring plastic ingestion by the northern fulmar (Fulmarus glacialis) (human litter and plastic >1 mm). The arctic foxes had ingested human litter at a low frequency (15%, 3 out of 20 foxes). Despite the low sample size, we do not regard ingestion of human litter as an immediate threat to the arctic fox population in Svalbard.

Climate variability and density-dependent population dynamics: Lessons from a simple High Arctic ecosystem.

Ecologists are still puzzled by the diverse population dynamics of herbivorous small mammals that range from high-amplitude, multiannual cycles to stable dynamics. Theory predicts that this diversity results from combinations of climatic seasonality, weather stochasticity, and density-dependent food web interactions. The almost ubiquitous 3- to 5-y cycles in boreal and arctic climates may theoretically result from bottom-up (plant-herbivore) and top-down (predator-prey) interactions. Assessing, empirically, the roles of such interactions and how they are influenced by environmental stochasticity has been hampered by food web complexity. Here, we take advantage of a uniquely simple High Arctic food web, which allowed us to analyze the dynamics of a graminivorous vole population not subjected to top-down regulation. This population exhibited high-amplitude, noncyclic fluctuations-partly driven by weather stochasticity. However, the predominant driver of the dynamics was overcompensatory density dependence in winter that caused the population to frequently crash. Model simulations showed that the seasonal pattern of density dependence would yield regular 2-y cycles in the absence of stochasticity. While such short cycles have not yet been observed in mammals, they are theoretically plausible if graminivorous vole populations are deterministically bottom-up regulated. When incorporating weather stochasticity in the model simulations, cyclicity became disrupted and the amplitude was increased-akin to the observed dynamics. Our findings contrast with the 3- to 5-y population cycles that are typical of graminivorous small mammals in more complex food webs, suggesting that top-down regulation is normally an important component of such dynamics.

Iterative model predictions for wildlife populations impacted by rapid climate change.

To improve understanding and management of the consequences of current rapid environmental change, ecologists advocate using long-term monitoring data series to generate iterative near-term predictions of ecosystem responses. This approach allows scientific evidence to increase rapidly and management strategies to be tailored simultaneously. Iterative near-term forecasting may therefore be particularly useful for adaptive monitoring of ecosystems subjected to rapid climate change. Here, we show how to implement near-term forecasting in the case of a harvested population of rock ptarmigan in high-arctic Svalbard, a region subjected to the largest and most rapid climate change on Earth. We fitted state-space models to ptarmigan counts from point transect distance sampling during 2005-2019 and developed two types of predictions: (1) explanatory predictions to quantify the effect of potential drivers of ptarmigan population dynamics, and (2) anticipatory predictions to assess the ability of candidate models of increasing complexity to forecast next-year population density. Based on the explanatory predictions, we found that a recent increasing trend in the Svalbard rock ptarmigan population can be attributed to major changes in winter climate. Currently, a strong positive effect of increasing average winter temperature on ptarmigan population growth outweighs the negative impacts of other manifestations of climate change such as rain-on-snow events. Moreover, the ptarmigan population may compensate for current harvest levels. Based on the anticipatory predictions, the near-term forecasting ability of the models improved nonlinearly with the length of the time series, but yielded good forecasts even based on a short time series. The inclusion of ecological predictors improved forecasts of sharp changes in next-year population density, demonstrating the value of ecosystem-based monitoring. Overall, our study illustrates the power of integrating near-term forecasting in monitoring systems to aid understanding and management of wildlife populations exposed to rapid climate change. We provide recommendations for how to improve this approach.

Ancient origin and genetic segregation of canine circovirus infecting arctic foxes (Vulpes lagopus) in Svalbard and red foxes (Vulpes vulpes) in Northern Norway.

