Survival Rate of Young Southeastern Brown Bats, Myotis austroriparius, in Florida

Abstract

Mortality of young Myotis austroriparius from birth to weaning was measured by censusing carcasses that fell from a nursery roost in northern Florida. Rates were determined from population counts and published data on natality and population structure. Shape of the mortality curve was the same in 2 years. Most deaths (75%) occurred in the first week of life, and the survivors that acquired their first permanent tooth had a high probability (97%) of living to weaning age. A total of 88.2% of the young survived to weaning age. Total preweaning survival in M. austroriparius is relatively low. This high loss and previously recorded features of M. austroriparius natality (twinning and altricial de- velopment of the young) are interpreted in the context of characteristics of the nursery roosts and climate, as follows. Low preweaning survival results from certain death when young fall into water under the roost, without parental retrieval. Selection of roosts over water is favored by predation pressure. Adults maintain high-risk activity almost all year, with only brief periods of hibernation, so adult mortality probably is relatively high. These two sources of heightened mortality have selected for twinning, but physiological limitations dictate that young are born at an early stage of develop- ment.