Unlocking The Mystery Behind The Survival Of Norwegian Bats

This is a guest post by Mari Aas Fjelldal. Original Norwegian text can be found in this Adressa article.

It’s a warm day in July when I knock on the door of a house in Trondheim. A person appears in the doorway and looks at me a bit uncertainly after spotting the huge, black antenna I am holding in my hand.

“Hi,” I say, trying to flash my biggest smile, “My name is Mari. I am a biologist and a bat-researcher at NTNU! I am very sorry to bother you, but I am looking for a bat and I believe it might be living here with you?”

This is how I’ve started many conversations with total strangers these last three summers, while writing my PhD at the Norwegian University of Science and Technology (NTNU). My fellow ecologists and I at the Stawski-lab have been investigating how the small insectivorous bats living in Norway manage to survive through the Norwegian summer with short, almost constantly illuminated nights and periodically awful summer weather.

How do these bats manage to build enough fat reserves to reproduce, give the pups enough milk to grow and then fatten up enough to survive a long and cold winter in hibernation when they only have a few semi-dark hours to hunt each night? And what do they do if they can’t forage for several nights if it rains for days on end?

The questions about how these little animals are adapted to a life in Norway are seemingly never-ending, and to find some answers my colleagues and I begin every summer by setting up tall, soft nets in the forest in the middle of the night. At my very first catch-night three years ago we cheered as a little whiskered bat got caught in the net after about half an hour of waiting. One of my colleagues gently freed the little creature from the net, put it in an airy cotton-bag and handed it to me:

“Here, put it under your sweater.”

I took the bag but stared perplexed back at my colleague. “Huh? Under my sweater? Why?”

“Because if the bat gets too cold it won’t be able to fly when we are going to release it again afterwards.”

My first meeting with Norwegian bats thus became more intimate than I had anticipated, using my own body heat to keep the little furry animal warm enough to fly. Maintaining a high body temperature can be extremely energetically expensive for little animals and can make it hard for them to survive in northern territories. But here we also find some of the answers to how bats in Norway are managing so well: many don’t bother to maintain a high body temperature when it gets cold.

Many people know that bats are animals that hibernate during the winter, but not many know that bats also hibernate both for long and short periods of time throughout the summer. This short-term hibernation can last from half an hour to several days if the weather is poor, and during the hibernation period the bats lower their body temperatures to match the air temperature, reducing the respiration, heartbeat and all physiological processes in the body to minimize the amount of energy they need to use. This is the bats’ ultimate superpower, and one of the reasons why we can find small insectivorous bats on every continent of the world except for Antarctica.

One of my research subjects asks “do my ears look big in this?” (Image Credit: Helene M. Hannestad, CC BY 2.0)

To find out more about the summer hibernation used by Norwegian bats we attach tiny transmitters to them that register their skin temperature before releasing them. In order to collect the body temperature data we need to install recording equipment close to where the bats live. Then the hunt begins, as I start running around with my big, black antenna and try to receive signals from the transmitters.

It turns out that the bats of Trondheim are rather urban, and very choosy with regards to their accommodation. They are residents of some of the city’s nicest neighborhoods with wooden houses and great views. This is where I have to pause and thank the human inhabitants of Trondheim, who have opened their doors to me and my team. Countless locals have listened with puzzled expressions as I’ve tried to explain why I was standing sweaty and nervous on their property with a huge antenna directed at the roof.

Every single houseowner I’ve encountered has shown real interest and great enthusiasm when they have been told that they share their home with a bat and have immediately let me install my research-equipment in the garden or backyard. I have also experienced great curiosity from people on the street who have dared to approach me while I have been swinging the antenna over my head in dense neighborhoods. “Are you working for the TV-license control agency? Are you measuring noise-levels? Are you looking for wolves?” When I explain that I am trying to locate a bat, I have received very helpful observations of bats at family cabins, in neighborhoods or, as a very nice lady could tell me, behind the Buddha-statue in her living room!

The positivity and interest I have met in every single person I have talked to have been completely overwhelming. The people of Trondheim have made our bat fieldwork a dream, and the level of excitement has sometimes surprised me. Bats have suffered from bad reputations after all – especially now during the pandemic. Conducting research on urban animals makes us dependent on being met with understanding by the people we encounter, and because of this the attitude of the people is so very important when we try to collect data. A “no” when I ask permission to install my research-equipment on someone’s private property would have been absolutely legitimate, but it would have resulted in a substantial loss of data, data which we need to try to answer some of the questions we are asking.

And so, thank you all known and unknown bat research assistants in Trondheim, you have all contributed to gathering information on some of Norway’s most mystical and (perhaps not entirely objectively) coolest mammals! The research belongs to the people, and it therefore makes me extra happy and proud knowing that the people has been involved in this data-collection.

The field-season is finished for the summer, but I look forward to more confusing looks when next summer rolls around.


Mari Aas Fjelldal is a behavioural ecologist completing her PhD at the Norwegian University of Science and Technology. You can read more about her research at this link.

Title Image Credit: Mari Aas Fjelldal, CC BY 2.0, Image Cropped

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