When to choose which tool. Multidimensional and conditional selection of nut-cracking hammers in wild chimpanzees

Investigating cognitively complex behaviours in their natural ecological context provides essential insights into the adaptive value of animal cognition. In this study, we investigated the selection of hammers used for cracking Coula nuts by wild chimpanzees, Pan troglodytes verus, in the Taï National Park, CÔte d'Ivoire, taking account of the availability of potential tools at the site and time of tool selection. Using GLMMs and focal follows of five adult females, we estimated the probability of an object being selected as a hammer according to its physical properties, transport distance and the location of the anvil on the ground or in trees. We found that chimpanzees took account of several variables at the same time (multidimensionality) when selecting nut-cracking tools and that their selection for hammer weight was adjusted to the state/value of other variables (conditionality). In particular, chimpanzees (1) preferred stones over wooden clubs and hard woods over soft woods; (2) selected heavy stones, but relatively lighter wooden hammers; (3) selected increasingly heavier hammers the closer they were to the anvil; and (4) selected lighter hammers when they were going to crack nuts on a tree. The latter two results represent instances of conditional tool selection based on the next steps in an operational sequence (transport and/or use of the tool in a stable or unstable location) and suggest that chimpanzees anticipated future events when they chose a tool. This large set of conditional rules suggests a high level of cognitive sophistication in a tool use task. Our results represent a compelling example of how powerful cognitive skills allow the optimization of an ecologically relevant foraging activity, supporting a food extraction hypothesis for the evolution of complex cognition in our closest relatives.