Canine circovirus (CanineCV) is a relatively new viral species, belonging to the family Circoviridae, whose pathogenic role is still uncertain. Since its first description in one domestic dog in 2011 from the USA, several reports have been documenting its distribution worldwide. Recently, CanineCV was also detected in wild animals such as wolves, foxes and badgers. In order to investigate the presence and the genetic characteristics of CanineCV in foxes of Arctic and Sub-Arctic regions, the presence of CanineCV DNA in internal organs (liver and spleen) of 51 arctic foxes (Vulpes lagopus) from Svalbard archipelago and 59 red foxes (Vulpes vulpes) from Northern Norway, sampled from 1996 to 2001 and from 2014 to 2018, respectively, was screened by real-time PCR. CanineCV was detected in 11/51 arctic foxes and in 10/59 red foxes, backdating the circulation of the virus at least to 1996 in the arctic fox population. The complete genome of 14 identified CanineCV was sequenced and analysed showing an identity higher than 80.8% with the reference strains available to date. According to the species demarcation threshold of 80% genome-wide nucleotide sequence identity for members of the family Circoviridae provided by International Committee on Taxonomy of Viruses (ICTV), all the CanineCV belong to a single species. Phylogenetic analysis revealed that all the CanineCV were subdivided into five main clusters with one including only CanineCV identified in foxes. Furthermore, CanineCV identified in arctic foxes and red foxes formed two distinct lineages. From these data, we hypothesize that the viral transmission did not occur between the two species of foxes as a consequence of the lack of contact between the two hosts or that the virus acquired mutations in the time elapsed between the samplings.

Environmental change reduces body condition, but not population growth, in a high-arctic herbivore.

Environmental change influences fitness-related traits and demographic rates, which in herbivores are often linked to resource-driven variation in body condition. Coupled body condition-demographic responses may therefore be important for herbivore population dynamics in fluctuating environments, such as the Arctic. We applied a transient Life-Table Response Experiment ('transient-LTRE') to demographic data from Svalbard barnacle geese (Branta leucopsis), to quantify their population-dynamic responses to changes in body mass. We partitioned contributions from direct and delayed demographic and body condition-mediated processes to variation in population growth. Declines in body condition (1980-2017), which positively affected reproduction and fledgling survival, had negligible consequences for population growth. Instead, population growth rates were largely reproduction-driven, in part through positive responses to rapidly advancing spring phenology. The virtual lack of body condition-mediated effects indicates that herbivore population dynamics may be more resilient to changing body condition than previously expected, with implications for their persistence under environmental change.

Arctic terrestrial biodiversity status and trends: A synopsis of science supporting the CBMP State of Arctic Terrestrial Biodiversity Report.

This review provides a synopsis of the main findings of individual papers in the special issue Terrestrial Biodiversity in a Rapidly Changing Arctic. The special issue was developed to inform the State of the Arctic Terrestrial Biodiversity Report developed by the Circumpolar Biodiversity Monitoring Program (CBMP) of the Conservation of Arctic Flora and Fauna (CAFF), Arctic Council working group. Salient points about the status and trends of Arctic biodiversity and biodiversity monitoring are organized by taxonomic groups: (1) vegetation, (2) invertebrates, (3) mammals, and (4) birds. This is followed by a discussion about commonalities across the collection of papers, for example, that heterogeneity was a predominant pattern of change particularly when assessing global trends for Arctic terrestrial biodiversity. Finally, the need for a comprehensive, integrated, ecosystem-based monitoring program, coupled with targeted research projects deciphering causal patterns, is discussed.

Contrasting consequences of climate change for migratory geese: Predation, density dependence and carryover effects offset benefits of high-arctic warming.

