Fruits, fingers, and fermentation: the sensory cues available to foraging primates

Survival and reproductive success hinge on the perception of environmental stimuli. In this regard, foraging efficiency depends on discerning predictive signals in food. A widespread occurrence of ethanol in fruits indicates a sustained historical exposure of frugivores to this compound. Accordingly, Dudley (2000, Quart. Rev. Biol. 75:3-15) proposed that ethanol could represent a prominent sensory cue to primates because of direct and indirectly associated caloric and physiological rewards. However, little is known regarding the extent to which ethanol correlates with such parameters. This information is essential to estimating the importance of detecting and detoxifying ethanol in fruits. Here I present a preliminary analysis of fruits from Southeast Asia; low levels of ethanol were present in fruits of all developmental stages (range: 0.005-0.48%). Moreover, ethanol correlated positively with concentrations of soluble sugars, suggesting that it could be a valuable foraging cue. Recent findings on the sensitivity of primate olfaction and gustation to ethanol are consistent with this notion. However, when primates smell fruits deliberately, it often occurs together with digital and/or dental evaluation of texture. Here I show that softening texture also characterizes the fruit ripening process, and that color is of ambiguous importance to primates possessing trichromatic vision. I discuss the relevance of these findings to the origins of primates and the ecology of key sensory systems and deduce that detecting and selecting fruits on the basis of cues other than color is a persistent theme in primate evolution. Ethanol has likely played a significant and underestimated role in the regulation of primate foraging behavior.