Monday, January 21, 2008

Winter bats

The article below is reprinted from the Ferrum Nature Society Bulletin. You can visit the Society web site at and subscribe to an e-mail list by sending me a message at Articles for contribution are welcome!

--Todd Fredericksen

The vast majority of bat species occur in the tropics. Some species, however, have adapted to temperate areas where they face a prolonged winter period with a shortage of food (mostly insects) and cold temperatures that require higher metabolic rates to maintain thermal homeostasis.

The most common solution to this problem is hibernation. Of the 18 species of bats in Virginia, nearly all hibernate to some extent. Some bat species, such as the Eastern Big-eared Bat (Plecotus rafinesquii), only hibernate in the northern part of their range. Many species will hibernate colonially in caves or mines, but some, such as the Big Brown Bat (Eptesicus fuscus), prefer to hibernate in buildings.

Most hibernating bats do not feed during the winter and live only off slow-burning brown fat stored during the summer. They often huddle in dense clusters for warmth. In some species, hibernation is interrupted by periods of activity when the bats will awaken to defecate or drink. Hibernating bats are easily aroused by cavers and the higher metabolic rates of awakened bats will deplete their fat reserves. It is best to avoid disturbing hibernating bats for this reason.

The time of entry or departure from hibernacula varies by species. The endangered Indiana Bat (Myotis sodalis) enters its hibernaculum in October and stays until April, while the Small-Footed Myotis (Myotis leibii) will hibernate only during the coldest part of the winter.

Many hibernating bat species breed in the fall, but the females store sperm in their reproductive tract, only becoming pregnant late in the hibernation period. This delayed fertilization leads to the birth of bat pups in April and May, a time coinciding with a high abundance of insect prey.

Another solution to the stress of winter is migration, which is a common strategy in vesper bats (Lasiurus). Many bats will also migrate to the southern part of their range to hibernate, a compromise solution between long-flights and colder temperatures. The Indiana bat migrates 300 miles to limestone caves in the southern Appalachians, but most bat species do not migrate as far. Since Virginia has a fairly mild winter, some bat species, such as the Silver-haired Bat (Lasionycteris noctivagans), will spend the summer in the northern U.S. and Canada and return to Virginia and the Carolinas to spend the winter. The extent of migration and the migratory routes of many bat species is unknown.

During the winter, one may be surprised to find bats flying about at dusk. These are most commonly Red Bats (Lasiurus borealis) or perhaps Big Brown Bats, light-sleeping species that awake when temperatures rise above 55°F to take advantage of winter moths and flies. In the past two years, I have observed bats flying during the first week of January when temperatures have hovered between 60-70° for several days.

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