Leptin was originally thought to be an antiobesity hormone, but increasing evidence suggests that its ancestral role was to mobilize neuroendocrine responses to starvation. Research on wild primates is critical for interpreting the high leptin values seen in Western human populations and captive animals. This study examined natural variation in serum leptin in wild vervet monkeys (Chlorocebus aethiops), testing the hypothesis that serum leptin in vervets varies with sex, adiposity, ecology, and reproductive state. Analyses made use of a unique dataset comprised of serum and morphometric measurements obtained from vervet monkeys in four Kenyan sites differing in altitude, temperature, rainfall, and access to human foods. Leptin and gonadal steroid concentrations were analyzed in serum samples from 116 adults. Low leptin levels in males and acyclic females support the contention that levels seen in captivity are not typical for wild primates. Measures of adiposity were not correlated with serum leptin, reflecting the extremely low fat storage in wild cercopithecine primates. Associations with habitat and season, however, indicate that leptin does register ecological variation in energy balance. Leptin levels were higher in sites and seasons with higher rainfall. Moreover, leptin varied significantly with reproductive state, with higher levels in pregnant than in acyclic females. Changes in leptin with gestation stage and duration of lactation suggest that transitory and reversible elevations were an important part of its ancestral role. These data show that in this wild primate population leptin is a sensitive index of natural variation in habitat and seasonally fluctuating reproductive state. Am J Phys Anthropol, 2008. © 2008 Wiley-Liss, Inc.