The present study questions whether hypothermia is an artifact due to captivity-induced stress or a thermoregulatory strategy for bats of the neotropical family Phyllostomidae. In Guanacaste, Costa Rica, Carollia perspicillata and Sturnira lilium exhibited a bimodal distribution of body temperatures when submitted to an ambient temperature of 21 degrees C. Body temperature was highly correlated with body mass in both species. C. perspicillata of mass > or = 20 g and S. lilium of mass > or = 17 g remained normothermic (body temperature > 37 degrees C), whereas at masses below 18 g and 13 g, respectively, > 80% of individuals were hypothermic (body temperature < or 32 degrees C). In two treatment groups for each species, we restricted food intake to ca. 20% of body mass on either night 1 or night 4 following capture. Hypothermia was significantly related to food-restriction, but not time in captivity. Metabolic rate (ml O2. g-1 h-1) at ambient temperature = 21 degrees C was MR = e(-2.11 + 0.101 Tb) (r2 = 0.7, P < 0.001) for C. perspicillata and MR = e(-2.62 + 0.115 Tb) (r2 = 0.89) for S. lilium. Free-ranging, radio tagged C. perspicillata exhibited daily depression of body temperature to 33-34 degrees C. We conclude that hypothermia is an thermoregulatory strategy that allows phyllostomid bats to adjust metabolic rate to feeding success and the level of fat stores.