Townsend’s Big-eared Bat

by Lewis Young

One look at an individual of this species and it’s easy to see where part of the name comes from. The ears are nearly 1 ½ inches long. Townsend’s comes from being named in honor of naturalist Charles H. Townsend even though it was first described in 1837 by William Cooper. Besides the large ears, other identifying characteristics include 2 large fleshy lumps on the nose and having the ears joined at the base. This is a medium sized bat with a nose-to-tail length about 4 inches, wingspan around 12 inches and weigh about 9 grams (0.3 oz). Overall coloration is brown with the belly being a lighter shade. The extremely large ears set it apart from all other bats in Montana except for Pallid and Eastern Red bats that have much different pelt colors.  

Although they never appear to be common in any part of the state, Townsend’s Big-eared Bats (Corynorhinus townsendii) are widely distributed in Montana with only the northcentral and northeastern short-grass prairie being unoccupied. They range from British Columbia to Mexico west of the Great Plains. Two isolated populations occur further east with one being in the Missouri, Arkansas and Oklahoma area, and another in the Virginia’s and Kentucky. The subspecies in these eastern populations are listed as endangered under the Endangered Species Act but the western populations are not listed.

Townsend’s Big-eared Bats are closely associated with caves, mines, and other similar features such as talus and erosion cavities found in badlands and river breaks that allow them to forage on various nocturnal flying insects near the foliage of trees and shrubs. They appear to specialize primarily on small moths but other insects such as lacewings, beetles, true flies and wasps have also been documented. Insects are captured in the air most often. However, their low wing loading allows them to fly slowly and even hover in the air and because of these flight capabilities they may be able to glean insects off vegetation. Nightly foraging bouts may be localized near the day roost or may cover more than 150 km (93 miles).

Echolocation is used to find and capture insects as well as to navigate around and through vegetation in the dark. Their large ears are thought to allow these bats to use quieter calls than other species. Their echolocation calls are distinctive and can be detected with electronic equipment called bat detectors.  However, since the calls are relatively quiet the bats must fly close to the detectors.

Roosts during the May through September active season include caves, mines, erosion cavities and structures like buildings and bridges. Maternity roosts are typically found in caves and abandoned mines but have also been found in buildings in some areas. Only five maternity colonies are known in Montana, with an estimated size in recent years of 25 to 100 adult females each.  Males roost separately from the females. Mating begins in autumn and continues into winter. Ovulation and fertilization are delayed until late winter/early spring. Gestation lasts 2-3 ½ months and one pup is born in late spring/early summer. Young can fly at 2 ½-4 weeks and are weaned by 6-8 weeks. Females are sexually mature their first summer but males are not sexually active until their second year. Nearly all adult females breed every year. Maximum longevity exceeds 21 years.

Hibernation sites in winter (called hibernacula) are also typically confined to caves and abandoned mines. These bats prefer relatively cold places for hibernation, often near entrances and in well-ventilated areas.

Individuals normally roost singly rather than in clusters and hang in an exposed position rather than wedging into crevices or cracks.  Being exposed makes them more susceptible to disturbance and after disturbance they often abandon their roost which greatly increases the likelihood of death. They also tend not to mix closely with other species that may occupy the same hibernacula. The large ears are snugly wrapped around the head and not so obvious. Hibernacula are cool and body temperature drops to just above the ambient temperature plus the heart rate drops to 10-20 beats/minute during hibernation unlike the 1300 when flying or 200 when resting. A few times each winter they will arouse from hibernation to excrete body wastes and may change location within the hibernacula or even change hibernacula. Fat reserves are only sufficient for a few arousals so any extra disturbances greatly increase the chance of death.

Although the viral disease White-nose Syndrome has killed millions of bats especially in the eastern U.S., Townsend’s Big-eared Bats are not yet known to be afflicted by it.

Townsend’s Big-eared Bats can be negatively impacted by a variety of human actions. Roosts are vulnerable to vandalism and disturbance by humans. Blockage of cave/mine entrances for human safety issues or to reduce human impacts to cave environments eliminates roosting sites unless bat friendly gates are installed. Similarly, exclusion of bats from buildings eliminates roosting sites. Application of chemical insecticides reduce food resources.

Although not easy to see because of their nocturnal habits, these bats contribute their share to the huge role bats play in insect control and are another interesting member of the diverse wildlife with which we share the planet.