Image Credit: Jasja Dekker, CC BY-SA 2.0, Image Rotated and Cropped

State dependence of arousal from torpor in brown long-eared bats (Plecotus auritus) (2022) Soras et al., Journal of Comparative Physiology B, https://doi.org/10.1007/s00360-022-01451-8

The Crux

When an animal is facing a lack of prey, or the weather is making it too difficult for them to keep on keeping on, they might choose to enter a state known as torpor. This occurs when the animal lowers its metabolic rate drastically, sometimes to less that 1% of its normal rate. It’s not a perfect solution though, as the costs of torpor include sleep deprivation and memory loss. Nevertheless, it’s a go-to for many small mammals, since they’re warmed up much more quickly than larger ones, and can snap out of torpor when they need to.

It might sound like this is cold-weather behaviour, but it can also occur in summer. Especially if you’re a nocturnal mammal living in part of the world where nights can be very short, or even non-existent, like Scandinavia. Long days means reduced hunting times, so using torpor might be necessary to get through summers as well as winters! This week’s researchers wanted to better understand how small bats survive in northern Norway by looking at how and when they awake from torpor.

What They Did

Long eared bats (Plecotus auritus) were caught in mist nets (thin, nearly invisible nets that bads and birds harmlessly fly into) in central Norway. They were measured for their initial weight, which was used to estimate their energy reserves going forward. 22 of these bats were brought back to cages at the team’s lab. These cages were set up to mimic normal light and temperature regimes.

The bats were later fed and subsequently placed in specialised chambers which could both control the ambient temperature, and monitor the bats’ metabolic rates through measuring the volume of oxygen consumed within the chamber. They then induced torpor by lowering the temperature, and monitored the metabolic rate and the temperature as the bats went into torpor, and as they rose from it when the temperature went back up.

Did You Know: Finding Fuzzballs in the Field

This is a lab experiment, but monitoring bat temperatures in the field is also possible thanks to the size of the transmitters the researchers can use these days. Unfortunately many bats have to hang around urban centres these days, which often means popping into citizens houses (with their permission of course). You can read more about Mari Fjelldal (second author on this paper) and her experiences looking for bats in people’s houses in the article linked below.

Read More: Unlocking The Mystery Behind The Survival Of Norwegian Bats

What They Found

Early on, it was evident that there were two distinct groups of bats. The first were bats that rose from torpor at much lower temperatures (around 22 degrees), at the lower end of the thermoneutral zone. This is basically the range at which an animal like a bat doesn’t have to do too much work to maintain a constant body temperature. The second group arose from torpor only then the temperature was turned up a bit more, around 31 degrees. This second group were generally on the smaller side (meaning they had less energy reserves going into the experiment).

Body mass had an effect on plenty of other results as well, one being that larger bats took longer to go into torpor. The metabolic rate of the bats tended to decrease as the chamber’s temperature decreased, except for when the temperature dropped below five degrees, at which point the bats’ metabolic rates actually increased. This is likely due to the bats trying to actively thermoregulate in order to bring their temperature up again.

Problems

The sample size here is fairly small, with only a total of 22 bats in use. However the patterns shown were pretty strong, so there were clearly enough bats to show some obvious trends in their responses to temperature!

So What?

The fact that better fed bats both delayed going into torpor, and rose from it much more quickly, indicates that it’s obviously not the first choice for these furballs. They are potentially trying to avoid those costs that were mentioned up top, like sleep deprivation and even a larger risk of being found by predators.

Yet the ability of the bats to enter torpor in both summer and winter is potentially a lifesaver, and even a species-saver, as getting through tough summer months on top of the winter seems to only be possible through the use of torpor.

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Dr. Sam Perrin is a freshwater ecologist who completed his PhD at the Norwegian University of Science and Technology who would love a bit of torpor right now, what with the only week of Norwegian summer having just ended. You can read more about his research and the rest of the Ecology for the Masses writers here, see more of his work at Ecology for the Masses here, or follow him on Twitter here.