Life Cycle of Bats
Details of the life cycle are known for only a few species. In northern temperate zone species, there is an annual cycle of sexual activity, with birth taking place between May and July. In males the testes, normally located in the abdominal region, descend seasonally into the scrotum, and active spermatogenesis occurs. In females sexual receptivity may be associated with egg maturation and release. Tropical bats may exhibit a single annual sexual cycle or may be diestrous (i.e., have two periods of fertility) or polyestrous (have many).
The sexual cycles of entire populations are closely synchronized, so almost all mating occurs within a few weeks. The periods of gestation, birth, lactation, and weaning are also usually synchronized. Gestation varies in duration: five or six months in flying foxes (Pteropus), more than five months in vampire bats (Desmodus), three months in some small leaf-nosed bats (Hipposideros), and 6 or 7 to 14 weeks in several small vesper bats (family Vespertilionidae). The length of gestation may be influenced by both ambient (surrounding) and body temperature.
In several North American and northern Eurasian vesper and horseshoe bats that hibernate, copulation occurs in the fall, and the sperm are stored in the female genital tract until spring. Ovulation, fertilization, and implantation occur after emergence from hibernation, when the female again has available an abundant food supply and a warm roost. Such favourable environmental conditions greatly enhance the young bat’s chances of survival.
Most bats bear one young, but the big brown bat (Eptesicus fuscus) may bear twins, and the Eastern red bat (Lasiurus borealis) bears litters of one to four.
At birth the young, which may weigh from one-sixth to one-third as much as the mother, usually have well-developed hind legs with which they hold on to their mother or to the roost. Their wings are very immature. The young are hairless or lightly furred and are often briefly blind and deaf. Female bats normally have one pectoral (at the chest) or axillary (at the armpit) mammary gland on each side. Several species that carry their young while foraging also have a pair of false pubic nipples, which the infant may hold in its mouth when its mother flies. The infants are nourished by milk for a period of about five or six weeks in many small bats and for five months in the Indian flying fox (Pteropus giganteus). By two months of age, most smaller bats have been flying and foraging for three or four weeks and have achieved adult size.
In many species females late in pregnancy migrate to special nursery roosts, in which large numbers of pregnant females may aggregate, usually to the exclusion of nonpregnant females, males, and bats of other species. In some cases the nursery roosts seem to be chosen for their high temperature, which may derive from the sun, from the bats themselves, or from decomposing guano. When foraging, some bats (Erophylla) leave their infants hanging quietly, one by one, on the cave wall or ceiling. In the case of the Mexican free-tailed bat and a few others, the closely spaced infants may move about and mingle on the wall. Some bats carry their young with them for a short period of time. Generally, each mother, on returning to her roost, seeks out her own offspring by position, smell, and acoustical exchange.
Some bats achieve sexual maturity in their first year, others in their second. Infant mortality appears to be high. Developmental and genetic errors and disease take their toll, but accidents seem to cause more serious losses—the young may fall from the ceiling or perhaps have serious collisions in early flight attempts. A fair number of bats probably fail to make the transition from dependent infants to self-sufficient foragers.
Adult bats, on the other hand, have low mortality. Predation is rarely serious, especially for cave-dwelling species. Disease, parasitic infestation, starvation, and accidents apparently take small tolls. There are records of several big brown (Eptesicus fuscus), little brown (Myotis lucifugus), and greater horseshoe bats (Rhinolophus ferrumequinum) that have lived more than 20 years, and a few have lived more than 30. Probably many bats in temperate climates live more than 10 years. Longevity has not been established for most tropical species, but a few are known to live for more than 10 years.
Several factors probably contribute to the unusual longevity of bats. Generally isolated roosts and nocturnal flight substantially protect them from predation, from some elements of weather, and from exposure to the sun. Their largely colonial way of life may ensure that entire populations experience contagious infection and subsequent immunity; indeed, such a pattern in the past may have hastened adaptation to disease. The persistent use of various seasonal roosts probably ensures isolation and security, food and water supplies, and access to mates. Many bats, moreover, reduce their body temperature at rest. Not only is there a probability that this conserves some cellular “machinery,” since metabolism is reduced, but fewer hours need to be spent in actively seeking food and water.