Effects of temperature and availability of insect prey on bat emergence from hibernation in spring

Abstract

Bats at northern latitudes may hibernate for up to 8 months during winter and must subsist on stored fat over this period. Environmental influences on bat emergence from hibernation are not well understood, and the degree to which bat emergence coincides with availability of their insect prey is not known. We used electronic detectors to monitor bat activity from 2001 to 2013 at a major hibernaculum, used primarily by Myotis lucifugus , in Wisconsin. We estimated dates of entry into hibernation in fall and recorded bat activity as bats emerged in spring over this 13-year period. Temperatures outside and inside the mine and airflow at the mine entrance were measured. Night-flying insects were monitored during spring from 2008 to 2013. Bat activity in spring was most strongly associated with warmer temperatures outside of the mine and was also correlated with airflow in the mine entrance. Changes in airflow were driven by temperature differentials between the mine and outside, and could serve as a signal for outside temperatures to bats inside of the mine. Insect abundance was also positively correlated with warmer outside temperatures. However, bat activity peaked before insects started to become abundant. Over the 13 years of the study, the date at which 5% of bat activity for the season was reached ranged from 10 April to 20 April, and for 50% of bat activity for the season the range was from 28 April to 4 May. The date of 5% bat activity for the season was not correlated with the date of entry into hibernation the previous fall, mean winter temperatures inside or outside of the mine, or the degree days (base 0°C) inside or outside of the mine. Our study was conducted before white-nose syndrome was documented in Wisconsin, thus our study documents hibernation behavior of healthy bats.