Extractive foraging of toxic caterpillars in wild northern pig-tailed macaques (Macaca leonina)

Extractive foraging in nonhuman primates may involve different levels of technical complexity in terms of the number of actions that must be performed and the manual dexterity involved. We describe the extractive foraging of caterpillars in wild northern pig-tailed macaques (Macaca leonina) at Khao Yai National Park, Thailand. The study group, observed from May to December 2016 (n = 146 days), comprised 60-70 habituated individuals, including 3-4 adult males, 20-23 adult females, and 36-47 immatures. Four adult males and five adult females, observed from September to November 2016 for a total of 24 days, were selected for focal animal sampling. Northern pig-tailed macaques were observed eating at least two families (Erebidae and Limacodidae) and three genera (Macrobrochis sp., Phlossa sp. and Scopelodes sp.) of caterpillars. While the monkeys ate short and small caterpillars with stinging setae and non-setae caterpillars without processing, they performed extensive caterpillar-rubbing behavior on large and long caterpillars with stinging setae. Based on 61 extractive foraging bouts, we found that caterpillar rubbing was hierarchically organized into five stages and 12 elements. Five stages of behavior sequence started with picking the caterpillar up, transporting it to a substrate, rubbing it to remove stinging setae, ingesting it, and then cleaning hands and mouth. Only adult macaques were observed using a leaf to rub stinging caterpillars.