Seed Dispersal Effectiveness of Chelonoidis carbonarius and C. denticulatus Tortoises Is Mediated by Body Size and Sex-Differences in Tortoise Movement

Abstract

Red-footed (Chelonoidis carbonarius) and yellow-footed tortoises (C. denticulatus) are highly frugivorous, terrestrial chelonians found throughout the Neotropics. We investigated the role of these tortoises as seed dispersers in plant communities in the northern Brazilian region of the Amazon Basin. We analyzed movement patterns of radio- and thread-tracked individuals, estimated gut retention time of seeds in captive tortoises, and analyzed fecal samples for abundance, diversity, and viability of seeds. Our results show tortoises to be effective seed dispersers: of 113 fecal samples examined, 92% contained seeds of 19 plant species. Seeds of 17 of these species were viable. Fecal clumps averaged 2.2 species (range 0–5) per unit with a median of 102 seeds per sample (range 0–1140). Larger-bodied tortoises dispersed more seeds. Integrating these data through movement simulations revealed that long gut retention times for seeds (10–28 days), combined with mean daily tortoise displacements of 103 m, generate seed shadow curves that peak between 300 m (female tortoises) and 400 m (male tortoises) and extended beyond 1 km from the seed source. This study demonstrates differential disperser effectiveness as a result of tortoise size and differences in movement patterns between sexes.