No abstract

For many animals, strategies for optimally timing reproduction involve monitoring not only the physical environment, but also the social context. To explore the potential for social factors to modulate reproductive seasonality, the in¯uence of social and environmental cues on birth timing was examined in greater spear-nosed bats Phyllostomus hastatus. Births were observed or dated from pup growth curves in three caves on Trinidad, West Indies, over 4 years. Nearly 40% of the variability in birth dates could be explained by environmental factors because birth dates differed signi®cantly between years and showed consistent differences between locations that receive differing rainfall amounts. Nevertheless, the timing of births within caves and in captivity indicated that social cues also affect the timing and synchrony of births within female social groups. Within each cave, social groups differed signi®cantly in mean birth dates. Two groups of greater spear-nosed bats brought into captivity and maintained without seasonal cues initially exhibited less birth synchrony than wild groups, but birth synchrony did not decline over 3 subsequent years. Further evidence for the in¯uence of social cues on reproductive timing came from four females that were transferred between the captive groups and then gave birth in synchrony with their new group and out of schedule with their original group. Social cues in¯uencing reproductive timing are unlikely to be volatile chemicals or other passively transferred cues, given that frequent prolonged physical contact did not increase birth synchrony between adjacent wild groups. Cues are more probably transferred actively between female group members, perhaps by grooming, or through mating with one attendant male.

Prior studies have shown that female cotton-top tamarins usually do not ovulate while living with natal groups, and most females do not ovulate until they are paired with an unfamiliar adult male. To examine the role of unfamiliar adult males on stimulating ovarian function, four cotton-top females were studied during three conditions: females living with their natal group for six weeks, living alone but exposed to a single unfamiliar adult male located 15 cm away from the female's cage for four weeks, and living with an unfamiliar adult male for six weeks. Behavior and urinary hormonal concentrations were measured during the three conditions. Exposure to the male consisted of visual, auditory, and airborne olfactory contact. First ovulation occurred during exposure to the unfamiliar male in three of the four females indicating that direct physical or sexual contact with the male is not required for onset of ovarian cycling. The fourth female did not ovulate even during six weeks of direct contact with the unfamiliar adult male. In addition, four parous females in either family groups (3) or singly caged (1) were examined for ovarian function 4-6 months after the death or removal of their mates. All females continued to cycle in the absence of the male indicating that the male was not needed to continue ovarian cycling. In fact, two of the females were pregnant at the time their males died and both delivered normal infants and resumed cycling. The results of this study indicate that an unfamiliar adult male may facilitate the onset of ovarian cycling without being in direct contact with the female and visual, auditory, or airborne olfactory cues may be involved. Once repeated ovarian cycling occurs the male is not required to maintain ovarian function.