Thursday, June 10, 2010

Mines and Bats

While at the Quincy mine, there were interpretive signs about a common resident of the mine. The little brown bat (sometimes called little brown myotis) (Myotis lucifugus) is a species of the genus Myotis (mouse-eared bats), one of the most common bats of North America. As suggested by the bat’s name, its fur is uniformly dark brown and glossy on the back and upper parts with slightly paler, grayish fur underneath. Wing membranes are dark brown on a typical wingspan of 8.7–11 inches. Their ears are small and black with a short, rounded tragus. Adult bats are typically 2.5–4 inches long and weigh 0.2–0.5 ounces. All teeth including molars are relatively sharp, as is typical for an insectivore, and canines are prominent to enable grasping hard-bodied insects in flight.

Little brown bats are insectivores and eat moths, wasps, beetles, gnats, mosquitoes, midges and mayflies, among others. Since many of their preferred meals are insects with an aquatic life stage, such as mosquitoes, they prefer to roost near water. They echolocate to find their prey. Often they will catch larger prey with a wingtip, transfer it to a cup formed by their tail, then eat it - smaller prey are usually just caught in the mouth. They often use the same routes over and over again every night, flying 3-6 yards high above water or among trees. An adult can sometimes fill its stomach in 15 minutes; young have more difficulty. If they do not catch any food, they will enter a torpor similar to hibernation that day, awakening at night to hunt again.

The little brown bat is found all over North America from northern Mexico to interior Alaska. It is the most abundant bat found in the United States and Canada. Although they roost in man-made structures, many can be found in caves and mines. Since little brown bats live in a temperate zone, they must find some way of dealing with winter. Most temperate bats either migrate or hibernate, but little brown bats do both. In summer, the males and females live apart, the females raising young. When fall comes, both sexes fly south to a hibernaculum, where they mate and then hibernate. Those that do not hibernate undergo a prolonged period of hibernation during the winter due to the lack of food. They hibernate in caves or mines as a community. Little brown bats mate in the autumn, before hibernation begins, and over winter the male's sperm is stored inside the female's body, and the infant is conceived in spring. When they arise in the spring, the females go to nursery colonies which may often be the same place where they were born.

These nursery colonies consist mainly of adult females and their young. The nursery colonies sometimes get to numbers as big as 1000 bats in one cave or mine. Some of the U.P. mines have many thousands. Gestation is 50-60 days. They usually have one baby per female each year, but sometimes they have twins. The young are born in an altricial state with their eyes closed and will hang in the nursery while their mothers forage at night. Their eyes open on their second day. They cling to a nipple constantly until they are two weeks old. At 3 weeks, they learn to fly. By 4 weeks, they are adult size.

The number of males in the nursery increases in September, and in October all of the bats migrate back to the caves to hibernate. They use the same hibernaculum and summer colonies year after year. While the females are at the nursing colonies during the summer, males roost in small groups in rock crevices or tree hollows.

Females may be sexually mature in the fall after their birth, but males may take a year longer. About half of females and most males breed in their first autumn. They can live up to 33 years, males living longer on average, though the average lifespan is shorter since about 50% of little brown bats die in their first year.

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