How bats survive Norwegian winter nights

Bats hunt at night, navigating in the dark using echolocation to find insects and other food. During the winter, bats in Norway have to manage as best they can by hibernating, but until now, not much has been known about how they manage to do this.

By Idun Haugan - Published 22.02.2024

You have probably seen them flying at dusk. They suddenly appear on summer evenings, when other flying creatures have settled down for the night. However, bats are not a common sight in Norway, because there aren’t that many of them. In addition, bats are not so easy to spot, because they only emerge from their hiding places once darkness descends.

Animals have many characteristics that we can learn from.  One example is echolocation, which bats have used for more than 50 million years to navigate in the dark. Bats emit sounds at ultrasonic frequencies that humans are unable to hear, and navigate by using the echoes they pick up from these sounds. The discovery of ultrasound resulted from studies involving bats.

Found almost everywhere
There are 1439 known species of bats in the world. The smallest weigh just 2 grams, and the largest weigh 1.5 kilograms and have a wingspan of 1.5 metres. Bats are the only mammal that can fly, and some species of free-tailed bats can fly at speeds of up to 100 km per hour.

This diverse species is found almost everywhere in the world and constitutes 20 per cent of all mammal species. Most species of bat are found around the equator. Two areas are particularly rich in bats: the Amazon and the islands between Asia and Australia. The further north and south you go, the fewer species there are.

Throughout history, bats have invoked both awe and fear. The Roman Basilius believed that bats were related to the devil. In Christian art, the devil and his companions are often depicted with bat wings, while angels are portrayed with bird wings.

This is a brown long-eared bat. Its large ears enable it to have extremely good hearing. Video: Rune Sørås

How do they survive the cold winters in Norway?
Seventy per cent of bat species feed on insects, but some species feed mainly on plants, and others eat small animals such as frogs, mice and lizards.

This means that the availability of food varies with the seasons. For species that live in cold regions with long winters (like Norway), there is little access to food for many months. As a result, bats that live in cold regions go into a kind of torpor to save energy.

Little is known about how the species that live in Norway hibernate, nor do we know much about where they go in winter. In many other countries, there are large, natural caves in which thousands of bats take shelter. In Norway, they probably live in rock crevices, but the number of bats that do this is unknown.

Eleven types of bat are found in Norway, all of which belong to the Vespertilionidae family of bats, more commonly known as “evening bats” or “vesper bats”. All 11 species are protected and 6 are red-listed, including the world’s northernmost species, called the northern bat (Eptesicus nilssonii).

Some people associated bats with supernatural properties and bats were therefore often used in amulets. Medicinal recipes used by Arab doctors often contained bat parts. These same parts could also be found in European apothecaries in the Middle Ages.

Hardly any research on Norwegian bats
In his doctoral thesis, biologist Rune Sørås has studied what happens to northern bats when they hibernate.

“I have studied how bats manage to survive in Norway by lowering their body temperature and reducing their energy expenditure when exposed to cold temperatures. Hardly any research has been conducted on bats in Norway, so we have helped create a new phase in bat research. In addition, bats have had fairly low status in nature management, although they have now received a little more attention due to the development of wind turbines,” Sørås says.

Much of the work involved in studying bats is often done by volunteers.

The Norwegian Zoological Society is an organization that is interested in all groups of animals, and gathers information about bats and actively studies them. The Norwegian Zoological Society website provides information about what to do if you find an injured bat (in Norwegian).

Norway is also a member of EuroBats, an organization and system of agreements for European countries that commit to taking care of their bat species.

A brown long-eared bat is easily recognizable when it hovers, due to its large ears.

Photo: Rune Sørås

Leonardo Da Vinci was inspired by the flying skills of bats. Drones are now being developed that mimic bats' efficient and elegant flying technique.

 What role do bats play in an ecosystem?
Bats have existed for over 50 million years and are masters at adapting to the environments they live in. However, in many places, bats are threatened by development  in their habitats and because the insects that most bats live on are disappearing due to pesticides and pollution. Another factor is that the environments where bats live are affected by climate change.

“The reason why it is important to study bats is that they play an important role in ecosystems. For example, they can help reduce damage to food crops because they eat insects that reduce crop yields,” says Sørås.

The fact that bats eat insect pests means that less pesticide is needed. A small bat can eat up to 1000 insects a day, including everything from moths to mosquitoes that can carry infectious diseases such as malaria, dengue fever and yellow fever.

This is called an ecosystem service. Bats also provide other ecosystem services, such as seed dispersal.

“By better understanding how bats function and live, we can better manage and protect them,” says Rune Sørås.

Bat hibernation is demanding
Many of the specimens Sørås studied in his research were captured in Nittedal, Trondheim and Orkland in Trøndelag County. In Orkland, Sørås found approximately 100 Brandt’s bats living in a small log cabin.

