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Across Africa, chimpanzees prey on army ants, typically using stick tools. Population differences in predation patterns reflect environmental as well as sociocultural factors. However, as for ecological constraints, we lack information about how the ants' defensive response unfolds over the course of a predatory attempt and how this influences chimpanzee reward. We studied these aspects of insectivory in the habitat of Nigerian chimpanzees through field experiments with previously used tools. The speed with which ants run up a tool decreased continuously during experimental dips into nests. This suggests that soldiers with long legs run up the tools first, before fanning out to deter the intruder. Workers attacking later on were increasingly smaller, thus running less fast. Yet, because more and more insects join the defence, harvesting yield (g dry weight) steadily increased for the first 17 min, to then drop markedly. We hypothesize that the length of dipping sessions by wild chimpanzees is limited by either diminishing return or discomfort caused by being continually bitten. Actual harvesting success cannot be reconstructed from ant remains in chimpanzee faeces without knowing what proportion of consumed insects is detectable in excreta. Through human self-experiments, we found that only 10% of ingested ants are found in a subsequent excretion. Based on counts of ant heads in chimpanzee faeces, this translates into 12.9 g dry weight ingested per dipping session, which is far more than elsewhere. Although prey availability and harvesting technique varies across sites, our data still suggest a much greater yield for Nigeria. One reason for this may be a particular aggressiveness of Dorylus rubellus. While this is the only army ant species preyed upon in Nigeria, it is not regularly eaten elsewhere. Standardized experiments and faecal analyses across study sites will be necessary to better understand how ecological constraints influence chimpanzee myrmecophagy. [ABSTRACT FROM AUTHOR]

Army ant predation by chimpanzees has been studied as an intriguing example of tool use and a possible case of cultural variation. However, the importance of army ant prey in chimpanzee diet and feeding ecology is still only poorly understood. We studied the availability and consumption of army ants in a population of the chimpanzee subspecies Pan troglodytes vellerosus in Nigeria. Army ants were collected from nests and trails (workers) and near artificial light sources (males). Three potential prey species were found: Dorylus rufescens, Dorylus gerstaeckeri and Dorylus kohli. Dorylus rufescens was by far more abundant than the other two species. Only remains from D. rufescens were present in chimpanzee faeces. This is the first report of consumption of this ant species by chimpanzees. However, because of the low availability of the other two species, it is unclear whether this pattern reflects a preference for D. rufescens. Although D. rufescens' availability varied with weather conditions, the occurrence as well as the absolute and relative numbers of Dorylus fragments in faeces did not. This finding, together with the considerable difficulties encountered by human observers in their efforts to locate nests by following trails, suggests that the chimpanzees in this population do not harvest army ants from trails and do not use trails to locate nests. The overall occurrence of army ant fragments in 42.3% of all faecal samples is the highest ever recorded in any chimpanzee population. This indicates that in this chimpanzee population, army ant prey is not a fallback during periods of sparse availability of plant food, but quantitatively important throughout the year. Future studies will be needed to clarify which cues and strategies chimpanzees use to locate army ant nests and to assess the role of myrmecophagy with respect to macro- or micronutrient demands. [ABSTRACT FROM AUTHOR]