The 82nd Annual Meeting of the American Association of Physical Anthropologists (2013)

Dietary toughness constrains juvenile feeding efficiency in Phayre's leaf monkeys (Trachypithecus phayrei crepusculus)


1Interdepartmental Doctoral Program in Anthropological Sciences, Stony Brook University, 2Department of Anthropology, Stony Brook University

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Studies of primate foraging ontogeny have found mixed evidence for juvenile competence. For some species, juveniles feed and forage no differently than adults, while in others, juveniles exhibit signs of reduced ability. Smaller size and lesser strength may constrain juvenile foraging competence, particularly for species relying on tougher diets. Here, we examined the influence of food toughness on diet and feeding rates for Phayre’s leaf monkeys (Trachypithecus phayrei crepusculus) at Phu Khieo Wildlife Sanctuary, Thailand. Bites per minute were counted and feeding data collected for adults (n=13.4 per month) and older and younger juveniles (n=8.3 and 7.4 per month respectively) in three groups. Food toughness was measured using a portable mechanical tester. Lastly, photogrammetric measures were used to estimate limb lengths in order to rank juveniles by size. We calculated individual monthly toughness profiles incorporating feeding time per food species. Using GLMMs, we found no age-related differences in overall dietary toughness. Similarly, preliminary analysis of a smaller juvenile subset provided no evidence for a size-related pattern in toughness profiles. To gauge the impact of toughness on feeding efficiency across age classes, bite-rate ratios were calculated for each age pair (e.g., adults vs. younger juveniles). For both age pairings involving younger juveniles, toughness was a significant positive predictor of bite-rate ratio (p<0.0001), that is, the degree of difference increased with tougher foods. Our results suggest smaller size does not limit the overall toughness of juvenile diet, however, toughness does take a greater toll on feeding efficiency in younger juveniles.

Data collection supported by the Leakey Foundation, NSF DDIG (BCS-0647837), NSF (BCS-0542035), the Wenner-Gren Foundation, and the Office of the Vice President for Research and the Department of Anthropology, Stony Brook University.

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