The reproductive pattern of the short-nosed fruit bat Cynopterus brachyotis in a tropical dry evergreen rainforest in Thailand (13°N) is characterized by seasonal bimodal polyoestry with postpartum oestrus. The first lactation period occurred in March–April, but the timing of the second varied between years: September–October in 1998 and July–August in 1999. Females may experience reproductive delays after the second parturition. Females attained sexual maturity at 6–7 months of age and gave birth for the first time at about 11–12 months, and are able to produce two young a year. Females born in April gave birth as early as March the following year. Those born in August–September first gave birth in July–August the following year. Males attained sexual maturity when they were a year old. Testis size was significantly positively correlated with photoperiod. The body weight of mature males varied cyclically, was higher during breeding periods and was significantly correlated with testis size. The first parturition coincided with the onset of the annual rains and the second occurred in the middle of the rainy season. Both lactation and weaning periods correspond with high food availability. A significantly higher percentage of reproductive females, earlier parturition, greater synchrony of lactation, higher body mass of lactating females, more abundant, larger and heavier juveniles, and heavier mature males were recorded in 1999 compared with 1998. The El Niño Southern Oscillation resulted in a prolonged and severe drought early in 1998, and induced mast fruiting in 1999 that led to higher reproductive success. Photoperiod may regulate the reproductive rhythm in male C. brachyotis, although experimental studies are required to verify this.
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