The 44 bats he examined were released unharmed after he had measured the metabolism of each individual. Metabolism is the chemical process in the body that converts oxygen, food and other materials into substances that the body needs in order to function.

Sørås has studied the temperatures at which bats start to hibernate, how much energy they use in connection with hibernation and how they use the energy.

“Understanding how bats manage their own energy budget under different climatic conditions is an important component in understanding what limits their prevalence and their ability to withstand environmental changes, such as climate change,” says Sørås.

People outside Europe often had a more positive view of bats. They were important in ancient Central American religions, and the Mayans had a hieroglyph with a symbol for bats.

Rune Sørås with a northern bat. Video: Rune Sørås

Bat hibernation is rather peculiar and quite demanding for the animals. When they hibernate, their breathing slows down, but they do not go to sleep. They need to drink, defecate and perhaps sleep a little on a regular basis. In other words, they alternate between hibernation and necessary activity, but one challenge during hibernation is actually sleep deprivation.

Bats have two types of hibernation, short-term hibernation, called torpor, that lasts less than a day, and long-term hibernation, which lasts for more than a day. Some bats only go into short-term hibernation, while others can do both.

“However, there is a lot we don’t know about what happens when bats are in a state of hibernation, so we wanted to find out more about it,” Sørås said.

Thin individuals hibernate quicker than fat ones
Sørås and his colleagues started out with a theory that bats will not be able to tolerate the temperature changes that climate change is likely to cause.

“We found that thin specimens went into hibernation quicker than those that were fat, and well-fed bats were active longer and waited longer before hibernating. Fatter bats also came out of hibernation earlier than the thinner ones. In other words, the length of hibernation depends on what kind of shape bats are in. Our studies show that bats are more adaptable than we originally assumed. They have a flexible and opportunistic metabolism that enables them to adapt to a wider climate spectrum than we first thought,” says Sørås.

In China and Japan, bats are a symbol of good luck, wealth and long life. (Source for all the fact boxes: Store norske leksikon.)

This is a forest bat. It can live to be 40 years old and is found in large parts of southern Norway.

Photo: Rune Sørås

Flexible bats in the High North
Søras's study provides insight into the physiological flexibility of bats living at high latitudes in the Northern Hemisphere. Thanks to their flexible and adapted use of hibernation and torpor to manage their energy budgets, bats can withstand long periods of limited food supply.

“On a global scale, researchers have largely overlooked bats' energy expenditure. So far, research has been limited to a few species, often focused around small geographical areas. In order to better predict, manage and protect species affected by climate change, more work is therefore needed to understand the physiological and behavioural responses of bats to different environmental conditions,” Sørås says.

Each bat has a distinctive pattern on its wings.

Photo: Rune Sørås

Wing prints
One of the peculiarities of bats that fascinates Rune Sørås the most is that they all have their own unique pattern on the skin of their wings. You could say that the wing prints of bats correspond to human fingerprints. While human fingerprints often have to go through police data analysis to be recognized, the patterns found on bats’ wings are easily visible and recognizable to the naked eye, making it relatively easy to identify individuals.

When Sørås caught specimens for research, many of them were immediately released back into the wild, such as pregnant females.

“There was actually one bat that I caught 8 times, and it was easily recognizable from its wing prints,” says Sørås.

Bat ringing, which is done in quite a few countries, can easily damage the skin of bats’ wings and affect their ability to fly.

“Bat ringing is a major topic of discussion in international bat research,” Sørås says.

This topic will be taken up in an upcoming publication, and Sørås is working on an idea to use artificial intelligence to identify individual bats from their wing prints.

We don't know much about where bats spend the winter in Norway. Here, Rune Sørås checks a rock cave that could be a good location for them to overwinter. Video: Sondre Olav Sivertsen/NTNU


Sørås's PhD dissertation: Energy management of heterothermic bats at northern latitudes: Understanding the physiological flexibility of bats and how this enables them to live in the northern edge of their distribution

Journal of Experimental Biology: High latitude northern bats (Eptesicus nilssonii) reveal adaptations to both high and low ambient temperatures Rune Sørås, Mari Aas Fjelldal, Claus Bech, Jeroen van der Kooij, Katrine Eldegard, Clare Stawski. (2023)

 Environmental Pollution: Species and reproductive status influence element concentrations in bat fur  Kieffer, Luc; Sørås, Rune; Ciesielski, Tomasz Maciej; Stawski, Clare Yvonne. (2023)

Journal of Thermal Biology: Determining the different phases of torpor from skin or body temperature data in heterotherms.Fjelldal, Mari Aas; Stawski, Clare; Sørås, Rune; Wright, Jonathan. (2023)

Interview with Rune Sørås in The Journal of Experimental Biology

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