Keen's long-eared bat (Myotis keenii) COSEWIC assessment and status report: chapter 6

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Biology

General

Like other Vespertilionids, M. keenii are heterothermic, and thus are well adapted to the seasonal temperature extremes and availability of food characteristic of temperate regions. They are likely capable of using torpor on a regular basis to conserve energy and survive periods of adverse weather, and of hibernating through the winter. Their reproductive chronology is also synchronized to take advantage of the short summer season. As for most myotis species, mating probably takes place in autumn just prior to hibernation or during the hibernation period. Mated females store and feed sperm through hibernation, and ovulation and egg fertilization occur in spring after arousal. Pregnant females return to traditional maternity roosts during April or May, depending on location. Gestation period is long (~ 40 - 60 days for Myotis), and typically only 1 large young (~ 25% of adult mass at birth) is raised per year (Tuttle and Stevenson, 1982; Kurta and Kunz, 1987). Length of gestation is variable in bats, and depends on how frequently the female uses torpor (Racey, 1981). Young usually become volant within about 3 weeks, but continue to be suckled for another two weeks, or until they become proficient at flying and foraging. Some juveniles are thought to mature sexually during their first summer and breed in their first autumn (Tuttle and Stevenson, 1982), although most probably don’t breed until the second autumn. Contrary to the norm among small mammals, most bats typically produce 1 pup per year, although they make up for this low reproductive rate by being relatively long-lived.

Reproduction

The Gandl K’in Gwaayaay study is the only one that reports detailed information on reproduction for this species. It was uncertain when females emerged from hibernation but few bats were present at the maternity roosts prior to the end of May in each of 2 years (Burles, 2001). Use of the roosts by females was consistently high throughout June and most were found to be pregnant. Parturition occurred during early July, and young were volant by early August. Use of the roosts declined during August as adults left for hibernation sites, although some juveniles continued to use the roosts into September. At least 80% of females captured during this study were reproductively active (either pregnant or lactating).

Reproductive chronology at Gandl K’in Gwaayaay varied between 1998 and 1999 (Figure 5) and was linked to weather conditions. Contrary to what might have been expected, however, gestation period was long and parturition late during the unusually warm, dry summer of 1998. The prolonged gestation was thought to be the result of energetic stress due to the lower availability of prey (Burles, 2001). The warm, dry conditions resulted in fewer insects being present, and those that were present may have spent more time flying, which would have made them less available to a gleaning bat such as M. keenii.

In 1999, reproductive chronology was more compressed, in spite of it being a cooler, wetter year. Parturition occurred during a short period of time between 5 and 12 July, and young were fledging by early August. The cooler, wetter conditions resulted in a greater abundance of Diptera and Trichoptera (Burles, 2001), and probably caused insects to spend less time flying and more time resting on vegetation, which would have made them more available to be gleaned.

The only records on longevity for this species come from the study at Gandl K’in Gwaayaay. Three individuals that were banded in 1991 by Firman et al. (1993) have subsequently been recaptured (Burles, 2001; Burles, unpublished data). A female recaptured in late May 2003 was banded as an adult, meaning that she was born in or before 1990, which made her at least 12 years and 11 months old at the time of recapture. Two others captured in 1998 were at least 8 and 7 years old at the time of recapture. These records are likely underestimates given that longevity records for most other Myotis species range from 13 to 22 years (Tuttle and Stephenson, 1982), and the record for M. lucifugus is 35 years (Davis and Hitchcock, 1994).

Survival

There is little information on survival rates for M. keenii, except for the longevity records mentioned above. Firman et al. (1993) banded a total of 27 individuals in 1991 but only 2 of 56 captured in 1998-99 were recaptures. This low recapture rate might suggest that survival is low, but recapture rates for bats tend to be misleading, as they quickly learn to avoid mist nets.

