Primates are characterized by an unusually long juvenile period generating costs in terms of delayed reproduction. Such a lengthened development, most distinct among humans, has been linked to high costs of growing large brains. Other researchers hypothesize that long juvenility is a response to the necessity of learning complex ecological or social skills or a consequence of juvenile risk aversion. Drawing from this theoretical perspective the proposed study aims at answering four main questions: (i) do juveniles use their time to acquire ecological skills, (ii) do they use spatial tactics to reduce risks, (iii) how do social relationships provide benefits, (iv) do growth rates reflect variation in juvenile skill acquisition. Research will be conducted at Phu Khieo Wildlife Sanctuary, Northeastern Thailand, following 22 juvenile Phayre's leaf monkeys (Trachypithecus phayrei) over a period of 20 months. Feeding and social data will be collected in addition to mechanical properties and nutrient content of food. Growth rates will be assessed non-invasively using photogrammetric methods to measure limb length. The intellectual merit of the proposed research is its new perspective on the role of skill acquisition. The role of food mechanical properties in foraging ontogeny has rarely been investigated in detail. This study also takes an important step by integrating behavioral and growth measures, factors rarely considered simultaneously in studies of nonhuman primates. Overall, most primate research has focused on adult subjects, thus neglecting the potentially different selective pressures operating well before adulthood. This project provides an opportunity to add much-needed data to any discussion concerning hypotheses for extended juvenility among primates. In terms of broader impacts this study will be one of the first to utilize non-invasive methods for measuring growth in wild primates. Most primate growth data are available only from captive animals not subject to resource shortages of natural environments. Paving the way for more practical and regular use of these non-invasive methods in field studies, particularly for arboreal species, will improve our understanding of growth and development and the interrelationship with behavior. The study will also form the basis for a Ph.D. involving local assistants. Through this involvement the project will help sanctuary workers to gain experience with all aspects of behavioral data collection and plant sampling. The co-PI will be an active assistant in a conservation and field methods course for Thai students, which will be conducted by the PI. Sharing ideas with local students, scientists and sanctuary workers will help to ensure a long-term commitment to both research and conservation at this site of extraordinary biodiversity including 8 primate species.
