Larger Doesn't Mean Longer: Neither Body Size Nor Seed Size Affect the Gut Retention Times of Aldabra Giant Tortoises

Abstract

Seed dispersal through endozoochory is a critical ecosystem function worldwide. Seed gut retention time (GRT; the duration that seeds are retained in the digestive tract) is an important part of the qualitative component of the seed dispersal effectiveness framework. GRT is a major determinant of when and how far away seeds are dispersed, aiding seeds in escaping predation in space as well as in time. In this study, we examined whether the size of the disperser and/or the size of the ingested seeds affect the GRT in Aldabra Giant Tortoises (Aldabrachelys gigantea) on Aldabra Atoll, Seychelles. We selected tortoises of varying body mass (mean ± 1 SD = 48.6 ± 27.2 kg, range = 0.6–104.0 kg) and fed them different-sized artificial seeds (plastic beads; 2, 4.5, and 10 mm diameter). Tortoises defecated the first beads a mean of 12.0 ± 2.7 d after ingestion, and the last beads 20.4 ± 6.0 d after ingestion. Mean GRT was 14.6 ± 3.7 d. We show that neither tortoise body size nor bead size had an effect on the patterns and time of defecation. We discuss the relevance of our result for seed dispersal and for rewilding projects that use Aldabra Giant Tortoises as substitute species for extinct giant tortoises on other oceanic islands.