The reproductive biology of bats is fascinating. Bats are broadly similar to humans and other primates in their reproduction and development. For example, bats have two teats, a relatively long gestation, and most bear a single young in which much maternal time and energy are invested. These traits contrast with most other mammals of similar body size, such as shrews and mice, some of which are capable of producing as many as 60 offspring per mother per year, but investing little in parental care. The reproductive strategy of bats, like that of humans and most other primates, is based on the quality of offspring, not the quantity.
Bats often are highly gregarious, forming large breeding colonies. Courting behaviors involve vocal exchanges and body language, and the copulation often includes soft chattering and mutual licking. Most bats apparently are promiscuous, but some South American species are known to form male-dominated harems.
For most Colorado bats, mating occurs in autumn in or near the hibernaculum. Sperm typically are stored by females throughout hibernation, sometimes up to seven months, in the uterus. Within a few days of leaving the hibernaculum, females ovulate one egg, and sperm are released. Fertilization and implantation take place shortly thereafter. Typically, females of a population form a maternity colony at a site different than the hibernaculum where breeding occurred. Gestation usually lasts from 40 to 50 days and results in a single offspring, usually in the late spring in Colorado.
Birth itself appears to be a rather precarious process. Hanging inverted, mothers of many species grab the newborn young as it emerges from the birth canal. Typically the newborn grabs the abdominal fur of the mother with its hind feet and pulls to facilitate movement. Other species of bats use gravity to assist the birthing, hanging by their thumbs, cradling the emerging young in their tails and swinging them into the interfemoral membrane, or uropatagium. Infants usually begin nursing almost immediately after birth. There have been reports of females helping others with the birth of young.
Several species of North American bats produce more than a single offspring annually. Red bats may give birth to twins, triplets or even quadruplets, and although big brown bats usually produce a single young annually in the western U.S., twins are the rule in the East.
Mothers of some species carry their suckling newborn young on feeding bouts. As the young rapidly grow, however, mothers leave them behind while foraging. Although relieving the mother of excess weight, leaving the nursling behind leads to a different problem -- relocating the correct offspring in a mass of other young on return to the maternity roost.
It was long believed that a female bat returning from foraging would nurse any young of the colony as her own, but studies of maternity roosts of Brazilian free-tailed bats -- even those consisting of millions of individuals -- show clearly that females search for their own young until successful. The longest search time recorded was nine hours. Young bats produce isolation calls, specialized high-frequency signatures that alert the mother to where her offspring is. A returning mother first identifies her youngster by its call, then "homes in" by recognizing its odor.
"In one experiment designed to test the ability of females to recognize their young, a mother little brown bat suckling her baby was presented with a choice of two loudspeakers, one carrying her baby's isolation call, the other a control signal," reported Brock Fenton in Just Bats. "The female cocked her ears at each speaker, looked under her wing and nudged her baby and ignored both stimuli. Clearly she had recognized the calls, but was not about to search for something that was not missing."
The offspring of some tree-roosting bats in Colorado, such as the red bat and the hoary bat, lead a much more solitary life. Young are moved from roost to roost by their mothers even when they are quite large. During these shuttle flights, mothers may carry a load more than half their body weight. The young, having reached a certain size are typically left in safe havens while the mother forages.
For pre-volant young, colony life is secure and rather pampered. Parental care results in high survival of young for the first few weeks after birth. A newborn's wings grow rapidly. Juveniles build their flight muscles by doing pushups and flapping while hanging stationary from the roost's ceiling. After about three weeks of this activity, juveniles begin to attempt flights away from the maternity roost. Their first flights and landings are awkward, precarious and exhausting. In fact, it is not uncommon near a maternity roost to see newly volant youngsters on the ground apparently giving up on flying and walking back to the roost site. The early practice flights of juveniles are not only precarious but also dangerous. Mortality of young bats is very high during this developmental stage. Carnivores such as skunks, raccoons, coyotes and domestic cats and dogs kill wayward juveniles. In addition, predators such as snakes, hawks, and owls have been observed grabbing bats as they exit a roost. Because of these hurdles to survival, only about half the young born each year survive to adulthood. If they survive the first few weeks of volancy, juveniles reach adult size and flight ability the next month.
In late summer or fall, maternity colonies usually disperse with a mass exodus of individuals to the hibernaculum where the males await. Female young of the year usually are reproductively active in the spring and return with other females to the maternity roost where they were born. Females show high affinity for roost sites, but males, which are not reproductive their first year, are likely to disperse widely and may never return to their birthplace.