Indiana Bat - Major Life Events

Major Life Events

Indiana bats begin to arrive at hibernacula (caves and mines in which they spend the winter) from their summer roosting sites in late August, with most returning in September. Females enter hibernation shortly after arriving at hibernacula, but males remain active until late autumn to breed with females arriving late. Most Indiana bats hibernate from October through April, but many at the northern extent of their range hibernate from September to May. Occasionally, Indiana bats are found hibernating singly, but almost all are found hibernating in dense clusters of 3,230 bats/m² to 5,215 bats/m².

Spring migration can begin as early as late March, but most Indiana bats do not leave their winter hibernacula until late April to early May. Females emerge from hibernacula first, usually between late March and early May. Most males do not begin to emerge until mid- to late April. Females arrive at summer locations beginning in mid-April. Females form summer nursery colonies of up to 100 adult females during summer. Males typically roost alone or in small bachelor groups during the summer. Many males spend the summer near their winter hibernacula, while others migrate to other areas, similar to areas used by females.

Females can mate during their 1st fall, but some do not breed until their 2nd year. Males become reproductively active during their 2nd year. Breeding occurs in and around hibernacula in fall. During the breeding season, Indiana bats undergo a phenomenon known as swarming. During this activity, large numbers of bats fly in and out of caves from sunset to sunrise. Swarming mainly occurs during August to September and is thought to be an integral part of mating. Bats have been observed copulating in caves until early October. During the swarming/breeding period, very few bats are found roosting within the hibernacula during the day. Limited mating may also occur at the end of hibernation.

Fertilization does not occur until the end of hibernation, and gestation takes approximately 60 days. Parturition occurs in late May to early July. Female Indiana bats typically give birth to one pup. Juveniles are weaned after 25 to 37 days and become volant (able to fly) at about the same time. Most young can fly by early to late July, but sometimes do not fly until early August. Humphrey and others reported an 8% mortality rate by the time young were weaned. However, they assumed that all females mate in the autumn, which is not the case, so not all the females would give birth. Thus, mortality of young may be even lower than 8%.

Indiana bats are relatively long lived. One Indiana bat was captured 20 years after being banded as an adult. Data from other recaptured individuals show that females live at least 14 years 9 months, while males may live for at least 13 years 10 months.