By November, bats in Britain are hidden away for the winter months in hibernation. There are only a few mammals in the UK that undergo hibernation in winter. Dormice and hedgehogs are the only others that go undertake this amazing physiological survival strategy. Understanding and monitoring bat hibernation activity and hibernacula sites is crucial for bat conservation in the UK and other temperate regions.
In temperate regions such as Britain there is a great decrease in the abundance of night-flying insects, the food of all British microchiroptera, during the coldest months of the year. Hunting for insects is an energetically costly process for bats and so it is not feasible for them to feed during winter. Hibernation is more than simply deep sleep; the bats' entire metabolism slows to a crawl. An active bat's heart beats around 900 to 1000 times a minute. In contrast, hibernating bats can allow their heart rate drop to 20 bpm and only take a breath 5 times per minute. This greatly reduces oxygen consumption and the bat will burn far fewer calories. Body temperature can drop to little higher than ambient temperature, not much above freezing in some cases. During this time of inactivity the bat lives off fat stores and can lose ¼ to ½ of its body weight whilst hibernating.
Bats do not tend to hibernate all the way through winter; they need to become active periodically to drink and excrete waste. On milder nights they may emerge to forage for insects. Body temperature rises fairly rapidly and waking often only takes around 20 minutes. Waking from hibernation can use up substantial energy reserves; bats may only have sufficient fat reserves to wake a handful of times each winter. Ensuring bat hibernacula are not disturbed is an important part of bat conservation efforts.
Bats choose sites in which to hibernate carefully according to environmental conditions and location. Although the exact requirements vary between species, hibernacula are cool and stable in temperature - normally around 5ºC. The site must be highly humid (near 100% ideally) to prevent dehydration. Depending on species, bats can hibernate in buildings, hollow trees, mine shafts and caves.
Recognition and protection of hibernation sites in the UK is an important (and legal) requirement of land and property developers as well as conservationists. Dataloggers are an ideal way to monitor important hibernation sites with the minimum of disturbance. They can record temperature and humidity conditions in a hibernaculum for months at a time without any human presence. A robust waterproof logging unit such as the Tinytag TGP-4500 is ideal for the damp environment. Wireless weather stations have also been used to monitor hibernation and other roosting sites alongside the outdoor weather conditions. A wirefree thermo-hygrometer sensor can transmit data to the nearby console - some models include built-in memory to log readings automatically.
It is argued that climate change may be actually beneficial for British bats. Warmer winters means insects are active for longer and bats survive hibernation in greater numbers and better condition. For example, the rare lesser horseshoe bat population in Wales is on the increase; this is attributed to greater survival rates due to milder winters. Conversely the appalling weather we have had in recent summers has been detrimental to some bat species such as pipistrelles. Cool wet weather means less available food for mothers and nursery roosts are too cold, so fewer young are reared successfully.