Data on the causes and rates of mortality are scanty. Skeletons of M. keenii, with fur attached, have been recovered from caves where bats hibernate on Vancouver Island, suggesting over winter mortality from depleted fat reserves or disease does occur. Over winter mortality is common in temperate region bats, particularly for juveniles. M. keenii is vulnerable to terrestrial predators; a number of museum specimens are cat kills (Nagorsen, unpublished data; Burles, unpublished data). The deer mouse (Peromyscus maniculatus) is a predator of hibernating bats in mine adits and caves in Ontario (Fenton, 1970) and mouse droppings and/or skulls (deer mouse or Keen's mouse, Peromyscus keeni) were found in 11 caves in the Weymer Creek area (Mather et al., 2000). As a crevice roosting bat, M. keenii could be particularly vulnerable to this type of predation.

Owls prey on bats, but the extent of owl predation in a coastal forest environment is unknown. There is only one resident owl on Haida Gwaii, the northern saw-whet owl (Aegolius acadicus). As many as 8 owl species occur in other parts of M. keenii ‘s range (Campbell et al., 1990).

Physiology

There is little specific information available on the physiology of this species. Temperate bats, however, are influenced by prey activities. Most insects become inactive when temperatures drop below about 10 °C, so bat activity often ceases below this threshold. M. keenii may be an exception, however, because of its ability to glean, and thus forage even when insects are not flying. High temperatures, on the other hand, may have a negative impact on M. keenii, as insects become more active and thus less available to be gleaned. Bats at higher latitudes where summer days are long are also more limited as to when they can forage because of their extreme vulnerability to predation under daylight conditions (Speakman, 1991).

The conditions under which bats hibernate are very specific. Hibernation sites with constant cool temperatures and a high humidity are considered to be essential for over winter survival. The greatest energetic costs of hibernating bats is their arousal, either because of extreme temperature fluctuations or because they have been disturbed. A high humidity is also required because most bats do arouse periodically to void themselves of metabolic wastes, and once this is done they need to replenish the water in their body. According to Davis et al. (2000b) and Mather et al. (2000), temperatures in the Vancouver Island hibernacula remained constant between 2.4 ° and 4 °C year around, with a constant relative humidity of 100%.

Movements/dispersal

Only two known M. keenii have been successfully radio tagged and followed for a total of 5 days (Mather et al., 2000). A nursing female captured in a small cave moved about 600 m to roost in a tree roost on a cliff the next day. The transmitter subsequently failed and she was not relocated. A male captured at the entrance of a cave was tracked to a rock crevice about 300 m away where it roosted for the next 2 days. This individual then moved about 300 m to a tree roost where the transmitter apparently fell off. 

The relatively short nightly movements seen on Vancouver Island are consistent with what has been found for other long-eared bats. Four long-eared Myotis (Myotis spp.) radio tracked near Smithers for 11 – 14 days each all returned to forage in the area that they were captured every night (MacKay et al., 2000). Similarly, Waldien and Hayes (2001) found that M. evotis in Washington State also foraged in relatively small areas.

No data are available on migratory movements between summer roosts and winter hibernacula, and virtually nothing is known of their dispersal patterns except that on Gandl K’in Gwaayaay, three females banded in 1991 were still using the same roosts when they were recaptured (Burles, 2001; Burles, unpublished data).

Nutrition and interspecific interactions

Diet for M. keenii occupying the maternity roosts at Gandl K’in Gwaayaay was assessed by Burles (2001) using faecal analysis. Lepidoptera were present in 78% of the 27 faecal pellets collected during 1998 and 1999, and made up 36% of the total number of individual prey items identified (Table 2).  Spiders (Arachnida: Araneae) were also present in 78% of all pellets analyzed, and was the predominant prey item identified in 1999. Diptera (22%) and Neuroptera (15%) occurred less frequently, while Hemiptera, Hymenoptera and Psocoptera were each found once (4%). Coleoptera and Trichoptera were not found in faeces, but they were also rare in light trap samples (Burles, 2001). The only other direct evidence of M. keenii diet comes from a single male collected in southeast Alaska (Parker and Cook, 1996). Its stomach contained 40% Trichoptera, 40% Araneae and 20% Diptera.

