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.
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