Ecology of a Pleistocene Relict, the Western Big-Eared Rat (Plecotus townsendii), in the Southern Great Plains

Abstract

In karst regions of western Oklahoma and Kansas, Plecotus townsendii is locally ubiquitous but low in density. Natality is low but reproductive effort and preweaning survival of young are comparatively high. Movement data show no consistent pattern of dispersal that would account for a low density. P. townsendii uses nursery roosts that are marginal or uninhabitable for locally sympatric cave-dwelling species. Reproductive success depends on the ability of individuals in small nursery colonies to maintain a high degree of thermal exchange during pregnancy and lactation. This is facilitated by the formation of dense clusters in which metabolic heat is shared directly by body contact. In our judgment, the selection of intrinsically warm nursery sites is essential for reproductive success. In early and late winter most P. townsendii are absent from caves; presumably they occupy alternate roosts and forage on warm evenings. These bats seem to require relatively cold temperatures to hibernate. In autumn and early winter some individuals hibernate singly near cave entrances where ambient temperatures are variable but cool cave rock stabilizes body temperatures. By midwinter, these sites reach subfreezing temperatures and many bats move deep into caves, where they may form clusters. Both sexes lose over half their body weight before spring. Owing to this large weight loss, susceptibility to predation in midwinter, and probable late winter foraging efforts, we suspect substantial winter mortality as the prevailing limiting factor. The relictual nature of populations in the southern plains is made comprehensible by the species' reproductive, dispersal, and hibernating performance. From this ecological performance we infer that P. townsendii occurred across the southern United States during the Wisconsin glaciation, when numerous trees grew on the southern plains and the climate was moist with short, cool summers and long, mild winters. The present isolated taxa use caves as refugia from intolerable post-Pleistocene winters. The small size of P. townsendii populations, rather severe ecological limitations, and high sensitivity to disturbance lead us to conclude that visitation of nurseries by humans could threaten the species' survival in the Great Plains.