Introduction Dietary analyses utilising visual methods to identify stomach and faecal contents have shown that urban red foxes (Vulpes vulpes) in the United Kingdom (UK) consume human-derived (anthropogenic) food to varying degrees. Anthropogenic foods have been implicated in poor health outcomes for synanthropic species that consume them, therefore it is critical to examine the degree of such foods in the UK fox diet. Methods We analysed the stable isotope ratios of carbon (δ13C) and nitrogen (δ15N) of whiskers collected from 93 foxes from across the UK to determine: 1) the viability of stable isotope analysis (SIA) for distinguishing between anthropogenic and natural food consumption in foxes; 2) the degree to which urban foxes consume anthropogenic food compared to rural foxes using a Bayesian mixing model; 3) whether factors such as sex, age and season predict individual levels of consumption of human-derived food in foxes. Main results 1) Urban fox diet was significantly different to rural foxes: urban foxes demonstrated significantly higher δ13C and lower δ15N, a pattern consistent with anthropogenic food consumption. 2) We estimated that food provided either directly or indirectly by humans contributed around 35% of the urban fox diet compared to approximately 6% in the rural fox diet. 3) Across rural and urban foxes combined, there were significant isotopic differences between males and females, with females demonstrating higher δ13C and lower δ15N. 4) No differences in δ13C and δ15N between subadults and adults were observed. 5) Values of δ15N differed significantly between samples from the summer and the winter, where summer had the highest δ15N and winter the lowest. Additionally, winter had the highest δ13C values of the seasons, although this difference was not significant. Conclusion Potential negative outcomes of anthropogenic food consumption are likely to disproportionately impact females than males and urban-dwelling foxes than rural foxes.

Abstract 1. Classic evolutionary theory suggests that sexual dimorphism evolves primarily via sexual and fecundity selection. However, theory and evidence is beginning to accumulate suggesting that resource competition can drive the evolution of sexual dimorphism, via ecological character displacement between sexes. A key prediction of this hypothesis is that the extent of ecological divergence between sexes will be associated with the extent of sexual dimorphism. 2. As the stable isotope ratios of animal tissues provide a quantitative measure of various aspects of ecology, we carried out a meta-analysis examining associations between the extent of isotopic divergence between sexes and the extent of body size dimorphism. Our models demonstrate that large amounts of between-study variation in isotopic (ecological) divergence between sexes is due to systematic heterogeneity, which may be associated with the traits of study subjects. We then completed meta-regressions to examine whether the extent of isotopic divergence between sexes is associated with the extent of sexual size dimorphism. 3. We found a modest but significantly positive association between size dimorphism and sex differences in trophic level. Furthermore, the strength of this positive association varied between ecological contexts, increasing in species whose diets provide the greatest scope for trophic variation and in those for which body size is of greater relevance to feeding. 4. Our results therefore provide further evidence that ecologically mediated selection, unrelated to reproduction, can contribute to the evolution of sexual dimorphism.

1. Dietary studies in birds of prey involve direct observation and examination of food remains at resting and nesting sites. Although these methods accurately identify diet in raptors, they are time-consuming, resource-intensive, and associated with biases from the feeding ecology of raptors like Gyps vultures. Our study set out to estimate diet composition in Gyps vultures informed by stable isotopes that provide a good representation of assimilated diet from local systems. 2. We hypothesized that differences in Gyps vulture diet composition is a function of sampling location, and that these vultures move between Serengeti National Park and Selous Game Reserve protected areas to forage. We also, theorised that grazing ungulates are the principal items in Gyps vulture diet. 3. Through combined linear and Bayesian modelling, diet-derived from 13C in Gyps vultures consisted of grazing herbivores across sites, with those in Serengeti National Park consuming higher proportions of grazing herbivores (> 87%). 13C differences in vulture feather subsets did not indicate shifts in vulture diet and combined with blood 13C, vultures fed largely on grazers for ~159 days before they were sampled. Similarly, 15N values indicated Gyps vultures fed largely on herbivores. 34S ratios separated where vultures fed when the two sites were compared. 34S variation in vultures across sites resulted from baseline differences in plant 34S values, though it is not possible to match 34S to specific locations. 4. Our findings highlight the relevance of repeated sampling that considers tissues with varying isotopic turnover and emerging Bayesian techniques for dietary studies using stable isotopes. Findings also suggested limited vulture movement between the two local systems. However, more sampling coupled with telemetry is required to fully comprehend this observation and its implications to Gyps vulture ecology and conservation.