Seasonal isotopic niche of a rodent : high between-individual variation but no changes in individual niche width during the rich-resource period

The dynamics of trophic niche width in animals at both population level and individual level is potentially influenced by temporal variation of food resources, by between-individual differences in food-resource rank preferences, and also by competition. Using stable isotope of carbon and nitrogen (δ13C and δ15 N) of fecal samples, we investigated the trophic niche dynamics and individual variation in food-resource use by the arboreal rat Rhipidomys macrurus, in the highly seasonal Brazilian savanna (Cerrado). We tested the hypothesis that dietary niche expansion during the rich-resource period (wet season) occurs via individual specialization and consequently lower individual niche overlap in contrast with niche retraction during the low-resource period (dry season) via increase in niche overlap and expansion of individual niches. The results indicated that R. macrurus is primarily frugivorous and presents a wider isotopic niche in the rich-resource period in comparison with the low-resource period. The increase in niche width was achieved by individual specialization (decrease in niche overlap), as expected. During the low-resource period, however, individual niche widths were not wider than during the rich-resource period. Additionally, individual body condition was lower in the wet season than in the dry season, suggesting higher competition in this period. We conclude that an increase in the population niche may involve only between-individual variation and not necessarily requiring changes in individual niche width. We propose that the combination of ecological opportunity (high resource diversity) in addition to a greater competition in the warm–wet season leads to expansion of the population trophic niche width via individual specialization