Wild chimpanzees select tool material based on efficiency and knowledge

Abstract

Some animals have basic culture, but to date there is not much evidence that cultural traits evolve as part of a cumulative process as seen in humans. This may be due to limits in animal physical cognition, such as an inability to compare the efficiency of a novel behavioural innovation with an already existing tradition. We investigated this possibility with a study on a natural tool innovation in wild chimpanzees: moss-sponging, which recently emerged in some individuals to extract mineral-rich liquids at a natural clay-pit. The behaviour probably arose as a variant of leaf-sponging, a tool technique seen in all studied chimpanzee communities. We found that moss-sponges not only absorbed more liquid but were manufactured and used more rapidly than leaf-sponges, suggesting a functional improvement. To investigate whether chimpanzees understood the advantage of moss- over leaf-sponges, we experimentally offered small amounts of rainwater in an artificial cavity of a portable log, together with both sponge materials, moss and leaves. We found that established moss-spongers (having used both leaves and moss to make sponges) preferred moss to prepare a sponge to access the rainwater, whereas leaf-spongers (never observed using moss) preferred leaves. Survey data finally demonstrated that moss was common in forest areas near clay-pits but nearly absent in other forest areas, suggesting that natural moss-sponging was at least partly constrained by ecology. Together, these results suggest that chimpanzees perceive functional improvements in tool quality, a crucial prerequisite for cumulative culture.

Keywords: Pan troglodytes schweinfurthii; efficiency; field experiment; sponges; subculture; tool use.

Figure 1.

Two examples of sponge tools manufactured during a log experiment. (a) Leaf-sponge made of Alchornea floribunda. (b) Moss-sponge made of Orthostichella welwitschii. (Online version in colour.)

Figure 2.

Comparison of absorbency for natural and experimental sponges. Each square represents the mean volume absorbed by one sponge across 10 repeated measurements. Circles represent model predictions. Lines are 95% confidence intervals. (Online version in colour.)

Figure 3.

Comparison of moss- and leaf-sponges manufacturing (i) and deployment (ii) time. Raw data are shown as squares and model estimates as circles with 95% confidence intervals. (Online version in colour.)

Figure 4.

Availability of sponge material across forest types.