Reproduction And Social Organization
Most bats have a breeding season, which is in the spring for species living in a temperate climate. Bats may have one to three litters in a season, depending on the species and on environmental conditions such as the availability of food and roost sites. Females generally have one offspring at a time; this is probably a result of the mother's need to fly to feed while pregnant. Female bats nurse their youngster until it has grown nearly to adult size; this is because a young bat cannot forage on its own until its wings have assumed adult dimensions.
Female bats use a variety of strategies to control the timing of pregnancy and the birth of young, so as to make delivery coincide with maximum food ability and other ecological factors. Females of some species have delayed fertilization, in which sperm are stored in the reproductive tract for several months after mating; in many such cases, mating occurs in the fall, but fertilization does not occur until the following spring. Other species exhibit delayed implantation, in which the egg is fertilized after mating, but remains free in the reproductive tract until external conditions become favorable for giving birth and caring for the offspring. In yet another strategy, fertilization and implantation both occur but development of the fetus is delayed until favorable conditions prevail. All of these adaptations result in the pup being born during a time of high local production of fruit or insects.
Bats exhibit every kind of breeding system that has been described for mammals. Some species are monogamous, with a particular male and female mating only with each other. Other species are polygynous, which means that one male may mate with several females. In these species, males may fight for control of preferred female roosting sites, or over aggregations of females, called harems. In hammer-headed fruit bats (Hypsignathus monstrosus) of Africa, the males assemble into leks, which are aggregations of displaying males at traditional sites that females visit for the purpose of selecting a mate. The males display by vigorous wing flapping, erecting patches of hair, and loud vocalizations. Still other bats have a promiscuous mating system, in which both males and females mate with more than one other individual.
While a mother and her young are the basic social unit, some species elaborate on this theme. For example, the social organization of the common vampire bat (Desmodus rotundus) is based on roosts shared by several females and their young. Roost-mates are often close relatives who groom each other, share wound sites on their prey, and regurgitate blood for consumption by their colleague.