Positional behavior is the measurable and observable link between the biology and behavior of an animal in its environment. In this dissertation, I examine ontogenetic patterns of positional behavior in infant, juvenile, and adult white-faced capuchins (Cebus capucinus) and mantled howling monkeys (Alouatta palliata) inhabiting the same tropical forest in Costa Rica. During growth and development ontogenetic changes in body size, limb proportions, and motor skills are likely to influence locomotion and posture through the arboreal canopy. I collected data on positional behavior, activity, prehensile-tail use, branch size, branch angle, and crown location during a 12 month period at Estación Biológica La Suerte in northeastern Costa Rica. The data set is comprised of 401.3 hours of data on Cebus capucinus and 554.3 hours of data on Alouatta palliata totaling 955.7 hours of data or 57,344 individual activity records.Life history timing and differences in rates of growth did not predictably influence the development of adult-like positional behaviors in Cebus and Alouatta. In both species, infancy was characterized by high proportions of dorsal, ventral, and side riding on the mother with smaller proportions of independent positional modes observed during play, explore, active posture, and feed/forage. Young Cebus resembled the adult pattern of positional behavior by six months of age while howlers exhibited significant differences in several positional behavior categories through 24 months of age. The positional repertoire of both species revealed similarities in the types of modes used during feed/forage and travel in juveniles and adults. For example, in juvenile and adult age categories of Cebus, feeding and foraging included high proportions of quadrupedal walk, sit, and squat in conjunction with climbing, leaping, and suspensory behaviors. In howlers, the degree to which coordination and increases in body mass during ontogeny as limiting factors in the development of adult-like positional competence is unclear. Data presented here suggest that the environment exerts different pressures on growing Cebus and Alouatta that may relate to diet, energy expenditure, foraging skill, and/or social learning.

The evolutionary origins of human handedness are still unknown. The study of lateralized behaviour in our closest relatives, the nonhuman primates, is useful to clarify how this trait appeared and evolved in our species. In the present study, lateralized behaviour was assessed in a population of 32 free-ranging Mexican mantled howler monkeys (Alouatta palliata mexicana) for thirteen spontaneous motor patterns, at individual and group levels, as well as the effect that age, sex and posture have on its strength and direction. The studied population of howler monkeys displayed only few significant lateral biases at the individual level with single motor patterns (Binomial tests, p≤0.05). No biases towards the use of a particular limb or side of the body were found at a population level. Therefore, even though some individuals showed significant limb/side preference with single motor patterns, no signs of task specialization, side specialization, or true handedness were found. Similarly, no effects of sex, age or posture were found on the direction or strength of lateralized behaviour. The general absence of limb/side preferences found in this population may be due to the constraints imposed by the arboreal life and/or the type of diet. Possible causal agents of the few significant individual biases found here may be the presence of handicaps and/or experience. Further research is needed in order to assess whether the lack of human-like handedness reported in this study is only specific to the studied population, a general phenomenon of the genus Alouatta or perhaps of all the Platyrrhini.