Montana Field Guide

Townsend's Big-eared Bat - Corynorhinus townsendii

Very large ears (30 to 39 millimeters) joined across forehead are a prominent feature in Townsend's big-eared bat; the tragus is long and pointed. The dorsal hairs are brownish at the tips, contrasting a little or considerably with the lighter underfur; ventral hairs are dark brownish-gray in color with brown to cinnamon tips. The hairs on the toes do not project beyond the toenails. There are two large, fleshy lumps on the snout, the basis for one of its common name, "lump-nosed bat." Total length is 90 to 113 millimeters; forearm length is 39.0 to 47.6 millimeters; adult mass is 5.0 to 13.5 grams. The greatest length of the skull is 15.2 to 17.4 millimeters; the skull has 36 teeth (Handley 1959, Kunz and Martin 1982, Nagorsen and Brigham 1993).

Townsend's big-eared bat differs from other Montana bats by its combination of extremely long, brownish ears that are joined at the base, the prominent lumps on the nose, the absence of large, white spots in the pelage (as with the spotted bat) and a dorsal pelage that is darker at the tips than the base (opposite that of the pallid bat, which is also larger-bodied).

Montana Range

Little information on movement is available for this species. Townsend's big-eared bat is present year-round in Montana, with summer or winter records from several localities (Hoffmann et al. 1969, Swenson and Shanks 1979, Hendricks 2000, Hendricks et al. 2000, Hendricks and Kampwerth 2001), but movements of individuals have not been reported or studied.

Habitat use in Montana has not been evaluated in detail, but seems to be similar to other localities in the western United States. Caves and abandoned mines are used for maternity roosts and hibernacula (Worthington 1991, Hendricks et al. 1996, Hendricks 2000, Hendricks et al. 2000, Foresman 2001, Hendricks and Kampwerth 2001); use of buildings in late summer has also been reported (Swenson and Shanks 1979). Habitats in the vicinity of roosts include Douglas-fir and lodgepole pine forests, ponderosa pine woodlands, Utah juniper-sagebrush scrub, and cottonwood bottomland. In hibernacula, ambient temperatures ranged from -1.0 to 8.0 degrees (30 to 46 when torpid Townsend's big-eared bats were present) (Hendricks and Kampwerth 2001). Temperatures at maternity roosts are poorly documented; the temperature was 12 degrees (54 in mid-July near a colony in an abandoned mine in Lake County), and 18 degrees (66 in August near a colony in a large and relatively open cave chamber in Lewis and Clark County). Most caves and mines in Montana appear to be too cool in summer for use as maternity roosts.

Townsend's big-eared bat feeds on various nocturnal flying insects near the foliage of trees and shrubs, but appears to specialize primarily on small moths (Kunz and Martin 1982); other insects in the diet include lacewings, beetles, true flies, and wasps. There are reports of gleaning insects from foliage, but most are captured in the air, often near foliage. In a California study, individuals hunted primarily around the perimeter of trees, usually 10 to 30 meters off the ground, between mid-canopy and near the top of the canopy (Fellers and Pierson 2002). The diet and foraging behavior of Townsend's big-eared bat in Montana have not been reported or studied.

Females form maternity colonies during the spring and summer. Colonies are typically composed of 20 to 180 females, each giving birth to one pup after a gestation period of 55 to 100 days (Pearson et al. 1952, Genter pers. obs.). Pups are able to fly in 3 weeks and are weaned at 6 weeks. Both sexes congregate at cooler caverns (called swarming sites) in late summer/early fall.

Townsend's big-eared bat emerges from day roosts in coastal California and central Oregon within an hour after sunset (Dobkin et al. 1995, Fellers and Pierson 2002); limited information from Montana indicates a similar emergence time (P. Hendricks and J. Carlson personal observation). In Oregon, individuals moved up to 24 kilometers from hibernacula to foraging areas (Dobkin et al. 1995). In California, foraging individuals traveled less than 10.5 kilometers from primary day roosts and tended to forage in the same areas each night. The mean center of activity for females was 3.2 kilometers from the roost, and 1.3 kilometers for males; 41 to 88% of tagged bats returned to their roost each night. Individual bats used nine alternate roosts (Fellers and Pierson 2002).

Townsend's big-eared bat tends to hibernate singly, but does occur in clusters during winter in some areas (Schmidly 1991). It tends not to associate closely in day roosts and hibernacula with other species of bats, although individuals of other species may be present elsewhere in the roost (Handley 1959, Kunz and Martin 1982, Genter 1986, Choate and Anderson 1997, Kuenzi et al. 1999). In Montana, Townsend's big-eared bat has been found at summer and winter roosts in the presence of other bat species (Swenson and Shanks 1979, Worthington 1991, Hendricks et al. 2000, Hendricks and Kampwerth 2001), although it usually hibernates in the open and alone, rather than in clusters or wedged in cracks.