Other long-eared Myotis are morphologically similar to M. keenii, so examination of their diet may provide further insight into that of M. keenii. It must be noted, however, that bat diet is influenced by prey abundance and availability, which will vary with region. On Vancouver Island, Kellner (1999) found that M. evotis/keenii fed primarily on Lepidoptera and Diptera, with Neuroptera, Trichoptera and Arachnida making up only a small part of their diet. In Oregon, Lepidoptera and Diptera dominated M. evotis diet, but Coleoptera also made up a significant portion of their prey (Whitaker et al., 1977; Whitaker et al., 1981). Spiders were an important part of their diet in western Oregon but occurred only incidentally in eastern Oregon. In Arizona M. evotis fed primarily on Lepidoptera, Coleoptera and Diptera, and spiders did not appear in their diet at all (Warner, 1985). The higher incidence of Coleoptera in their diet in both eastern Oregon and Arizona may be a reflection of their greater availability in these areas. The consumption of spiders by M. evotis appears to be limited to wetter coastal regions similar to that occupied by M. keenii, which may also be a result of their greater availability in these regions.

In eastern North America, M. septentrionalis fed mostly on Lepidoptera and Coleoptera, and to a lesser extent Diptera and Neuroptera (Griffiths and Gates, 1985). These authors found green plant material in some faeces, which they took as evidence that these bats were gleaning insects from vegetation.

At Gandl K’in Gwaayaay, there was no evidence of competition for food between M. keenii and M. lucifugus, perhaps because of their different foraging behaviour (Burles, 2001). M. keenii’s greater maneuverability presumably allowed it to forage within the forest, and its ability to glean allowed it to forage on different prey (i.e., spiders), while M. lucifugus’s faster flight probably limited it to aerial hawking for insects in more open areas. Lepidoptera made up a large proportion of the diet of both species, however, and while their different foraging strategies likely minimized interference competition for this prey, exploitation competition may have been occurring. In the southern portion of its range where M. keenii is sympatric with M. evotis, another gleaner, competition for food could be a significant factor.

Information about interactions with other species is limited. At Gandl K’in Gwaayaay, M. lucifugus roosted in close proximity to M. keenii, although not in the same roosts. The crevices used by the latter were generally narrower than those used by the former. In southeast Alaska, however, a M. keenii was collected from a maternity colony of M. lucifugus (Parker and Cook, 1996), suggesting that they may occasionally roost together. Similarly, in Kingcome Inlet a M. keenii was captured in a nursery colony of Yuma bats (M. yumanensis), in the attic of a house. In the Weymer Creek hibernacula, M. keenii was found in the same caves as other Myotis species, although they apparently did not roost together.

Behaviour/adaptability

The ability of long-eared bats to glean prey from vegetation has been clearly demonstrated under laboratory conditions (Faure and Barclay, 1992; Faure et al., 1993), and the prevalence of spiders in the diet of M. keenii is evidence that gleaning is an important foraging strategy for them in the field (Burles, 2001). Bats that glean typically have sensitive hearing, which allows them to locate prey by passively listening for prey-generated sounds. This strategy is thus energetically efficient because there is no need to vocalize, an activity that is energetically expensive (Norberg and Rayner, 1987). It is also more efficient because insects with ears are less likely to detect them, and insects at rest have greater difficulty avoiding predators. Gleaning bats also have an advantage because they can continue to forage even after low temperatures prevent insects from flying.

On Haida Gwaii, the ability to glean likely broadened the prey base of M. keenii, allowing them to continue to forage during a cool, rainy summer when prey were less abundant and insects were less likely to be flying (Burles, 2001). A similar situation was found in the mountain regions of Alberta, where the ability to glean allowed M. evotis to forage even during cold mountain nights when insects ceased to fly (Barclay, 1991). In both locations, the ability to glean appears to be a key factor that allowed these long-eared bats to survive and successfully breed when other bats could not. The ability to forage by gleaning then, may provide long-eared bats with an advantage in regions where cool and/or wet conditions limit insect activity or abundance (Burles, 2001).

This species' ability to tolerate human disturbance is unknown. However, the persistence of the maternity colony at Gandl K’in Gwaayaay for more than 40 years despite human activity on the island is noteworthy. Moreover, this colony was affected by the collection of 26 M. keenii (mostly adult females) in the early 1960s for the Cowan Vertebrate Museum at University of British Columbia and the Canadian Museum of Nature.