Under future climate change scenarios, many species will be faced with natural habitats which progressively fail to meet their niche requirements. For vervet monkeys and other long-lived, slowly reproducing mammals, physiological plasticity is likely to be the first and most robust response to rapidly changing environmental conditions. In order to investigate how physiological plasticity may allow free-living vervet monkeys to respond to environmental change, we implanted miniature data loggers, during surgery (anaesthesia by blowdart with 5 mg/kg ketamine and 0.25 mg/kg midazolam, maintained with 0-2% isoflurane inhaled in oxygen), to measure body temperature and activity continuously in adult animals (3-6 kg) for one year. Data loggers were removed during surgery, with the same anaesthetic protocol, at least one year later, and animals were returned to their natural habitat. Despite black-globe temperatures ranging from below freezing to over 50°C over the year, and a mean 24-h variation in black-globe temperature of 30.0°C, vervet monkeys maintained body temperature around a mean of ~38°C with a 24-h amplitude of the body temperature rhythm of only 2.9 ± 0.1 °C (n=6). The 24-h amplitude of the body temperature rhythm, however, was significantly higher in mid-winter (3.2 ± 0.4°C) than in mid-summer (2.5 ± 0.1°C; paired t-test, t=5.47, P<0.01). In summer, when the primary thermoregulatory challenge was heat dissipation at high environmental temperatures, vervet monkeys had ad libitum access to a natural source of water and long day-lengths that allowed a flexible activity schedule. In winter, short day lengths limited the amount of time available to achieve fundamental activities, with activity also influenced by thermoregulatory requirements at low environmental temperatures and social stress of the coinciding mating season. Heterothermy in winter was associated with low environmental temperatures and low food availability. In free-living mammals, homeothermy appears to be a luxury, evident only in individuals that are well nourished, hydrated and not energetically compromised. Whether the lability of body temperature in vervet monkeys under conditions of environmental stress reflects an inability to maintain homeothermy or a regulated adjustment of the thermoregulatory system to conserve energy remains to be elucidated.