Why Are Hibernacula so important?
Over winter, many of our native mammals ‘power down’ and rest to conserve energy. For some, this is a reduction in activity and tendency towards staying in their shelters. For bats and hedgehogs (and dormice in the south of England) this is a huge physiological change, resulting in a dramatically reduced metabolism, with decreased body temperature, breathing and heart rates. This change is known as hibernation, and is a distinct adaptation to conserve energy.
How Does Hibernation Work?
Hibernation is used to save energy over winter. Bats reduce their body temperature to between 2-8°C, slowing their heart rate to as little as 10 beats per minute and breathing once per hour. Hibernation is not a continuous state, and bats may wake up, move location, or even hunt day flying insects if the weather is warmer. Waking up is energetically expensive, and being disturbed can dramatically reduce a bat’s ability to survive through hibernation.
The conditions inside the hibernacula - a roost used specifically for hibernation - are critically important. They need a steady temperature between 2-8°C, high humidity to minimise water loss, and safety from predators. It can take up to half an hour for bats to rouse from their hibernation state, so they need to be comfortable and safe. Each species has slightly different preferences…
Noctules (Nyctalus noctula) and Leisler’s Bats (N. leisleri) tend to hibernate in deep tree holes.
Pipistrelles (Pipistrellus sp.) often use crevices in buildings, cliffs or under loose tree bark which may seem exposed, but contain a suitable microclimate.
Underground hibernacula like caves, mines and tunnels tend to be used by Myotis species - Daubenton’s (M. daubentonii), Natterer’s (M. nattereri), Whiskered (M. mystacinus) and Brandt’s (M. brandtii) - and Brown Long-eared Bats (Plecotus auritus).
The Scottish Climate
The females of most Scottish bat species will have one pup a year (Noctules occasionally have twins). Pups are born in June and have to grow and learn to forage at an astonishing rate. Within just three months they need to be flying well enough to hunt not just to survive today, but to lay down enough fat reserves for their first hibernation. Many don’t survive their first winter.
Adults don’t have it much better. Females start the new season pregnant, give birth, and then nurse their hungry pups. Some species will regularly move roosts carrying their youngsters which is a feat in itself! Males have an easier summer, but in the late autumn will expend lots of energy competing for and mating with females - a large energetic ask when food is beginning to become scarce.
Our short, temperate summer means bats have a short window to meet their energy needs for the winter. The right conditions in their chosen hibernacula are essential for making sure their quickly gathered reserves are enough to see them through to spring.
The entrance to a preserved underground hibernation site in Midlothian.
Underground Hibernation Sites
Bats in Scottish, underground hibernacula are usually found on their own or in small groups. When skilled and licensed bat professionals survey hibernacula it’s rarely possible to fully assess the full hibernating population. We will see the bats roosting on walls or shallow crevices, but there are many more in deeper areas.
Add in the lack of safe access to areas of many hibernacula, and surveys need to be standardised to ensure representative information is collected each year. When we support survey efforts we take care to:
Have the same number of surveyors
With similar mixes of experience
Taking the same route through the hibernaculum, and
Taking the same amount of time to complete the survey.
A long, hard survey may result in just a few bats being recorded, but across the entire country we can monitor year-on-year data for trends. This let’s us monitor how key hibernacula are being impacted by other changes and pressures on our bat populations.
PLEASE NOTE: Great care is taken to ensure disturbance of bats is kept to and absolute minimum during hibernaculum surveys. Surveys are carried out twice each winter, with several weeks gap between them. Noise is kept low, and bats are never touched or handled. We will even take care to not stand below hibernating bats or breath on them to ensure surveyor’s body temperatures don’t impact them.
This article is adapted from David’s Bat Blog, which was first published in April 2015 edition of Recorder News by BRISC - Biological Recording in Scotland.
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