Fallback foraging as a way of life: using dietary toughness to compare the fallback signal among capuchins and implications for interpreting morphological variation.

The genus Cebus is one of the best extant models for examining the role of fallback foods in primate evolution. Cebus includes the tufted capuchins, which exhibit skeletal features for the exploitation of hard and tough foods. Paradoxically, these seemingly "specialized" taxa belong to the most ubiquitous group of closely related primates in South America, thriving in a range of different habitats. This appears to be a consequence of their ability to exploit obdurate fallback foods. Here we compare the toughness of foods exploited by two tufted capuchin species at two ecologically distinct sites; C. apella in a tropical rainforest, and C. libidinosus in a cerrado forest. We include dietary data for one untufted species (C. olivaceus) to assess the degree of difference between the tufted species. These data, along with information on skeletal morphology, are used to address whether or not a fallback foraging species exhibits a given suite of morphological and behavioral attributes, regardless of habitat. Both tufted species ingest and masticate a number of exceedingly tough plant tissues that appear to be used as fallback resources, however, C. libidinosus has the toughest diet both in terms of median and maximal values. Morphologically, C. libidinosus is intermediate in absolute symphyseal and mandibular measurements, and in measures of postcranial robusticity, but exhibits a higher intermembral index than C. apella. We propose that this incongruence between dietary toughness and skeletal morphology is the consequence of C. libidinosus' use of tools while on the ground for the exploitation of fallback foods.