The big brown bat (Eptesicus fuscus) is widely disturbed, and is the most abundant, commonly seen bat in North America. The big brown bat ranges from upper South America through the United States and into Canada. Big brown bats are even found in the Caribbean Islands. The mammal's ears and wings are always black, but the fur color varies geographically, with the stomach area always being paler. Females are larger than the males. The bats can be recognized in flight because they fly in straight lines at a height of 20-30 feet. Big brown bats mate in the fall and sperm is stored inside the female's body over the winter. After a 2 month gestation period, young are born in mid-summer. The newborns are naked and immobile, but mature quickly. Eyes and ears open within hours of birth, and juveniles can began foraging 3-4 weeks after birth. In Eastern North America, twins are common, but in other parts of their range only a single baby is born. Big brown bats begin hunting shortly after sundown and spend about 90 minutes a night foraging. On an average night, a big brown bat will consume 50-100% of its body mass in insects. The bats play an important part in keeping insects in some urban areas under control, eating at a rate of 5-20 insects a minute. Big brown bats dependency on insects made the use of DDT, as an insecticide, highly detrimental to them. Insects that had been affected by DDT were consumed by the bats, creating a build up of the chemical in the bats body. While big brown bats eat a wide variety of insects, they prefer to eat beetles. An adult brown bat will weigh 14-35 grams. Prior to hibernation up to 30% of the bats body weight is fat reserves. While big brown bats tend to forage separately, they do roost together. Roosting sites include dark buildings, such as attics, barns or bridges and trees. Big brown bats can be found hibernating in places such as buildings, caves, mines and houses. Colonies of up to 300 females have been recorded. The females will move roosting sites if they are disturbed. The bats use facial glands that secrete a musky odor to mark their roost and hibernation sites. The bats do not like hot temperatures and will move if it becomes warmer than 95° F. Cold temperatures seemed to be tolerated more easily with bats surviving subfreezing body temperatures. Some big brown bats have been known to live up to 19 years in the wild.
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