Climate change is most rapid in the Arctic, posing both benefits and challenges for migratory herbivores. However, population-dynamic responses to climate change are generally difficult to predict, due to concurrent changes in other trophic levels. Migratory species are also exposed to contrasting climate trends and density regimes over the annual cycle. Thus, determining how climate change impacts their population dynamics requires an understanding of how weather directly or indirectly (through trophic interactions and carryover effects) affects reproduction and survival across migratory stages, while accounting for density dependence. Here, we analyse the overall implications of climate change for a local non-hunted population of high-arctic Svalbard barnacle geese, Branta leucopsis, using 28 years of individual-based data. By identifying the main drivers of reproductive stages (egg production, hatching and fledging) and age-specific survival rates, we quantify their impact on population growth. Recent climate change in Svalbard enhanced egg production and hatching success through positive effects of advanced spring onset (snow melt) and warmer summers (i.e. earlier vegetation green-up) respectively. Contrastingly, there was a strong temporal decline in fledging probability due to increased local abundance of the Arctic fox, the main predator. While weather during the non-breeding season influenced geese through a positive effect of temperature (UK wintering grounds) on adult survival and a positive carryover effect of rainfall (spring stopover site in Norway) on egg production, these covariates showed no temporal trends. However, density-dependent effects occurred throughout the annual cycle, and the steadily increasing total flyway population size caused negative trends in overwinter survival and carryover effects on egg production. The combination of density-dependent processes and direct and indirect climate change effects across life history stages appeared to stabilize local population size. Our study emphasizes the need for holistic approaches when studying population-dynamic responses to global change in migratory species.

Circumpolar status of Arctic ptarmigan: Population dynamics and trends.

Rock ptarmigan (Lagopus muta) and willow ptarmigan (L. lagopus) are Arctic birds with a circumpolar distribution but there is limited knowledge about their status and trends across their circumpolar distribution. Here, we compiled information from 90 ptarmigan study sites from 7 Arctic countries, where almost half of the sites are still monitored. Rock ptarmigan showed an overall negative trend on Iceland and Greenland, while Svalbard and Newfoundland had positive trends, and no significant trends in Alaska. For willow ptarmigan, there was a negative trend in mid-Sweden and eastern Russia, while northern Fennoscandia, North America and Newfoundland had no significant trends. Both species displayed some periods with population cycles (short 3-6 years and long 9-12 years), but cyclicity changed through time for both species. We propose that simple, cost-efficient systematic surveys that capture the main feature of ptarmigan population dynamics can form the basis for citizen science efforts in order to fill knowledge gaps for the many regions that lack systematic ptarmigan monitoring programs.

Gastrointestinal parasites of arctic foxes (Vulpes lagopus) and sibling voles (Microtus levis) in Spitsbergen, Svalbard.

The arctic fox (Vulpes lagopus), an apex predator with an omnipresent distribution in the Arctic, is a potential source of intestinal parasites that may endanger people and pet animals such as dogs, thus posing a health risk. Non-invasive methods, such as coprology, are often the only option when studying wildlife parasitic fauna. However, the detection and identification of parasites are significantly enhanced when used in combination with methods of molecular biology. Using both approaches, we identified unicellular and multicellular parasites in faeces of arctic foxes and carcasses of sibling voles (Microtus levis) in Svalbard, where molecular methods are used for the first time. Six new species were detected in the arctic fox in Svalbard, Eucoleus aerophilus, Uncinaria stenocephala, Toxocara canis, Trichuris vulpis, Eimeria spp., and Enterocytozoon bieneusi, the latter never found in the arctic fox species before. In addition, only one parasite was found in the sibling vole in Svalbard, the Cryptosporidium alticolis, which has never been detected in Svalbard before.

Temporal trend of mercury in relation to feeding habits and food availability in arctic foxes (Vulpes lagopus) from Svalbard, Norway.

We investigated the temporal trend of mercury (Hg) in arctic foxes from Svalbard, Norway sampled in the period 1997-2014 (n = 109, from 11 trapping seasons). We used linear models to investigate the effect of trapping season, feeding habits (δ13C), food availability from marine and terrestrial ecosystems (reindeer carcasses and sea ice cover), sex, age and body condition on liver total Hg (THg) levels. Liver THg levels increased in arctic foxes with 7.2% (95% CI: 2.3, 9.6) per year when the concentrations were adjusted for variation of δ13C, sea ice cover, and reindeer carcasses, whereas the raw annual trend was 3.5% (CI: -0.11, 7.2). However, the THg levels in arctic foxes from Svalbard are still lower than other marine mammals. We also demonstrate that arctic fox terrestrial food consumption is important for lowering the overall THg levels in this species.