Crude population density in Oklahoma was estimated at one bat per hectare (Humphrey and Kunz 1976, Kunz and Martin 1982), about 3 to 4 times greater than that reported in California (Pearson et al. 1952). Natality rates for colonies of adult females typically exceed 90%, but may be as low as 35% (Kunz and Martin 1982, Fellers 2000); pre-weaning post-natal mortality of adults generally is 4 to 5%. Adult survivorship is relatively high (about 70 to 80% in females in California). Regional population increases in California may be dependent on the establishment of new nursery colonies (Pearson et al. 1952), since colony size has been reported to remain static year after year. Predation has been suggested as the primary limiting factor in Kansas and Oklahoma (Handley 1959), although lack of suitable roosting habitat seems more likely to limit population size in this region (Humphrey and Kunz 1976). Predators of Townsend's big-eared bat are poorly documented, but include the black rat and eastern woodrat (Clark et al. 1990, Fellers 2000), as well as the black rat snake, spotted skunk, domestic cat, and ringtail (Pierson et al. 1999); predators can significantly depress reproductive success in some maternity colonies. No demographic data or estimates of population size are available for any population in Montana, nor have any predators been documented.

No published studies are available on the reproductive biology of this species in Montana, and other documentation is very limited. Only five maternity colonies are known in Montana, with an estimated size in recent years of 25 to 100 adult females each. Lone adult females captured in early August in the Pryor Mountains were non-lactating (P. Hendricks and J. Carlson personal observation); flying juveniles appear in the same region sometime between late June and early September (Worthington 1991).

Based upon studies in other areas of the species' range mating begins in autumn and continues into winter. Ovulation and fertilization are delayed until late winter/early spring. Gestation lasts 2.0 to 3.5 months. A single young is born during a five week period, beginning mainly in late May in California, June in west Texas, and the second week of July in Washington (Pearson et al. 1952, Easterla 1973, Kunz and Martin 1982). Young can fly at 2.5 to 3.0 weeks, and are weaned by 6 weeks. In central California, summer colonies start to break up in August when the older young are just over 3 months old. Females become sexually mature their first summer; males are not sexually active until their second year. Young fly at 1 month of age and are weaned at 2 months. Most adult females breed every year. Maternity colonies are often smaller than 100 adult females, but up to 550 adult females are present in some (Easterla 1973, Hymphrey and Kunz 1976, Pierson et al. 1991, Szewczak et al. 1998, Fellers 2000, Sherwin et al. 2000, Fellers and Pierson 2002). Males roost separately (apparently solitary) during this time. Maximum longevity is estimated to be about 16 to 17 years (Kunz and Martin 1982).

The response by Townsend's big-eared bats to human activities is largely undocumented in Montana. The maternity colony at Lewis and Clark Caverns has persisted for over a century, even though it is exposed daily to tour groups. In eastern Montana, numerous abandoned coal mines have been completely closed in recent decades, several of which were used as hibernacula; these mines are no longer accessible to bats. Abandoned mine reclamation has also been underway in western Montana during the same time. During the last decade, mine surveys prior to closure have been undertaken by land management agencies to determine the potential of abandoned mines as bat habitat. In some cases bat-friendly gates were installed at known Townsend's big-eared bat roosts, and the roosts have continued to be used after gate installation (Hendricks 1999, Hendricks and Kampwerth 2001). Some caves in the Pryor Mountains and Little Rocky Mountains with documented use by Townsend's big-eared bat are protected with bat-friendly gates (Worthington 1991, Hendricks et al. 2000). Abandoned mines should be surveyed for Townsend's big-eared bats or other bat species prior to any reclamation activity. Surveys should follow protocols in the conservation assessment and conservation strategy (Pierson et al. 1999). Installation of bat-friendly gates should be considered as a protective measure for all Townsend's big-eared bat roosts. Other land management activity (cave management, pesticide spraying, timber harvest, other vegetation conversion) at or near known roosts should also be conducted according to the best management practices outlined in the conservation assessment and strategy. Maternity roosts and hibernacula should be routinely monitored to establish population trends across the state. Undiscovered maternity colonies and hibernacula undoubtedly exist in Montana. All observations of Townsend's big-eared bat roosts should be reported to the appropriate land management agency, the Montana Natural Heritage Program, or the Montana Department of Fish, Wildlife and Parks.