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This dissertation investigates the relationship between Madagascar’s environment and the stress physiology of the red-bellied lemur, Eulemur rubriventer. Unique lemurid traits are proposed to have evolved in response to selection pressures unlike those in other primate habitats, and appear to be particularly suited to helping animals cope with harsh and unpredictable environments. Several hypotheses as to why lemurs evolved characteristics such as small group sizes, low basal metabolic rates, and cathemerality rest upon the untested assumption that these species are ecologically and/or reproductively stressed. This study simultaneously analyzes seasonal changes in climate, ecology, and the behavior and stress hormones (cortisol) of Eulemur rubriventer, as well as differences in these parameters across habitats with different ecological matrices. The goals of this dissertation are to: (1) evaluate the influences of seasonally varying food availability and climate upon lemur behavior and physiology, to determine whether such changes are sufficient to exert strong selective pressure; and (2) assess the additional influences of habitat composition and quality to evaluate the effects of unpredictability and habitat disturbance. Eulemur rubriventer are sensitive to seasonal environmental changes, as indicated by their time budgets, diets, and fecal cortisol levels. Overall they adopt a time minimizing strategy whereby energy is conserved by resting a majority of the time. Ripe fruit scarcity periods elicit the launch of an energy maximizing strategy whereby fecal cortisol levels and time feeding increase, and time resting decreases. Sensitivity to these influences varies across habitats. The behavioral and stress responses of groups in the undisturbed habitat (UND) were more seasonal and pronounced than those from their disturbed habitat (DIST) counterparts. Lower cortisol levels in DIST may result from a less seasonally predictable environment requiring frequent short-term responses (with possible energy deficits during critical reproductive stages yielding 3 deaths out of 5 births and out-of-season reproduction). Alternatively, the attenuated behavioral and hormonal response to environmental change in DIST may indicate a severely stressed population with insufficient energy to launch an appropriate coping response. The hypothesis that animals in DIST have adapted to frequent unpredictability due to disturbance is rejected because all animals behaviorally and hormonally respond to fruit declines, indicating that this species undergoes ecological stress. / text