Circannual rhythms of body weight in pallid bats

Body weights of pallid bats collected in the field were substantially greater during October and December than between April and September. When maintained in the laboratory under constant conditions of photoperiod and temperature, bats manifested circannual rhythms in body weight. These endogenous cycles were observed in adult and juvenile individuals of both sexes and in bats housed in long or short photoperiods (14 or 10 h of light/day). The period of the cycle averaged 10 mo. Photoperiod influenced the timing and magnitude of the body weight cycle; bats kept in short photoperiods achieved peak body weights earlier than animals housed in long day lengths. We propose that day length is one of the proximate factors for synchronization of circannual cycles of pallid bats.