Pre-Weaning Mortality in a Florida Maternity Roost of Myotis austroriparius and Tadarida brasiliensis

Abstract

The roost environment is a factor determining the survival and mortality rates in newborn bats. We compared mortality in two species of bats that shared a common roost but that differed in developmental strategies. The precocial species, Tadarida brasiliensis, appeared to exhibit less preweaning mortality than did the altricial species, Myotis austroriparius. Pre-weaning mortality in M. austroriparius was most pronounced shortly after birth. Many North American bat species produce a single precocial offspring each year, a demographic response related to the brevity of the season during which offspring can be produced and reared (Tuttle and Stevenson, 1982), restricted food supply (Humphrey, 1975), or aerodynamic constraints (Myers, 1978). The southeastern brown bat M. austroriparius however, commonly produces altricial twins (Foster et al., 1978). Central Florida populations of M. austroriparius do not hibernate and are therefore exposed to in-flight mortality factors over a larger portion of the year than other temperate Myotis. Maternity roosts of M. a. austroriparius are generally found in limestone caves over water where pre-weaning mortality is significant because females cannot retrieve fallen young from the water (Rice, 1957). Thus, twinning is thought to be an adaptive response to a high annual mortality of M. austroriparius (Foster et al., 1978). In contrast, female free-tailed bats, T. brasiliensis, give birth to a single precocial young each year (Pagels and Jones, 1974; Sherman, 1937), and generally roost in man-made structures throughout the year in Florida (Bain, 1981). Roost mortality is unknown in the Florida Tadarida populations, but was estimated at 1.8% for Texas cave populations (Davis et al., 1962). We monitored mortality in an attic-dwelling colony of M. austroriparius and T. brasiliensis during the spring and summer of 1983. These data supplement information provided for M. austroriparius by Foster et al. (1978), and compare mortality in two species, M. austroriparius and T. brasiliensis, under identical roost conditions.