Where do bats go in the winter?

The warmer months of May – September are known as the ‘bat season’ or ‘active season’ for bats. During this time, in Britain, they are widely found roosting in our houses and buildings (as well as in trees, bridges etc.) and can transition from roost to roost on a regular basis, spending the nights out foraging for insects, of which are more abundant during the warmer months. Groups of female bats gather to form maternity roosts, where they give birth to and raise their fully dependent pups, only leaving during the night to forage. This is also the time when bats get busy mating! At the end of September / beginning of October, however, bats start to disappear from sight, and throughout the winter months they often cannot be found in places where they were once abundant. So… where do the bats go?

Lesser Horseshoe bat (Rhinolophus hipposideros). Image copyright © Steve Wadley.

Insect prey becomes scarce during the colder months and, as such, bats across Britain retreat to hibernation sites. At these sites, which often consist of cave walls or ceilings (or similar structures such as cellars/tunnels), they can wait out the harsh winter curled up in a state of ‘torpor’. During torpor bats remain stationary, lowering their body temperature to a level that is usually between 1°C and 12°C, but varies between species, and drastically slowing down their breathing and metabolic activities. This means that the bats need to expend far less energy compared to when they are foraging, mating etc.; activities that use up a lot of energy. During this time they use stored body fat to stay alive. This of course means the bats can survive the winter on minimal food, so the reduced abundance of insect prey in winter does not pose such an obstacle to survival.

Lesser Horseshoe bat in torpor. Image copyright © Steve Wadley

[endif]--It therefore also makes sense that a bat chooses a hibernation site with a microclimate of a constant temperature that is close to the bat’s ideal temperature for torpor, so the bat does not have to expend excess energy warming up its body. Bats therefore hibernate in a wide range of locations based on their specific requirements for torpor. This includes caves, cellars, mines and tunnels as aforementioned, but also in more exposed sites such as in trees e.g. under tree bark. More than one species can be found within the same cave, but utilising different areas, for example Barbastelle bats (Barbastella barbastellus) tend to prefer the cooler cave entrances whereas Horseshoe bats (Rhinolophidae) opt for warmer locations deeper in the cave. ![endif]--

Lesser Horseshoe bat. Image copyright © Steve Wadley

[endif]--It must be noted that bats do not go the entire winter without ‘waking up’ from hibernation; they actually arouse from torpor for a period of time every few days/weeks. Again, this frequency depends on the species, for instance Pipistrelle bats (Pipistrellus) arouse much less frequently than Horseshoe bats (Rhinolophidae). When the bats do arouse, they spend time drinking water and foraging for nearby insects before re-entering a state of torpor. ![endif]--

Lesser Horseshoe bat. Image copyright © Steve Wadley

When spring arrives and the temperatures rise, insect prey starts to become abundant again. Bats start to return back from the hibernation sites that they have adopted for the winter and then the next ‘bat season’ is upon us!

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