Fringed bat COSEWIC assessment and status report: chapter 6

Biology

General

The general biology of the Fringed Bat is similar to that of most temperate region verspertilionid bats. It has the ability to enter torpor to conserve energy, a long life span, and a low reproductive rate. Roosting sites are critical for protection from predators, providing shelter, and suitable physiological conditions. Echolocation is used for orientation in flight and prey detection. Almost all of the data on this species’ general biology are derived from studies done in the United States.

Reproduction

Reproductive biology was described by O’Farell and Studier (1972, 1973) for a maternity colony in a building in New Mexico. The precise timing for copulation is unknown, but mating occurs sometime after this species leaves its maternity roost in autumn. Females arriving at the maternity colony in early April were carrying sperm in their uteri. Ovulation and fertilization occurs nearly a month after the females arrive at the maternity colony. Females give birth to a single young and produce only one litter in the breeding season. The gestation period is about 50 to 60 days. The parturition period was synchronized for the New Mexico colony with births from 25 June to 7 July. The young develop rapidly reaching adult size by 3 weeks. By 16.5 days they are capable of limited flight; by 20.5 days they are capable of flight comparable to the adults. Reproductive data for the Canadian population are scanty. A pregnant female was captured 27 June; nursing females have been taken from 9 to 29 July (Roberts and Roberts 1993; Holroyd et al. 1994). The nursery colony at Vernon had young bats 19 July when examined by Maslin (1938). Females may breed in their first year, but males evidently do not breed until their second year.

Survival

Data on survival rates and mortality are few. However, in some temperate bats, first year mortality rate may approach 1%. Tuttle and Stevens (1987) reported a banded Fringed Bat recovered at a mine in Oregon that was a minimum of 18.3 years old. Constantine et al. (1979) described rabies in several Fringed Bats in the United States. There are no reports of predation in the literature.

Physiology

In the south-western United States, Jones (1965) netted Fringed Bats at ambient temperatures of 9 to 25°C. According to Studier and O’ Farrell (1972), hyperthermia begins at ambient temperatures of 24°C and this species is fully hyperthermic at ambient temperatures above 28°C. When exposed to ambient temperatures of 24°C or less, breeding females will use torpor or regulate their body temperature by increasing their metabolic rate. Energy demands are greatest for this species during lactation (Studier et al. 1973) and nursing females remaining homeothermic will maintain a body temperature about 2°C lower than that of pregnant or post-lactating females. In maternity roosts, this species undergoes daily movements within the roost to find the ideal temperature regime for thermoregulation.

Studier and O’Farrell (1972) found that Fringed Bats in New Mexico had deposited insufficient fat reserves to hibernate when they abandoned the nursery roost in autumn. They concluded that this bat is periodically active throughout winter. However, Fringed Bats have been found hibernating in Oregon and North Dakota (Martin and Hawks 1972; Perkins et al. 1990). No winter records exist for the Fringed Bat in British Columbia. Winter surveys of 18 caves and mines within the Canadian range found no hibernating Fringed Bats (Nagorsen et al. 1993).

Movements/dispersal

There are no measurements on the distances that this bat commutes from its day roost and nocturnal foraging sites. However, radio tracking studies (Cryan et al. 2001) have shown that it generally occupies a limited area of about 4 km². Roosts, particularly maternity colonies, in permanent structures such as buildings are generally occupied throughout the entire summer (O’ Farrell and Studier 1975). But Fringed Bats roosting in trees and rock crevices frequently switch their roosts, remaining at a roost site for only a few days (Cryan et al. 2001; Weller et al. 2001). Distances moved between roost sites are short. In north-western California, Weller et al. (2001) found that the mean distance between tree roosts was only 254 m (range 7-641 m). In the Black Hills of South Dakota where both tree and crevice roosts were used, the Fringed Bat never moved more than 2 km between roosts (Cryan et al. 2001). Nursing females will transport their non-volant young when moving to a new roost.

Studier and O’Farrell (1972) speculated that in New Mexico the Fringed Bat migrates in late summer from the summer maternity sites to its winter range. Hoffmeister (1970) documented a shift in occurrence records for summer and winter in Arizona that he attributed to migratory movements. However, there are no specific data on long distance or migratory movements by this bat. Nothing is known about migratory movements in the Canadian population.

Nutrition and interspecific interactions

There are no dietary data for the Canadian population. In New Mexico, Black (1975) found that beetles (Coleoptera) occurred in 73% of the faecal pellets he sampled from this species; moths (Lepidoptera) were found in 36% of the pellet samples. In Oregon moths, lacewings (Neuroptera), and various flightless invertebrates such as spiders (Araneida), crickets (Gyrillidae), and harvestman (Phalangida) were identified in stomach remains (Whitaker et al. 1977, 1981). These diet studies were all based on small sample sizes. An analysis of a large sample (68) of faecal pellets from Arizona revealed that the diet was mostly moths, beetles, flies (Diptera), and lacewings (Warner 1985).

Bell (1980) interpreted the slow manoeuvrable flight and moderate to low intensity, frequency-modulated, echolocation calls of this species as adaptations for a gleaning foraging strategy. Studier and O’Farrell (1980) speculated that the puncture resistant flight membranes of the Fringed Bat are an adaptation for gleaning on the ground or thick thorny vegetation.

With 12 to 14 species including 7 species of Myotis, the southern interior grasslands of British Columbia support the most diverse bat community in Canada with some species occurring in high densities (Fenton et al. 1980). Most bats in this region forage opportunistically in the valleys over lakes, marshes and open grassland or open forest. The gleaning strategy of the Fringed Bat may reduce dietary overlap with some of the abundant aerial foragers such as the Little Brown Bat (Myotis lucifugus) and Yuma Myotis (Myotis yumanensis).

Behaviour/adaptability

The Fringed Bat is colonial. Maternity colonies in buildings may contain up to 1,000 to 1,200 individuals (O’Farrell and Studier 1975). Adult males are rarely found in these large maternity colonies. Maternity colonies in tree or rock crevice roosts are smaller. Cyran et al. (2001), for example, reported a maximum group size of 27 Fringed Bats for tree and crevice roosts. They noted that maternity colonies maintained their group integrity with the same bats associating through roost changes. Although Fringed Bats move frequently between tree or rock crevice roosts, they demonstrate local fidelity returning annually to the same area (Cyran et al. 2001). There may also be some fidelity to night roosts as Collard et al. (1990) recaptured 13 Fringed Bats at the Suzie Mine including 4 individuals banded the previous year at the mine. An individual banded at the Suzie Mine in August 1982 was recaptured at the mine in April 1990 (Nagorsen and Brigham 1993).

According to O’Farrell and Studier (1973), adult female Fringed Bats are easily disturbed in the maternity colony. They are particularly vulnerable to disturbance during late pregnancy before parturition when they become secretive and nearly impossible to approach. The ability to switch roosts coupled with a familiarity of alternate roosting sites, however, may enable the Fringed Bat to relocate to new roosts if roosting habitat is lost.

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