Winter activity of Australian tree-roosting bats: influence of temperature and climatic patterns

Many bats hibernate to overcome a critical energy shortage in winter. However, unlike strictly seasonal hibernators, bats sometimes alternate prolonged torpor bouts with increased activity coinciding with periods of mild weather. Winter activity may be especially common in hibernating tree-roosting bats because they are less insulated from external conditions than cave-roosting species and must also leave their roosts to drink, socialize or mate. To test this hypothesis, I measured the activity of a community of tree-roosting bats by recording echolocation calls at four sites on 18 nights throughout winter in south-eastern Australia. I also measured insect activity using light traps at each site. I recorded some bat activity on most nights, typically at dusk. Greater bat and insect activity occurred on nights of warmer temperatures, moderate wind speeds and falling barometric pressure, indicating the approach of low-pressure cold fronts. Thus, winter activity of tree-roosting bats is linked to short-term mild conditions caused by a specific, reoccurring weather pattern. Climatic shifts that reduce the frequency of such weather events, such as the El Nino Southern Oscillation phenomenon and future human-induced climate change, may also reduce winter activity and/or increase the cost of necessary arousal periods by hibernating tree-roosting bats.