Population sizes have been declining in many species of bats, leading to many species receiving federal or state protection under the Endangered Species Act. There are various reasons for these population declines, but most biologists believe that habitat loss and/or degradation is one of the most important causes. Bat researchers have been working hard during the past decade to determine which aspects of a species’ habitat are preferred by individuals and allow for successful reproduction.
Biologists have known for some time that fetal gestation rate and developmental rate of juvenile bats is influenced by temperature. If temperature within a bat’s roost (where it spends its daytime hours) is warm, then fetal bats and juvenile bats develop faster than if temperatures are relatively cool. Biologists hypothesize that reproductive females choose warmer roost sites, allowing for earlier birth and weaning of their young. By minimizing gestation and weaning time, reproductive females provide their young with a greater proportion of time during the remaining summer months to become successful foragers and to gain adequate fat reserves. Without adequate fat reserves, young bats that enter their first winter of hibernation may experience relatively high mortality due to energy reserves being depleted prior to the onset of spring.
Recent research has provided indirect evidence that reproductive female bats do choose the warmer of available roost sites. For example, for species that roost in trees (e.g. in tree hollows or under loose bark), females often choose relatively large trees, that are taller than surrounding trees, and that likely allow for increased exposure to solar insolation. Although such indirect evidence offers valuable information about habitat requirements, it does not directly test whether the bats are choosing the warmest (assuming that roost sites do not get too hot) of available roost sites.
To help answer this question, NexSens micro-T temperature logger units are being used to generate a multi-point thermal map of a habitat area used by one of the rarest bats in North America: the eastern small-footed bat. Dr. Jacques Veilleux of Franklin Pierce College (Rindge, New Hampshire) began a research project during the summer of 2005 which aimed to document the habitat preferences of eastern small-footed bats (state endangered) at the Surry Mountain Lake Dam, located in southwestern New Hampshire. Eastern small-footed bats were found roosting in rock crevices associated with the crushed rock ‘rip-rap’ that forms the upper layer of the north and south facing dam slopes. On most occasions, pregnant females chose to roost in rock crevices on the far western side of the south facing dam slope; the side that receives sunlight earliest in the morning. It seemed reasonable to hypothesize that female bats were choosing to roost at the dam location that warmed earliest and became warmest during the daytime.
Since the dam consists of two slopes (each identical in nearly all respects except for the direction of the slope, and therefore exposure to the Sun), a natural experiment presented itself this hypothesis and to provide the first direct evidence of habitat choice based on temperature differences across the range of a bat population’s available habitat. A total of 36 micro-T sensors were installed this spring within the two slopes, 18 on the north and south facing slopes, respectively. Sensors were placed at approximately 100m intervals along the east/west and at 20m intervals along the north/south gradients of each slope. Temperature data will be collected once per hour, for approximately 3 months, generating a thermal map of the dam slopes. It is predicted that: 1) the south facing slope will be significantly warmer than the north facing slope, and 2) the west side of the south facing slope will be significantly warmer (especially early in the day) than the east side of the same slope.
If the female bats are shown to prefer roost sites that are warmer than other available roost sites, biologists will have a much better understanding of what components of the bat’s habitat are critical when considering conservation and management actions. In addition, knowing the thermal preferences of a particular species may allow researchers to examine additional habitat areas that may offer similar beneficial microclimates, in order to locate additional populations of such rare and endangered species.