Little is known regarding mating behavior of most molos-sids. Chaerephon pumila is reported to roost and mate in stable harem groups of about 20 females attended by a single male. Evidence suggests that Tadarida brasiliensis mates promiscuously during a brief period in spring when males and females assemble at specific sites. Many reports of the use by molos-sids of low-frequency vocal communication, the abundance of scent glands, and the existence of obvious structures for social displays such as head crests all suggest that molossids engage in a diversity of social interactions and mating systems that are, as yet, unstudied.
Females of most species appear to give birth to a single young annually. However, some species are reported to be polyestrus, giving birth twice (Molossus ater and M. molossus) or three times (Chaerephon pumila) annually, in parts of their geographic ranges. Cheiromeles is additionally unique among mollosids in giving birth to twins during a single annual reproductive period. Where known, gestation is usually two to three months in length, and the period from birth to weaning typically lasts five to six weeks. Studies on milk composition and reproductive energetics in Brazilian free-tailed bats demonstrate an extremely high-fat content in the milk of fe-
males, allowing for the rapid growth of their young. During the period of peak lactation, it is estimated a lactating female Brazilian free-tailed bat has energy demands of 106 kj/day to meet her needs and those of her growing young, requiring that the female consume approximately 70% of her body weight in insects each night.
During pregnancy and lactation, females typically roost in maternity colonies, separated from adult males. But, even in the largest maternity colonies that contain tens of millions of individuals, females relocate and selectively nurse their own young. The mating system is not known for all species, but most are thought to be polygynous.
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