What does seed handling by the drill tell us about the ecological services of terrestrial cercopithecines in African forests?

The contribution of primates to the seed rain in tropical forests has been long recognized, and there is evidence that these services are not compensated for by other frugivores in the event of primate extirpations. Given the vulnerability of large rainforest frugivores to hunting and the crisis dimensions that commercial hunting is taking in much of tropical Africa, studies on the seed handling of terrestrial cercopithecine primates are especially timely and needed in order to understand their role in maintaining plant community dynamics. Unlike arboreal species, terrestrial primates handle seeds at the post-dispersal stage of fruits and their populations are among the first to decline due to hunting pressure. We analyzed, for the first time, the year-round seed handling of the drill Mandrillus leucophaeus, an endangered terrestrial primate, by examining feeding remains and feces collected over 3284 km of patrols in Korup National Park, Cameroon. Drills defecated intact 110 seeds of diverse plant, fruit and seed traits primarily during periods of fruit abundance, and increased their seed predation during the pronounced fruit-scarce dry season. Considering the drill's diverse fruit diet, ground foraging, long day ranges and seed handling, we suggest that forest-dwelling terrestrial cercopithecines differ in their impact on seed deposition patterns compared with sympatric arboreal primates. Our findings emphasize the need to better assess the impact of terrestrial cercopithecines on plant community dynamics in Africa, warn of potential ecological process disruptions in the event of their extirpation and make a case for rethinking primate conservation priorities so that they do not only reflect a species' dire population status but also the impact its loss would entail for the ecosystem.