Temperate vespertilionines hibernate—sometimes alone, sometimes in the hundreds. Commonly, individuals awaken periodically during the winter. If the outside temperature is warm enough, they will travel outside of the hibernaculum,

A red bat (Lasiurus borealis) on a small tree branch. (Photo by Merlin D. Tuttle/Bat Conservation International/Photo Researchers, Inc. Reproduced by permission.)
A little brown bat (Myotis lucifugus) in flight. (Photo by © Joe McDonald/Corbis. Reproduced by permission.)

which may be a cave, attic, or tunnel, and look for food. Following hibernation, male bats in this subfamily will typically spend the summer alone, while females will group together in maternity colonies to bear and raise their young. A maternity colony of pipistrelles (Pipistrellus pipistrellus), for example, may include more than 100 females. Unlike most vespertilionines, pipistrelle nursing colonies may include some males. After the young are independent, the bats abandon these sites.

Mouse-eared bats generally return to the same summer and winter roosts, which may be as much as 125 mi (200 km) away from each other. Another change of roosts is occasionally made in summer or in winter. Roost changes are also typical of other vespertilionine species.

Most species begin mating in the fall, but very little is known about courtship behaviors. The little brown bat (Myotis lucifugus), which is one of the most well-known bats in the New World, engages in no courtship. Mating simply involves the male grasping the female by the nape of her neck during copulation, performed upside down. The two separate after copulation, often to find additional partners.

In the fall, many temperate vespertilionines disappear from northern habitats, assumedly to migrate south, although some species' seasonal movements are little known. The hoary bat is one species whose migration pattern is generally understood, although specifics are still lacking. These bats begin migrating in late summer to early fall, with often-large, mixed-sex groups traveling to the Gulf states and Mexico to spend the winter. However, the destination of some individual populations is still in question. For instance, populations of the red bat (Lasiurus borealis) of North America that spend their summers in the upper Great Lakes may not fly as far south as more

A big brown bat (Eptesicus fuscus) foraging in flight. (Photo by Animals Animals ©Fred Whitehead. Reproduced by permission.)

southern populations, perhaps migrating only as far as the Ohio River valley for the colder months instead of the Gulf states or Mexico. Part of the mystery surrounding many bat migratory movements stems from their hibernacula, which are often hidden, remote, and undiscovered. Return migrations for many species, including the hoary bat, begin when gravid females head north. The males follow shortly thereafter.

Vocalizations are used for communication and carry a variety of information. Acoustic studies on hoary bats show that they use mainly multiharmonic signals with considerable intra- and inter-individual variability in five signal variables (call duration, call interval, highest, lowest frequency, and frequency with maximum energy) to recognize each other and communicate with one another. Echolocation behavior is influenced by the presence of conspecifics. When bats hunt together, call duration decreases and call interval increases. While hunting, the pallid bat flies slowly, and close to the ground, with rhythmic dips and rises. Instead of echolocating, the desert-adapted pallid bat relies on sounds made by its prey to locate and capture a meal, often crickets or scorpions.

Some species (pallid bats) are known to produce a musky skunk-like odor from glands on the muzzle. There have been no experimental studies to determine the function of this odor—it may be a defense mechanism for repelling predators.

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