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Photo by Bruce Bonebrake
Photo Copyright Bruce Bonebrake
Spotted Bat
Euderma maculatum
NatureServe conservation status
Global (G-rank): G4
State (S-rank): S4
External links
Phenology
Poche (1981) presented results of mist-netting at Fort Pearce Wash (Utah-Arizona boundary) stating: "Ten spotted bats were netted during the winter study period--November 1974 to March 1975, and January 1976. None were captured in November [1974] or March [1975]...." However, his Table 1 in the same report indicated the capture of 11, not 10, active spotted bats in every month during the winter period specified in the text except March (1975) and December (1974), not November (1974). (The same table also indicates the capture of spotted bats in March of 1976.) Thus it appears from Poche's table and text that he captured active spotted bats in every month except one--either November or December. The coldest ambient temperatures at which Poche mentions capturing active spotted bats were -3 C and -5 C. Even more interestingly, Poche found that, although the greatest absolute numbers of spotted bats captured were in summer months, the numbers captured relative to effort (bats per net-night) were higher for some winter months than for some summer months. From these data it appears that spotted bats are active throughout the year, including winter (at least intermittently), at Fort Pearce Wash. It should be remembered, of course, that Fort Pearce Wash is one of the lowest and warmest locations in Utah, and therefore the seasonal activity pattern of the spotted bat at this locality may not be representative of this species elsewhere in the state. [Furthermore, despite the title of Poche's report, "Ecology of the spotted bat (Euderma maculatum) in southwest Utah", Poche's Figure 1 shows that two of his five mist-netting sites in Fort Pearce Wash were not in Utah, but rather in Arizona.]
The only report of hibernation in this species, anywhere in its range, is that of four spotted bats found hibernating in Crocodile Cave, Kane County, in February 1930 by Bernell McAllister of the Soil Conservation Service (Hardy 1941): "He observed that they were hibernating on the walls of the cave above the large pool of water. They hung by the thumb with the head up and not downward as do other bats, according to Mr. McAllister." These, incidentally, were also the first spotted bats known to have been observed in Utah. [Easterala (1965), reviewing knowledge of the spotted bat in Utah, repeated this story, citing Hardy (1941); however, Easterla augmented the date to "4 February 1930". In the absence of any explanation of this greater knowledge regarding the date, the day specified by Easterla must be viewed with doubt.]
Although many authors (e.g., Easterla 1971, Barbour and Davis 1969) have reported this to be a very late-flying species of bat, typically appearing between midnight and dawn, Poche (1981) found at Fort Pearce Wash, Utah and Arizona, that "[s]ixty-two percent of all spotted bats captured for this study were netted within three hours following sunset and 87% within four hours of sunset"; 39% of his spotted bats were captured between 1 and 2 hours after sunset. (Apparently included in Poche's 1981 report were the data published earlier [Poche and Bailie 1974] that documented the capture of four spotted bats at Fort Pearce Wash in May 1974 at times ranging from 2215 to 2230 hours.) Based on his Fort Pearce Wash study, Poche (1981) staed: "Times of captures in nets and observations of flight from roosts indicate spotted bats generally emerge from roosts approximately 1.5 hours after darkness (90 to 140 minutes after sunset). In June, a male with a known roost was observed three successive nights. On all nights, the bat emerged 120 minutes after sunset (plus or minus five minutes)."
Easterla (1965), however, captured four spotted bats just north of Bryce Canyon National Park, at 7,589 ft. elevation, in Garfield County, all on different nights, but always between midnight and 7:00 a.m., which agrees with the late-night activity pattern widely reported by others.
Species range
Spotted bats are found throughout the mountainous regions of western North America, from central Mexico to southern British Columbia (Luce and Keinath 2007). Their presence likely extends across all of Utah, though current records are sparse in the northern and western parts of the state (Oliver et al. 2000, UDWR data).
Migration
Long-distance migration has not been demonstrated in this species either in Utah or elsewhere; however, so little is known of the biology of the spotted bat that the possibility that at least some individuals or populations migrate cannot be ruled out. The species occurs, at least in summer north and west of Utah into Canada, and, if long-distance migration does occur in spotted bats, some could migrate to or through Utah. The fact that the one report of hibernation in this species (Hardy 1941) was in a cave in Kane County, Utah, suggests that at least Utah populations hibernate rather than migrate.
Poche hypothesized local migration of this species in summer in southwest Utah. Poche (1981) believed that, based on his "study from southwest Utah [and adjacent northwest Arizona], the species primarily inhabits xeric communities but occasionally wanders into higher elevations when temperatures are too stressful in desert environments", but, he noted, "[p]erhaps variations exist over the bat's geographic range."
Habitat
Within the Utah range, the majority of records are from deep, narrow, rocky canyons, particularly those bounded by precipitous cliff faces. Crevices in cliff walls are the primary roosting sites (e.g., Poche 1981). Individuals forage over open sagebrush steppe, desert scrub, or montane meadow habitat (Easterla 1965, Poche 1981, Foster et al. 1996), sometimes considerable distances from roosting habitat (Poche 1981).
