V. Reproduction and Growth

Little is known about the reproductive cycles of bats because their nocturnal lifestyles and secretive natures have posed challenges to the study of bats in the wild. Similarly, the vast diversity of species has defeated most attempts to make generalized statements about the life histories of bats. Among species that have been extensively studied, many have annual cycles of sexual activity. The cycles of entire populations are synchronized so that nearly all mating, birthing, and rearing activities occur within a narrow time frame of days or weeks. Most bat species are promiscuous, meaning that each individual mates with several others. In many species, pregnant females migrate to special nursery roosts, where they are joined by hundreds of other pregnant females. These roosts are usually warmer than non-nursery roosts-a feature that may speed up the rate with which the baby bats develop inside and outside of the womb.

The gestational periods of bats are relatively long, ranging from 40 days to 8 months. Most bats give birth once a year. Many have only one offspring, but some have twins, and the hairy-tailed bats have triplets. The reproductive cycles of hibernating bats are often interrupted. Some hibernating bats, such as the little brown bats, mate in the fall, then hibernate throughout the winter months. The sperm remains dormant in females until these bats arouse in early spring, at which time fertilization takes place. In other bats, such as the straw-colored fruit bat and the Mexican fruit bat, fertilization occurs immediately after mating, but then the fertilized egg stops developing for several months.

Bats are born bottom-first-an arrangement that minimizes the chances of the wings getting tangled in the birth canal. Newborns are enormous, often weighing in at 25 or 30 percent of their mother's weight. For the first few days of its life, the baby remains upside-down, suckling at its mother's breast. Since most bats are born singly, it would disturb the mother's balance if her newborn hung on one side. To remedy this, the baby hangs at an angle across the chest, its mouth grasping one breast and its hind legs hooked under the opposite armpit.

In all bat species, only the females take care of their offspring. The mothers must feed throughout the rearing period but cannot hunt efficiently while carrying their young, so young bats are often left on their own in the nursery roost for several hours each day. When the mother returns to the nursery roost, she must find her baby in the crowd of seemingly identical young. She is guided by a general memory of the area where she left her baby and by the baby's distinctive scent and shrill cries. A mother Mexican free-tailed bat can pick out her offspring from a crowd of as many as 3000 seemingly identical young per square meter (as many as 300 per square foot) of cave space.

In 1994 scientists reported the capture of ten adult male Dayak fruit bats in Malaysia and discovered that these bats had breasts full of milk. It is not known if these males breast-feed, but if they do, they will not only be an exception to the rule that male bats do not care for their young, but they will be the only species of mammal where the male is known to breast-feed.

Young bats grow rapidly. Some species learn to fly and forage in about 18 days. Others require considerably more parental care: the most dependent youngsters are common vampire bats, which continue to nurse for six to nine months after birth.

As the bats mature, they must avoid an assortment of predators, including snakes, hawks, owls, weasels, raccoons, and wild and domestic dogs and cats. Like other animals, bats can succumb to various diseases, and fatalities from airborne accidents do occur. However, any bats that survive such trials can have exceptionally long lives-from roughly 10 to 20 years, depending on the species. The world's longest-lived mammal for its size, the little brown bat has a life span exceeding 32 years.