Animal seed dispersal may affect the distribution and structure of vegetation, and has crucial implications for forest regeneration. The reduction of seed dispersers in fragmented landscapes is therefore an important theme in conservation biology. We evaluated the efficiency of one of the largest Mesoamerican primate species – spider monkey (Ateles geoffroyi) – as a seed disperser and analyzed if this mutualistic interaction is altered in forest fragments. During 15 months in the rainforest of Lacandona, Chiapas, we collected foraging data using focal animal observations and we collected fecal samples to identify seeds dispersed and to perform germination experiments. Using 3 troops from continuous forest and 3 from fragments we assessed fruit and seed handling, diversity and composition of defecated seeds, seed germination, and dispersal distance of defecated seeds to nearest conspecific trees.

Results/Conclusions Approximately 95% of swallowed seeds were defecated intact. The percentage of seeds swallowed was higher in continuous forests (70 %) than in forest fragments (51%), while the proportion of spat out seeds was higher in forest fragments (24% % vs 13%). Overall, spider monkeys dispersed seeds from 52 species, 39 genera and 23 families, and similar numbers of seed species were found in feces from continuous forest (37) and forest fragments (38). Most defecated seeds were from Ficus, but large seeds (ca. 4.5 cm length) were also found (e.g. from species such as Spondias spp. and Attalea butyracea).  More than 90 % of the fecal samples (N= 865 out of 957) contained seeds, but the average percentage of feces without seeds was greater in fragments (17.5 %) than in continuous forest (4.5 %).  Defecated seeds had greater germination percentages than control seeds (range: 33% to 100% vs 2.5% to 28.6%) from mature fruits in five out of six species in which germination was evaluated. The average seed dispersal distance to the nearest conspecific was 20 m, being notably higher in Spondias mombin (65 m). Our results indicate that spider monkeys are efficient seed dispersers based on seed handling behavior, the number of species dispersed, germination success, and dispersal distance. These primates likely have a strong influence on the dispersal and recruitment success of a large number of tree species, but this efficiency may be limited in small forest fragments, wherein the percentage of seeds swallowed decreases and fruit shortage can force them to use proportionally higher amounts of leaves and other food items.