Unique genetic features of canine adenovirus type 1 (CAdV-1) infecting red foxes (Vulpes vulpes) in northern Norway and arctic foxes (Vulpes lagopus) in Svalbard.

Canine adenovirus type 1 (CAdV-1) is the aetiological agent of infectious canine hepatitis (ICH) in domestic dogs (Canis familiaris). In spite of the widespread use of vaccination, CAdV-1 continues to circulate in the dog population. Although a high number of serological screenings have indicated that CAdV-1 is widespread in fox species, little is known about the potential role of foxes as reservoirs of CAdV-1. Furthermore, very little data exist on the molecular features of this virus in foxes. To add to existing knowledge on CAdV-1 circulating in wild carnivores, tissue samples from CAdV-seropositive red foxes (Vulpes vulpes, n = 10) from the northern mainland of Norway and arctic foxes (Vulpes lagopus, n = 10) from the Svalbard archipelago, Norway, were investigated using a molecular approach to detect CAdV-1 DNA and important structural and non-structural genes of the detected viruses were sequenced and analysed. Amplicons characteristic for CAdV-1 were amplified from 14 out of 20 foxes (7 red foxes and 7 arctic foxes) and spleen and lymph node tissues resulted optimal targets for the viral DNA detection. The nucleotide sequences showed unique features that distinguished the viruses detected in this study from the CAdV-1 to date identified in wild carnivores and dogs. Greater attention should be given to genetically different CAdV-1 circulating in wild carnivores that may be transferred to dogs, potentially causing disease and reducing the effectiveness of available vaccines.

Mandibular shape in farmed Arctic foxes (Vulpes lagopus) exposed to persistent organic pollutants.

We investigated if dietary exposure to persistent organic pollutants (POPs) affect mandibular asymmetry and periodontal disease in paired male-siblings of Arctic foxes (Vulpes lagopus). During ontogeny, one group of siblings was exposed to the complexed POP mixture in naturally contaminated minke whale (Balaenoptere acutorostarta) blubber (n = 10), while another group was given wet feed based on pig (Sus scrofa) fat as a control (n = 11). The ∑POP concentrations were 802 ng/g ww in the whale-based feed compared to 24 ng/g ww in the control diet. We conducted a two-dimensional geometric morphometric (GM) analysis of mandibular shape and asymmetry in the foxes and compared the two groups. The analyses showed that directional asymmetry was higher than fluctuating asymmetry in both groups and that mandibular shape differed significantly between the exposed and control group based on discriminant function analysis (T2 = 58.52, p = 0.04, 1000 permutations). We also found a non-significantly higher incidence of periodontal disease (two-way ANOVA: p = 0.43) and greater severity of sub-canine alveolar bone deterioration similar to periodontitis (two-way ANOVA: p = 0.3) in the POP-exposed group. Based on these results, it is possible that dietary exposure to a complexed POP mixture lead to changes in jaw morphology in Arctic foxes. This study suggests that extrinsic factors, such as dietary exposure to POPs, may affect mandibular shape and health in a way that could be harmful to wild Arctic populations. Therefore, further studies using GM analysis as an alternative to traditional morphometric methods should be conducted for wild Arctic fox populations exposed to environmental contaminants.

Emission Changes Dwarf the Influence of Feeding Habits on Temporal Trends of Per- and Polyfluoroalkyl Substances in Two Arctic Top Predators.