Food habits
Easterla (1965) reported the stomach contents of two spotted bats collected on 15 and 20 August 1964 in Garfield County: "Only small moths, probably Noctuids, about one centimeter in length were found. This bat seems to pick off head, legs and wings before onsuming a moth. To quote Dr. Byers [who performed the analysis of stomach contents for Easterla], 'Both samples have one very striking characteristic: there are no wing fragments, no tarsi (feet), no heads (antennae, mouthparts, eyes, etc.). The only conclusion I can draw from this is that the bats capture their prey, then, perhaps on the wing but more likely by going to some resting place, methodically chew off head, legs and wings. This is interesting in that these parts are the most densely sclerotized (i.e., hardened) parts of an insect and those most often well preserved in stomachs of vertebrates. All the sclerotized parts I found, in both samples, were pieces of abdomen or thorax, occasionally a leg fragment.'" (It should be noted that Byers was probably right in suggesting that it is more likely that the bats take their prey to a roost where they then dismember the organism, as do many species of bats, rather than processing the food item in flight.) Easterla went on to comment that these findings agreed with those reported in a study conducted in northern New Mexico in which 100% moths, 8-12 mm in length, were found in 18 fecal pellets of spotted bats. He then speculated: "The apparent selectivity in the eating habits of the spotted bat could possibly limit its distribution." Though, of course, possible, it is highly unlikely that prey selectivity is limiting to the spotted bat; small moths are ubiquitous and abundant, and the family Noctuidae "is the largest family in the order [Lepidoptera, butterflies and moths], with some 2700 N. American species [north of Mexico], and many are common moths" (Borror and White 1970).
Poche (1981) said: "A study of the stomach contents of eight Euderma revealed the primary prey species to be Lepidoptera. Moth wings, heads and legs comprised [sic] 66% of stomach contents." It is unfortunately not entirely clear from this statement or others in his report whether the study to which he was referring was his own Fort Pearce Wash study or someone else's study, to which he failed to give citation. It does, however, seem probable that he was reporting results from his own Utah-Arizona study, for later in the same report he added: "Further lab analysis of Euderma stomachs from southwestern Utah [and Arizona?] will be undertaken in the fall of 1978." (Whether this was ever done is not known; however, the work to be completed in the fall of 1978 was not included in the 1981 report that "forecast" it.) It is of interest that all of the specific items that Poche mentioned from the eight spotted bat stomachs, whatever their geographic origin may have been,--the moth wings, heads, and legs that composed 66% of the sample--were exactly the kinds of prey remains that Byers and Easterla reported to be so surprisingly missing from the two Garfield County spotted bat stomachs.
Poche and Bailie (1974) reported their observations of the release of a spotted bat that they had captured at Fort Pearce Wash on a previous night; they released the bat "approximately 55 minutes before dark" and observed its behavior: "While in flight, the Euderma appeared to be feeding on small insects within two meters of the ground. Suddenly it dropped to the ground and seized and ate a grasshopper; and within ten seconds it was again in flight. A second plunge to the rocky terrain was observed. ... The descents to the ground were preceded by split-second hovering."
Ecology
This species is strongly associated with cliffs for roosting, with one study calling them "nearly obligate cliff-roosters" (Pierson and Rainey 1998). Roost sites in northern Arizona are described as "sheer cliffs near perennial water sources" (Chambers et al. 2011). While found in various plant communities, most captures occur in open areas, with wet meadow and riparian habitats being preferred foraging sites (Wai-Ping and Fenton 1989, Williams et al. 2006, Chambers et al. 2011). Researchers previously suggested that spotted bats do not travel more than approximately 10 miles between roosts and feeding areas, requiring cliffs close to foraging habitat (Fenton et al. 1987, Wai-Ping and Fenton 1989, Luce and Keinath 2007). However, a study in northern Arizona showed spotted bats having large home ranges, traveling up to 38.5 km from roosts, confirming the link between cliffs and foraging areas (Chambers et al. 2011). The primary diet for spotted bats are grasshoppers and moths.
Reproductive characteristics
Easterla (1965) reported the capture of three lactating females on 10, 15, and 18 August 1964 in Garfield County. Foster (1995, unpublished) captured three females, two of which were lactating, on 27 July 1995 in San Juan County.
Poche and Ruffner (1975) mentioned that in six of seven males captured at Fort Pearce Wash between 12 and 15 August 1974 the testes were scrotal. Poche (1981) captured pregnant females at Fort Pearce Wash (Utah-Arizona) on 20 June 1974 and 16 June 1976. He also reported: "Two [spotted bat] juveniles were captured during this study [at Fort Pearce Wash, Utah-Arizona]--one on 5 September 1974 and the other on 1 September 1975." This, however, seems to conflict with his statement earlier in the same report: "Three juveniles were captured in August 1975." Moreover, his Table 1 in the same report indicates that he captured only one spotted bat of any age in August 1975.
Poche (1981) concluded (based on evidence from scattered locations, some of them in Utah): "it appears that Euderma breeds in the early spring--late February to early April--and gives birth late May to early July."
Threats or limiting factors
Potential threats to spotted bats include the loss of wet meadow habitats, pesticides, and human disturbance at roosts (Luce and Keinath 2007). Habitat alteration in areas where cliffs and moist habitats coincide could lead to the loss of local populations (Wai-Ping and Fenton 1989, Pierson and Rainey 1998, Williams et al. 2006, Chambers et al. 2011).
Spotted bats primarily feed on moths (Luce et al 2007) and forage over both public and private lands (Chambers et al. 2011). Broadcast spraying of pesticides for insect control poses two threats: direct poisoning and reduction of food sources (Luce and Keinath 2007). While spotted bats may be somewhat protected from human disturbance due to their use of inaccessible or remote cliff roosts (Luce et al 2007, Chambers et al. 2011), the increasing popularity of recreational rock climbing means cliff inaccessibility is no longer a guarantee of safety.
