Agoutis (Dasyprocta punctata): The Inheritors of Guapinol (Hymenaea courbaril: Leguminosae)

Abstract

Hymenaea courbaril (guapinol) is a large-seeded neotropical tree that owes much of its present widespread distribution to seed dispersal by agoutis. Guapinol fruit and fruiting traits influence the fate of a pod’s seeds by affecting agouti scatterhoarding behavior. Agoutis transport pods of experimental fruit crops 0−200+ m. In Costa Rican lowland dry forest, the distance a pod is carried and the rate pods are removed from a crop are strongly influenced by season and by the condition of a pod’s fruit pulp. In turn, distance strongly affects seed survival: near the parent tree, where guapinol seeds and other foods are concentrated, 99% of seeds and seedlings are killed by peccaries, agoutis and mice. However, where guapinol seeds and other foods are at a low density, mortality of buried seeds and seedlings is about 50%. Likewise, pods left below the parent tree are in danger of being opened by seed-crushing collared peccaries and, until the last few decades, white-lipped peccaries. Agouti scatterhoarding is the only process that moves Santa Rosa guapinol seeds from zones of very high seed-seedling mortality to zones of lower mortality. In the absence of agoutis, guapinol would probably be locally extinct wherever peccaries and guapinol-eating small rodents were common. However, the present interaction between guapinol and its seed predators and dispersers is serendipitous rather than coevolved: guapinol fruit traits probably have changed little since they evolved in the Oligocene among a species-rich fauna of large herbivorous dispersal agents. Since the Pleistocene megafaunal extinctions, guapinol’s survival in much of its range may have been due to the possession of fruit traits that allowed successful seed dispersal by agoutis. Similarly, agoutis now disperse the seeds of other largeseeded or hard-fruited tree species whose fruit and seed traits presumably evolved in part as a consequence of megafaunal seed dispersal. These plant species possess two traits. The seeds must be eaten by agoutis, and must be sufficiently protected to survive the slow process of agouti scatterhoarding. Agoutis (the largest scatterhoarders of seeds) surpass all other extant neotropical mammals in dispersing large seeds and seeds from hard fruits, because the upper size limit to dispersal is set by what scatterhoarders can lift rather than what they can swallow, and because they gnaw rather than crush open fruits. Dispersal patterns, and hence the genetic structure of tree populations, must have changed greatly when territorial scatterhoarding rodents took over the dispersal of guapinol and other trees from seed-swallowing megaherbivores. The two species of peccaries, which are as widespread as are agoutis, overlap with agoutis extensively in diet but kill rather than disperse large seeds. They are parasites of many of the agoutimegafauna-flora interactions.