Behavioral patterns of animals correlate with biotic (i.e., resources and conspecifics) and abiotic (i.e., weather and lunar cycle) factors. We studied the influence of ambient light, rain, sex, and resource availability on the activity pattern of the neotropical chestnut short-tailed bat (Carollia castanea; Phyllostomidae) in a tropical lowland forest in Panama. Time of emergence was tightly correlated with local sunset, in contrast to the time of return, which occurred sporadically over a span of hours. Activity by individuals peaked at the beginning of the night, coinciding with maximum availability of ripe fruits from understory pepper plants (Piperaceae), their main food source. Bats continued to forage during light and moderate rain and only stopped at heavy rain. Nightly activity level was similar in nonreproductive female and male C. castanea but the temporal distribution of activity differed. Females were more active in the 1st half of night, whereas activity of males was more evenly distributed throughout the night. In contrast to fruit-eating bats in the canopy, C. castanea did not exhibit a significant reduction in flight activity (lunar phobia) during bright nights around the full moon. We conclude that ecological conditions (availability of food and predation risk) and physiological constraints (small body size associated with high metabolic rate) are the most important factors that account for the observed activity patterns.