Perimyotis subflavus (Menu, 1984)
Basis for Listing
The tricolored bat, also known as the eastern pipistrelle, ranges over most of the eastern United States and southeastern Canada. It was first discovered in Minnesota at St. Peter in 1934 (Swanson and Evans 1936). It has never been found in large numbers, and no maternity colony has yet been found in the state. Since its designation in Minnesota as a species of special concern in 1984, the tricolored bat has been found to occur regularly, although in low numbers, in caves and mines in the southeastern part of the state. A single hibernating individual was found in 1990 and two were found in 2003 in northeastern Minnesota, several hundred miles north of the previously documented northernmost locality in the state (Knowles 1992). Between 1995 and 1999, seven dead tricolored bats were found in Lincoln County during studies of avian mortality related to wind energy development (Osborn et al. 1996; Johnson et al. 2003). Due to its small population size in the state, its susceptibility to disturbance during hibernation, and the potential for persecution, the tricolored bat remains listed as a special concern species.
The tricolored bat is the smallest of Minnesota's 7 bat species. Its dorsal color varies from yellowish or grayish brown to reddish brown, and the underside is somewhat paler. The tricolored bat can be distinguished from Myotis species by its tri-colored pelage: the bases and tips of individual hairs are dark, while the middle sections are light. The ears, muzzle, and membranes on the forearms are light-colored and often appear pinkish, compared to the dark brown or black of both species of Myotis. The tragus (fleshy projection in the ear) is short and blunt. The basal third of the tail membrane is furred dorsally, but on some individuals these hairs are sparse. Typically, adults are 75-90 mm (2.9-3.5 in.) long and weigh 4-8 g (0.14-0.28 oz.).
Tricolored bats hibernate in caves, mines, and tunnels. While this species is often found hibernating in the same sites as large populations of other bats, such as little brown bats (Myotis lucifugus) and northern myotis (M. septentrionalis), tricolored bats tend to occupy the deeper portions of the hibernaculum where temperatures and humidity are higher (Hazard 1982). In the summer, tricolored bats generally roost singly, often in trees, but some males and non-reproductive females also roost in their winter hibernaculum (Carter et al. 1999). Maternity colonies have not yet been located in Minnesota, but elsewhere they have been found in trees, rock crevices, and barns or other buildings (Whitaker 1998).
Biology / Life History
Tricolored bats hibernate from October into April. During this time, they enter a state of torpor in which their body temperature drops to that of the surrounding air temperature. Human activity in caves where bats are hibernating can be detrimental, causing disturbed bats to awaken frequently during the winter. Such disturbance may result in bats emerging from the hibernaculum early, before there is an adequate supply of insects for them to feed on, or they may fail to awaken altogether. Disturbance during hibernation is especially damaging to juveniles, who are already less likely to survive the winter because they have had less time than adults to accumulate fat reserves (Stebbings 1969). Tricolored bats mate in the fall, and females give birth to litters, usually of two young, in the spring. While the young are growing, the mothers roost in small maternity colonies. After about four weeks, the young are able to fly and will accompany their mothers on foraging flights. They become independent after another week or two (Whitaker 1998). Tricolored bats forage early in the evening, and may catch up to half their body weight in insects each hour. They forage mainly over water, and tend to avoid deep woods or open fields. Tricolored bats eat moths, flies, beetles, and ants (Barbour and Davis 1969).
Conservation / Management
Protection of bat hibernacula from human disturbance is a top priority. Suitable wintering sites should be gated or otherwise made inaccessible from September until June. Research on the summer habitat use of this species will be important in determining whether lack of suitable summer habitat is limiting the distribution of tricolored bats in the state. Further survey work to determine the true range of this species in Minnesota is necessary in light of the recent finding of dead tricolored bats in southwestern Minnesota.
Conservation Efforts in Minnesota
In the mid-1980s, a survey of the distribution and status of Minnesota bats was funded by the DNR's Nongame Wildlife Program. In 1988, 8-10 tricolored bats were found hibernating in the Heinrich Brewery Cave in Minneapolis (Birney and Nordquist 1989). Subsequently, the Nongame Wildlife Program funded the installation of a bat gate over the cave entrance to prevent disturbance to hibernating bats. Nongame wildlife staff in southeast Minnesota conduct a survey of the Brightsdale Tunnel in Fillmore County, where tricolored bats have been found since 1985, on a three-year rotation. Approximately 800 bats, including individuals of three other species, overwinter in this tunnel, making it one of the largest known bat colonies in the state. To date, bat-friendly gates and doors have been installed at seven cave/mine entrances in the state. In addition, winter surveys are conducted periodically to note any changes in the number of hibernating individuals.