Spotted bat (Euderma maculatum) COSEWIC assessment and status report: chapter 6

Biology

General

The general biology of the Spotted Bat is poorly documented. Nonetheless its general life history strategy presumably is similar to that of other temperate region verspertilionid bats--the ability to enter torpor, 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.

Reproduction

Reproductive data for the Spotted Bat are scanty. Most Canadian bats mate in autumn, but the timing of copulation in the Spotted Bat is unknown. The only data on male reproduction is Poché’s (1975, 1981) observations of males captured in March and April with mature sperm in their cauda epididymis and males with scrotal testes taken in May and June. Females produce a single young in the breeding season (Watkins 1977). The young are generally born in May or June and nursing animals have been found from June to August in the United States (Watkins 1977). There are no specific dates for parturition in Canada, but the limited data suggest that the young are born in late June or early July (Nagorsen and Brigham 1993). In Washington, the young are volant by mid-August (Sarell and McGuiness 1993). The age at sexual maturity is unknown.

Survival

There are no longevity records but the Spotted Bat is presumably long-lived similar to other temperate region vespertilionid bats. Nothing is known about survival rates or the population age structure in this species. Information on mortality is limited to a few anecdotal observations. Black (1976) described predation by an American Kestrel (Falcosparverius) on a Spotted Bat that was flying during the day; owls presumably occasionally take this species when it is foraging at night. Rabies has been reported in two Spotted Bats (Constantine et al. 1979) in the United States. A rabid Spotted Bat found near Bastion Mountain in British Columbia in 1994 is the only known case of rabies for this bat in Canada.

Physiology

Spotted Bats forage at temperatures from 4.4 to 18.3°C (Leonard and Fenton 1989). This species uses torpor to reduce its energy demands at low temperatures. Poché (1981) observed that captive Spotted Bat’s were capable of only awkward flight when their body temperature dropped to 23°C. A male captured in January with a body temperature of 0.8°C increased its body temperature to 32.5°C within 10 minutes after being exposed to an ambient temperature of 20°C. To what extent this species can prolong bouts of torpor into extended hibernation is unknown. In Utah it has been captured in mist nets in mid-winter (Poché 1981; Ruffner et al. 1979). Spotted Bats have only been detected in Canada from April to October (Nagorsen and Brigham 1993).

The Spotted Bat is also exposed to high ambient temperatures in summer. According to Poché (1981) roosting Spotted Bats exposed to 35°C extend their wings and tail membrane and move their ears forward to expose more surface area for cooling. At 38°C, panting occurs indicating heat stress. Captive Spotted Bats exposed to 25 to 30°C, drank about 1.2 cm³ of water daily.

Movements/dispersal

No data exist on dispersal movements. In the southern Okanagan Valley, radio tracking studies by Wai-Ping and Fenton (1989) demonstrated that Spotted Bats move 6 to 10 km from their day roost to foraging sites. Round-trip movements up to 77 km from the day roosts to foraging areas were reported for the Grand Canyon National Park in northern Arizona (Rabe et al. 1998). From May to July, this bat shows a strong fidelity to its roost, returning to the same roost site night after night (Wai-Ping and Fenton 1987, 1989). By early August, the daily movements become less predictable. This change in roost fidelity may reflect behavioural changes associated with nursing and volant young or possibly migratory movements. Nothing is known about seasonal or long distance migratory movements.

Nutrition and interspecific interactions

Analyses of stomach contents and faecal pellets have shown that in the wild this species feeds mostly on moths (5- 2 mm in length) (Easterla 1965; Ross 1967; Poché 1981; Wai-Ping and Fenton 1989). Despite one observation that was interpreted as evidence of gleaning (taking prey from surfaces), Spotted Bats appear to capture their prey while in fight using echolocation calls to detect and track flying moths (Woodsworth et al. 1981; Wai-Ping and Fenton 1989). Spotted Bats forage in open areas 5 to 30 m above ground flying in elliptical paths of 200 to 300 m.

Because its echolocation calls (Figure 7) demonstrate several adaptations for hunting moths, Fullard and Dawson (1997) suggested that the Spotted Bat is a ‘moth-specialist’. Its low frequency echolocation calls have a dominant frequency of 9-12 kHz (within the range of human hearing) and several harmonics of higher frequency. Many moths have ‘ears’ sensitive to bat echolocation calls and will take evasive action to avoid capture when they detect a bat’s echolocation calls. Fullard et al. (1983) found that moths from the Canadian range of Spotted Bat are most sensitive to ultrasonic frequencies of 30 to 75 kHz suggesting that they be unable to detect the search calls of the Spotted Bat. Experimental studies by Fullard and Dawson (1997) revealed that the low frequency, soft, short calls of the Spotted Bat are either inaudible to moths or detectable at close range (distances of less than 1 m) making them vulnerable to capture. The long wave length of Spotted Bat calls (a characteristic of low frequency sounds) makes the calls most effective for detecting objects about 10 mm in diameter; this is consistent with the size of moths recovered in prey remains (Leonard and Fenton 1984). Nevertheless, because no extensive dietary studies have been done on this bat, the extent that its diet is restricted to moths is unknown.

Figure 7. Sonogram of representative calls of the Spotted Bat (Euderma maculatum). A, B: search phase, C: approach phase, D: terminal feeding buzz. Inset shows variation in the fundamental note of calls A and B. Taken from Leonard and Fenton (1984).

Figure 7. Sonogram of representative calls of the Spotted Bat (Euderma maculatum)

Even if it is a moth-specialist, the Spotted Bat would have a diverse and abundant prey base. According to Don Lafontaine (pers. obser.) the Cordilleran ecozone of British Columbia supports about 700 species of noctuid moths; more than half of these species would occur in the low elevation steppe and grasslands of areas such as the Okanagan Valley. Species such as cutworms are especially abundant and diverse in grassland communities. A light trap (attractancy range of 6 m) set in these habitats would be expected to capture several thousand moths on a warm night.

With 12 to 14 species, 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 in the valleys over lakes, marshes, and open grassland or forest. Some overlap with foraging Spotted Bats would be expected. However, competition for food resources with the Spotted Bat is probably limited as these bats tend to be opportunistic feeders taking a variety of invertebrate species in addition to moths.

Behaviour/adaptability

Although most researchers have concluded that this bat roosts solitarily, its social structure is unknown and the Spotted Bat may roost alone or in small groups. The limited data suggest that it is not found in large aggregations. Evidently echolocation calls of the Spotted Bats are also used in communication. It reacts strongly to playback calls of other individuals (Obrist 1995). When foraging, agonistic vocalizations are produced in response to intruders entering a bat’s foraging area (Leonard and Fenton 1983; Storz 1995) and some researchers (Easterla 1973) have observed chases in flight. Although there is a tendency for neighbouring Spotted Bats to space out their feeding areas, the extent to which this bat is territorial, however, is not clear. Leonard and Fenton (1983) concluded that individuals maintained exclusive feeding territories separated by at least 50 m; however, Wai-Ping and Fenton (1989) observed extensive overlap in the foraging areas of neighbouring individuals.

Adept at crawling into small crevices and openings in rock outcrops, and able to climb vertical rock faces (Poché 1975) the Spotted Bat is well adapted for crevice roosting. Garcia et al. (1995) and Sarell and Haney (2000) noted that this species is intolerant of noise and disturbance at its roosting sites. However, most of this evidence appears to be anecdotal. The Spotted Bat is adaptable in its use of foraging habitat feeding over disturbed habitats such as agricultural fields and golf courses (Leonard and Fenton 1983; Gitzen et al. 2001). 

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