Apes finding ants: Predator–prey dynamics in a chimpanzee habitat in Nigeria

Abstract

Some chimpanzee populations prey upon army ants, usually with stick tools. However, how their prey's subterranean nesting and nomadic lifestyle influence the apes' harvesting success is still poorly understood. This is particularly true for chimpanzees (Pan troglodytes ellioti) at Gashaka/Nigeria, which consume army ants (Dorylus rubellus) with much higher frequency than at other sites. We assessed various harvesting and search options theoretically available to the apes. For this, we reconstructed annual consumption patterns from feces and compared the physical characteristics of exploited ant nests with those that were not targeted. Repeated exploitation of a discovered nest is viable only in the short term, as disturbed colonies soon moved to a new site. Moreover, monitoring previously occupied nest cavities is uneconomical, as ants hardly ever re-used them. Thus, the apes have to detect new nests regularly, although colony density is relatively low (1 colony/1.3 ha). Surprisingly, visual search cues seem to be of limited importance because the probability of a nest being exploited was independent of its conspicuousness (presence of excavated soil piles, concealing leaf-litter or vegetation). However, chimpanzees preferentially targeted nests in forests or at the base of food trees, that is, where the apes spend relatively more time and/or where ant colony density is highest. Taken together, our findings suggest that, instead of employing a search strategy based on visual cues or spatial memory, chimpanzee predation on army ants contains a considerable opportunistic element.

Sustained myrmecophagy in Nigerian chimpanzees: Preferred or fallback food?

“…The necessity hypothesis states that tool-assisted ant harvesting is a response to the scarcity of preferred food while the opportunity hypothesis predicts that sustained patterns of tool-assisted insect gathering are triggered by high encounter rates with the insects. Our research supports the opportunity hypothesis, because it suggests that Dorylus rubellus are not actively searched for but opportunistically encountered (Pascual-Garrido et al, 2013). Moreover, they are not only far more abundant than the other Dorylus species at the site, but perhaps more easily available because of an enhanced aggressive-defensive nature (Allon et al, 2012).…”

“…However, army ant dipping wands are only found if searched for on the forest floor, given the subterranean nesting and nomadic lifestyle of these insects. On average, a colony remains in a given burrow for 11.4 days (median 9, range 2-36, SD = 8.3, n = 9), after which the ants migrate on average 119 m (median 113, range 46-261 m, SD = 53.9, n = 43) to a new site (Pascual-Garrido et al, 2013). The likelihood of finding dipping wands thus depends on search time.…”

“…The apes seem to find them mostly opportunistically, at places where they spend relatively more time and/or where ant colony density is highest, given that burrows in forests or at the base of food trees are predominantly targeted (Pascual-Garrido et al, 2013).…”

“…Instead of employing specific search strategies based on visual cues or spatial memory, the apes seem to encounter these underground burrows opportunistically (Pascual-Garrido et al, 2013; see below). The ants respond aggressively upon disturbance, quickly attacking a foreign object such as a wooden tool (Fig.…”

“…For army ants in particular, we have earlier reported how chimpanzee predation success is related to their defensive and migratory behaviour (Allon et al, 2012;Pascual-Garrido et al, 2013). Here, we focus on the annual patterns of army ant gathering and relate them to habitat ecology with the aim to assess the importance of myrmecophagy in the diet of the rarest and least known chimpanzee subspecies.…”