How bats survive Norwegian winter nights

You have probably seen them flying around 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. 

Bats are also not so easy to spot, as 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 1,439 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 kilometres per hour.

This diverse species is found almost everywhere in the world. It 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.

How do they survive the cold winters in Norway?

70 per cent of bat species feed on insects. 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. We also do not know much about where they go during the 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.

11 types of bat are found in Norway. They all 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. This includes the world’s northernmost species, called the northern bat (Eptesicus nilssonii).

Hardly any research on Norwegian bats

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," he says.

Hardly any research has been conducted on bats in Norway, so a new phase in bat research has been created.  

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

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

 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. The insects that most bats feed 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 1,000 insects, such as moths, in one night. 

It can also eat 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,” the researcher says. 

Bat hibernation is demanding

Many of the specimens Sørås studied in his research were captured in Central and Eastern Norway. In Orkland, Sørås found approximately 100 Brandt’s bats living in a small log cabin.

The 44 bats he examined have been duly released unharmed.

He 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 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 to understand what limits their prevalence and their ability to withstand environmental changes, such as climate change,” he says.

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 bit at regular intervals. In other words, they alternate between hibernation and necessary activity. A 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 says.

Thin bats hibernate quicker than fat ones

Sørås and 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," he says.

He adds that their studies show that bats are more adaptable than researchers have assumed. They have a flexible and opportunistic metabolism that enables them to adapt to a wider climate spectrum than researchers have thought.

Flexible bats in the High North

Søras' 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," he says.

Sørås believes that to better predict, manage, and protect species affected by climate change, more work is needed to understand the physiological and behavioural responses of bats to different environmental conditions.

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 our fingerprints often have to go through police data analysis to be recognised, the patterns found on bats’ wings are easily visible and recognisable to the naked eye.  This makes it relatively easy to identify the 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 recognisable from its wing prints,” says Sørås.

References:

Kieffer et al. Species and reproductive status influence element concentrations in bat fur, Environmental Pollution, vol. 333, 2023. DOI: 10.1016/j.envpol.2023.122092

Fjelldal et al. Determining the different phases of torpor from skin or body temperature data in heterotherms, Journal of Thermal Biology, vol. 111, 2023. DOI: 10.1016/j.jtherbio.2022.103396

Sørås et al. High latitude northern bats (Eptesicus nilssonii) reveal adaptations to both high and low ambient temperatures, Journal of Experimental Biology, 2023. DOI: 10.1242/jeb.245260

Sørås, R. 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, Dissertation at NTNU, 2023.

Interview with Rune Sørås in The Journal of Experimental Biology.

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Read the Norwegian version of this article on forskning.no

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