We monitored concentrations of per- and polyfluoroalkyl substances (PFASs) in relation to climate-associated changes in feeding habits and food availability in polar bears (Ursus maritimus) and arctic foxes (Vulpes lagopus) (192 plasma and 113 liver samples, respectively) sampled from Svalbard, Norway, during 1997-2014. PFASs concentrations became greater with increasing dietary trophic level, as bears and foxes consumed more marine as opposed to terrestrial food, and as the availability of sea ice habitat increased. Long-chained perfluoroalkyl carboxylates (PFCAs) in arctic foxes decreased with availability of reindeer carcasses. The ∼9-14% yearly decline of C6-8 perfluoroalkyl sulfonates (PFSAs) following the cease in C6-8 PFSA precursor production in 2001 indicates that the peak exposure was mainly a result of atmospheric transport of the volatile precursors. However, the stable PFSA concentrations since 2009-2010 suggest that Svalbard biota is still exposed to ocean-transported PFSAs. Long-chain ocean-transported PFCAs increased 2-4% per year and the increase in C12-14 PFCAs in polar bears tended to level off since ∼2009. Emerging short-chain PFASs showed no temporal changes. Climate-related changes in feeding habits and food availability moderately affected PFAS trends. Our results indicate that PFAS concentrations in polar bears and arctic foxes are mainly affected by emissions.

Vole abundance and reindeer carcasses determine breeding activity of Arctic foxes in low Arctic Yamal, Russia.

BACKGROUND: High latitude ecosystems are at present changing rapidly under the influence of climate warming, and specialized Arctic species at the southern margin of the Arctic may be particularly affected. The Arctic fox (Vulpes lagopus), a small mammalian predator endemic to northern tundra areas, is able to exploit different resources in the context of varying tundra ecosystems. Although generally widespread, it is critically endangered in subarctic Fennoscandia, where a fading out of the characteristic lemming cycles and competition with abundant red foxes have been identified as main threats. We studied an Arctic fox population at the Erkuta Tundra Monitoring site in low Arctic Yamal (Russia) during 10 years in order to determine which resources support the breeding activity in this population. In the study area, lemmings have been rare during the last 15 years and red foxes are nearly absent, creating an interesting contrast to the situation in Fennoscandia. RESULTS: Arctic fox was breeding in nine of the 10 years of the study. The number of active dens was on average 2.6 (range 0-6) per 100 km2 and increased with small rodent abundance. It was also higher after winters with many reindeer carcasses, which occurred when mortality was unusually high due to icy pastures following rain-on-snow events. Average litter size was 5.2 (SD = 2.1). Scat dissection suggested that small rodents (mostly Microtus spp.) were the most important prey category. Prey remains observed at dens show that birds, notably waterfowl, were also an important resource in summer. CONCLUSIONS: The Arctic fox in southern Yamal, which is part of a species-rich low Arctic food web, seems at present able to cope with a state shift of the small rodent community from high amplitude cyclicity with lemming dominated peaks, to a vole community with low amplitude fluctuations. The estimated breeding parameters characterized the population as intermediate between the lemming fox and the coastal fox ecotype. Only continued ecosystem-based monitoring will reveal their fate in a changing tundra ecosystem.

Exposure to Persistent Organic Pollutants Reduces Testosterone Concentrations and Affects Sperm Viability and Morphology during the Mating Peak Period in a Controlled Experiment on Farmed Arctic Foxes (Vulpes lagopus).

We investigated testosterone production and semen parameters in farmed Arctic foxes by dietary exposure to persistent organic pollutants (POPs) for 22 months. Eight male foxes were given a diet of POP-contaminated minke whale blubber, whereas their eight male siblings were fed a control diet containing pig fat as the main fat source. The minke whale-based feed contained a ∑POPs concentration of 802 ng/g ww, whereas the pig-based feed contained ∑POPs of 24 ng/g ww. At the end of the experiment, ∑POP concentrations in adipose tissue were 8856 ± 2535 ng/g ww in the exposed foxes and 1264 ± 539 ng/g ww in the control foxes. The exposed group had 45-64% significantly lower testosterone concentrations during their peak mating season compared to the controls (p ≤ 0.05), while the number of dead and defect sperm cells was 27% (p = 0.07) and 15% (p = 0.33) higher in the exposed group. Similar effects during the mating season in wild Arctic foxes may affect mating behavior and reproductive success. On the basis of these results, we recommend testosterone as a sensitive biomarker of POP exposure and that seasonal patterns are investigated when interpreting putative endocrine disruption in Arctic wildlife with potential population-level effects.