Individual specialization can influence important ecological and evolutionary traits and both inter- and intra-individual variation in resource use can drive niche shifts in natural populations. We evaluated the predominance of these two factors for determining seasonal differences in the trophic niche of the didelphid marsupial Gracilinanus agilis (Burmeister, 1854) in the highly seasonal Brazilian savanna. In the three sampled sites, the population of G. agilis increased its dietary niche width in the warm–wet season, when food resources are more abundant, and there were no differences between sexes and no interaction between season and sex. However, the evaluation of intra-individual variation indicated that females reduce the number of items consumed during the warm–wet season, whereas males show no seasonal differences. Inter-individual variation nonetheless followed the overall population pattern because both sexes increased their spread with respect to food-item consumption in the warm–wet season. Additionally, we found positive relationships between body length and diet only in the warm–wet season, when larger animals fed more on invertebrates and less on fruits than the small ones. Our results show a previously unknown pattern for mammals, in which the trophic niche is wider during the high-resource season as a result of inter-individual variation along the body-size axis. © 2014 The Linnean Society of London, Biological Journal of the Linnean Society, 2014, 111, 737–747.