How chimpanzees integrate sensory information to select figs

Abstract

Figs are keystone resources that sustain chimpanzees when preferred fruits are scarce. Many figs retain a green(ish) colour throughout development, a pattern that causes chimpanzees to evaluate edibility on the basis of achromatic accessory cues. Such behaviour is conspicuous because it entails a succession of discrete sensory assessments, including the deliberate palpation of individual figs, a task that requires advanced visuomotor control. These actions are strongly suggestive of domain-specific information processing and decision-making, and they call attention to a potential selective force on the origin of advanced manual prehension and digital dexterity during primate evolution. To explore this concept, we report on the foraging behaviours of chimpanzees and the spectral, chemical and mechanical properties of figs, with cutting tests revealing ease of fracture in the mouth. By integrating the ability of different sensory cues to predict fructose content in a Bayesian updating framework, we quantified the amount of information gained when a chimpanzee successively observes, palpates and bites the green figs of Ficus sansibarica. We found that the cue eliciting ingestion was not colour or size, but fig mechanics (including toughness estimates from wedge tests), which relays higher-quality information on fructose concentrations than colour vision. This result explains why chimpanzees evaluate green figs by palpation and dental incision, actions that could explain the adaptive origins of advanced manual prehension.

Keywords: Bayesian updating; Ficus; Pan troglodytes schweinfurthii; colour vision; manual prehension.

Figure 1.

Figs of Ficus sansibarica and their evaluation by chimpanzees in Kibale National Park, Uganda. The mastication and swallowing of figs is preceded by successive sensory assessments: (a) vision, (b) digital palpation and/or olfaction and (c) incisor evaluation. Figs can be discarded at any stage of the sensory sequence (photographs by Nathaniel J. Dominy [top right only] and Alain Houle, reproduced with permission).

Figure 2.

Estimates of fig mechanical properties included measures of (a) Young's modulus and (b) fracture toughness.

Figure 3.

Sensory assessments and corresponding information plotted as increasing functions of fructose concentration. (a) fig colour on the basis green–redness [L/(L + M); green low, red high] and yellow–blueness [S/(L + M); yellow low, blue high], the two physiological subsystems of primate colour vision. (b) The elastic deformation of a fig is determined by its Young's modulus. (c) The energetic equivalent of the critical stress intensity, K_IC, relates to incisal evaluation and the ease of fracture. Enclosed circles (in red) signify consumed figs (photographs by Alain Houle, reproduced with permission).

Figure 4.

(a) Measures of the differential entropy () and (b) information gain () for the probability distribution describing the known fructose concentrations of figs (the following are additive) before observations (the prior), combined with observations of yellow–blue frequencies (+yb), combined with observations of Young's modulus (+m), combined with observations of toughness (+t), combined with observations of K_IC. Filled circles show differential entropy and information gained from the known sequence of sensory modalities, and open circles represent a baseline sequence where it is assumed that all data are visual and have correlations equivalent to that of yellow–blue frequencies and fructose.