Between forest and city: stable isotope evidence for anthropogenic impacts on the dietary ecology of the vulnerable Wied’s marmosets in Brazil

Anthropogenic disturbance is a major driver of environmental change, altering resource availability and the feeding ecology of primates, particularly in rapidly changing tropical landscapes. The vulnerable Wied’s marmoset ( Callithrix kuhlii ), endemic to the Brazilian Atlantic Forest, inhabits increasingly human-modified environments. However, how these changes affect its feeding ecology remains poorly understood. Here, we used carbon (δ 13 C) and nitrogen (δ 15 N) stable isotope analyses to investigate the dietary ecology of free-ranging Wied’s marmoset populations across 14 municipalities representing a gradient of forest cover within human-modified landscapes in southern Bahia, Brazil. We analyzed hair samples from 107 individuals across 30 social groups, alongside isotopic data from potential dietary resources. Linear mixed models were used to assess the effects of individual characteristics (age and sex), total monthly precipitation, forest cover, and human population density on marmoset hair isotopic values across all 14 municipalities. For the subset of six sites for which isotopic data from potential dietary resources were available, we then ran stable isotope mixing models to estimate dietary composition and evaluate how resource contributions changed in marmosets across the forest cover gradient. We found age-related differences in δ 13 C values, suggesting variation in resource use across life stages. Both δ 13 C and δ 15 N values were negatively associated with forest cover, with individuals in less forested sites exhibiting higher isotopic values. Mixing models indicated that fruit and insects were the main dietary components, although their relative contributions varied greatly across sites. Populations in more forested sites showed higher fruit consumption, whereas those in less forested areas relied more heavily on insects. The isotopic signatures of populations inhabiting less forested landscapes may also suggest increased insect consumption and/or the opportunistic use of anthropogenic food resources. These findings indicate that C. kuhlii exhibits dietary flexibility associated with variation in forest cover and resource availability, while highlighting the importance of forest cover in maintaining native food resources and dietary composition. Our study provides new insights into how forest cover and associated habitat modification influence the dietary ecology of small arboreal primates in rapidly changing tropical environments.