Five big brown bats huddling together in a cave (Ontario, Canada). Photo credit: Willis Bat Lab.

Five big brown bats huddling together in a cave (Ontario, Canada). Photo credit: Willis Bat Lab.

White-nose syndrome is a scourge of North American bats. Sweeping across the continent, the fungal disease has decimated vulnerable species by forcing victims to emerge briefly from hibernation – a process known as arousal – too frequently, exhausting their precious energy reserves with fatal consequences. One idea suggests that infected bats interrupt their hibernation more often to drink, as they probably lose more water through damage caused by the infection to their skin. But one species seems to be less vulnerable to white-nose syndrome, big brown bats (Eptesicus fuscus), which are capable of hibernating in drier locations than other species, possibly keeping the fungus at bay. But how do big brown bats evade dehydration when hibernating in relatively dry locations? While a graduate student with Craig Willis at the University of Winnipeg, Canada, Kristina Muise, and colleagues, wondered whether huddling together could offer hibernating big brown bats protection from dehydration.

In October 2018, Yvonne Dzal drove the 20-strong female bat colony in individual bags from Erin Gillam's lab in North Dakota State University, USA, to the University of Winnipeg. The team then settled the bats in groups of 10 in two cool damp incubators (8°C, 98% relative humidity), equipped with a frequently refreshed bowl of water and infra-red cameras to record the animals’ activity. They also fitted tiny electronic thermometers between the bats’ shoulder blades, to record their body temperature; the animal's skin temperature falls to ∼10°C while they are hibernating but rises to 20°C and higher when the animals are active, revealing what the bats are up to. Then, the scientists turned the humidity down to 50% in one incubator, to create a dry environment, before allowing all of the bats to settle down and hibernate for 3.5 months.

Scrutinising the movies of the mammals’ lengthy winter hibernations, Muise could see the bats huddled together in one corner of each incubator, with individual animals emerging periodically from hibernation, sometimes visiting the water bowl to drink. However, when she compared the length of time between arousals from hibernation, she found the drier conditions did not force the bats to arouse more often. Thirst didn't seem to be driving the bats to emerge from hibernation more frequently.

But when she analysed whether the bats took the opportunity to visit the water bowl during their arousals from hibernation, the bats in the drier incubator drank 52% more often than the bats in the humid incubator, with one dry-incubator bat visiting the bowl 28 times during the 3.5 month period, in contrast to the humid-incubator bats, which only visited 12 times at most. The bats in the dry incubator needed to drink more than the bats in the humid incubator, but they were not thirsty enough to arouse more frequently to slake their thirst. So how were the bats in the dry incubator able to prevent themselves from running low on water?

This time, Muise recorded how tightly the bats cuddled together while hibernating and realised that the bats in the drier incubator formed a more compact huddle than the bats in the humid incubator. ‘This behaviour may have allowed bats in the dry treatment to reduce their surface area exposed to the dehydrating environment and allowed them to tolerate drier conditions than other species’, says Muise.

Big brown bats are able to adjust how they huddle to prevent themselves from becoming dehydrated when hibernating in drier environments, which may protect them from the white-nose syndrome fungus that flourishes in more humid hibernation locations.

Muise
,
K. A.
,
Dzal
,
Y.
,
Fletcher
,
Q. E.
and
Willis
,
C. K. R.
(
2024
).
Hibernating female big brown bats (Eptesicus fuscus) adjust huddling and drinking behaviour, but not arousal frequency, in response to low humidity
.
J. Exp. Biol
.
227
,
jeb246699
.