Temporal and spatial fluctuations in food abundance can affect the feeding habits of vertebrates in tropical dry forests. We explored the effects of season and forest degradation in dietary patterns of the Spiny pocket mouse Lyomis pictus (Heteromyidae) in a tropical dry forest in Mexico. We used C (¹³C, ¹²C) and N (¹⁵N, ¹⁴N) stable isotope analyses to test the hypotheses that 1) L. pictus would increase its use of arthropods during the rainy season when seeds are less available on the forest floor, 2) L. pictus would increase the use of arthropods in degraded forest compared to conserved forest, and 3) the use of seeds from grasses by L. pictus would be more significant in the degraded forest.

Results/Conclusions Our first two hypotheses were proven wrong because assimilated biomass was derived almost exclusively from seeds in the dry and rainy seasons, and the importance of arthropods as food was marginal in both conserved and disturbed forests. We found a significant increase in the use of grass seeds in the disturbed forest in both the dry and rainy seasons. Seed hoarding during the season of high seed availability probably allows L. pictus to subsist on a seed-based diet throughout the year in conserved and disturbed forests. The significant presence of grasses in the disturbed forest would allow L. pictus to complement its diet with their seeds. Our study showed that season and forest degradation did not affect the use of plant and animal food by L. pictus. Given the wide distribution of this species, it remains to be tested whether the feeding patterns found in dry forest also occur in other ecosystems with different vegetation types (e.g., deserts and cloud forests).