Reproduction and growth of two species of congeneric fruit bats, Cynopterus minutus and C. brachyotis, were studied in Brunei, northern Borneo. C. minutus is smaller and inhabits rainforest; C. brachyotis is larger and inhabits non-rainforest habitats (secondary forest, mangrove forest, deforested zone). The two species were sympatric only at Batang Duri, a village near rainforest.
The climate of the study area is characterized by uniformly high temperatures and rainfall, but relatively there are two seasons of more rain and two of less rain. Reproduction of both species is seasonal with continuous bimodal polyoestry. Within each species, the females are in reproductive synchrony, with young born in two distinct seasons (3–4 months each) of parturition 5–7 months apart. Each parturition is followed by postpartum oestrus. For each species, gestation spans about 5–6 or 5–7 months, with embryonic development delayed in the early stages. Births of neither species were timed to the seasons of greater rainfall or to the general season of flowering/fruiting of trees. Instead, births were centred about the two seasons of less rainfall and probably timed to the fruiting of certain species of trees, including mangoes for C. brachyotis.
The weight data for C. brachyotis indicated a difference in diet between adult males and females. The bimodal pattern of weight increase/decrease for adult males corresponded to the bimodal cycle of ripened mangoes in Brunei. Although weight increase/decrease for adult females did not correspond to the bimodal cycle of ripened mangoes, none the less the two birthing seasons did, following shortly after or beginning at the two times of abundant ripened mangoes. Similarly, adult males and females of C. minutus appeared to have different diets and/or different foraging patterns.
Both sexes of C. minutus (and probably also of C. brachyotis) attain maturity at about 7 months, with females giving birth for the first time at about 12 months. Females conceive shortly upon attaining sexual maturity and are thereafter in states of essentially continuous reproduction with two young born annually (2 litters/yr, 1 neonate/birth). Relative to other fruit bats (Mickleburgh, Hutson & Racey, 1992), C. minutus and C. brachyotis have high rates of reproduction.