Food Mechanical Properties and Dietary Ecology in Sympatric Pithecia and Chiropotes During a Period of Preferred Food Scarcity

Knowledge of the relationship between feeding biomechanics and craniofacial evolution in primates has benefited greatly through the collection of primate food fracture properties in the wild. Much recent work advocates for the use of mechanical properties, including Young’s modulus ( E , a measure of stiffness) and toughness ( R ), while other studies employ structural properties, including measures of puncture and crushing resistance. It is therefore a pertinent question as to whether these properties explain differences in feeding ecology equally well. We examine the  E  and  R  of fruit tissues eaten by sympatric  Pithecia  and  Chiropotes  at Brownsberg Nature Park and compare our results to previous observations of niche partitioning in these taxa. Specifically, it has been demonstrated that  Chiropotes  breaches unripe fruits with higher puncture resistance, whereas  Pithecia  masticates seeds with higher crushing resistance. We found that maximum  E  of fruit exocarp breached was higher in  Chiropotes , consistent with previous observations of puncture resistance. We revealed even greater differences in exocarp  R , suggesting that perhaps toughness is the primary challenge for  Chiropotes  in accessing unripe fruits.  Chiropotes  was also found to prefer larger fruits, which they accessed using more extensive paramastication, including downward pulling to tear open fruit husks. Seeds masticated by  Pithecia  had greater  E  than in  Chiropotes , also supporting previous work. Our results are consistent with the hypothesis that sympatric pitheciines diverge dietarily with respect to the intensity of anterior and postcanine tooth use, and suggest that both structural and mechanical properties can be used reliably to quantify primate diets.