Winter Habitat Selection by North Temperate Cave Bats

Abstract

Discriminant function analysis was used to quantify both microhabitat (within-cave) and macrohabitat (among-cave) locations selected by five species of cave-dwelling bats in the limestone caves and mines of Maryland, Pennsylvania and West Virginia. Microhabitat parameters reflected microclimate, physical structure and behavioral interactions at the roosting site. Macrohabitat parameters described cave macroclimate, structure, size and external habitat features. Three discriminant functions accounting for more than 95 % of the variance were derived in the microhabitat analysis. Function I was correlated with microclimate and the tendency to cluster, function II with the exposure of the site, and function III with the draftiness of the location. There was a large degree of overlap along many of the univariate parameters, particularly temperature. Multivariate classification of the five species was 63.5% correct. Parameters that were important on a macrohabitat scale differed among species. Zones of suitable microhabitat available within a cave or mine were important factors, but presence or absence of a species at a cave or mine could not be accounted for solely by differences at this level. Cave size is shown to be important to several species in this guild. Correct multivariate classifications varied by species from 82.0 to 96.0%. INTRODUCTION Bats are primarily tropical in distribution, with only three families (Vespertilionidae, Rhinolophidae and Molossidae) occurring in temperate regions for extended periods (Koopman and Cockrum, 1967). This subset of the order Chiroptera is composed entirely of insectivorous species which must deal with seasonally fluctuating food supplies. Two ways of coping with the problem of a temporally reduced food supply are hibernation and migration (Dwyer, 1964; Davis, 1970). Selection of a site that meets the physiological requirements for hibernation is adaptive and should be favored by natural selection. Several studies of bats show that mortality rates are highest at the end of hibernation (Eisentraut, 1947; Ransome, 1968; Stebbings, 1969), when fat reserves are low and starvation may occur. Factors that contribute to an increased probability of starvation are selection of a site that is outside the thermal neutral zone (Lyman, 1970), a long winter that extends the hibernating period (Beer and Richards, 1956), and frequent disturbance of hibernating bats causing an increased metabolic rate (Hall, 1975). Caves and mines are used as winter hibernation sites (hibernacula) by large numbers of temperate bats. A number of habitat variables has been postulated to affect selection and use of specific microhabitats within these sites. These variables can be broadly grouped into microclimatic, structural and behavioral categories. Microclimatic parameters include temperature (Eisentraut, 1934; Hock, 1951; Hanus, 1959), relative humidity (Anciaux, 1948) and airflow (Kallen, 1964). Parameters of physical structure are cave wall conformation (Twente, 1955; Krzanowski, 1959; Hall, 1962) and site exposure (Krzanowski, 1959; Bezem et al., 1964; Daan and Wichers, 1968; Daan, 1973). Behavioral adaptations are cluster formation and site specificity (Twente, 1955; Hall, 1962; Daan, 1973). Previous studies of hibernation site selection have focused on microhabitat or within-cave selection, particularly with respect to microclimate (see Davis, 1970; Fenton, 'Present address: Program in Ecology, ill Ferguson Building, The Pennsylvania State University, University Park 16802.