An endogenous circannual rhythm of reproduction in a tropical bat, Anoura geoffroyi, is not entrained by photoperiod

Abstract

Most species of mammals live in the tropics, and many breed seasonally, but little is known about the regulation of their seasonal cycles. Males of a tropical bat, Anoura geoffroyi (Order Chiroptera, Family Phyllostomidae), from 10° latitude in Trinidad, were studied to test the role of photoperiod in regulating seasonal reproduction in the deep tropics. Groups of males were subjected to five treatments: 1) constant photoperiod; 2) a 12-mo cycle of civil twilight photoperiods mimicking those occurring at 10° latitude; 3) civil twilight photoperiods of 10° latitude, but accelerated to a 9-mo cycle; 4) civil twilight photoperiods characteristic of 30° latitude, but accelerated to a 9-mo cycle; and 5) constant photoperiod, but with the timing of dark onset varied to match the timing of darkness at 10° latitude, and accelerated to a 9-mo cycle. In all treatments, the first cycle of testis growth and regression matched that expected in the wild population, as reported previously for some of these groups. Subsequently, the testis cycle of bats in constant conditions free-ran for 20 mo with a peak-to-peak period of 7.3 ± 0.3 mo. Period lengths in the four nonconstant groups, 7.2-7.7 mo, were not significantly different from that under constant conditions. Bats failed to entrain to any photoperiod cycle, including those mimicking changes at 10° or 30° latitude. They also failed to entrain to the cycle in which day length was held constant while time of sunset was varied, as occurs at the equator. This study provides the first evidence for deep tropical mammals of a truly seasonal and endogenous reproductive rhythm that is not regulated by photoperiod. By extension, these results imply that effective seasonal timing can be achieved in the deep tropics by use of a nonphotoperiodic predictive cue.