Bat boxes potential ‘death traps’ for winged mammals during heatwaves

By Stephen Beech

Bat boxes are potential death traps for the winged mammals during heatwaves, new research warns.

They can overheat and die in bat boxes during extreme hot weather, say scientists.

They explained that staying cool during heatwaves is challenging for small creatures, but the problem is even more extreme for nocturnal creatures - such as bats - that are unable to move to cooler locations while slumbering.

Study author Ruvinda de Mel said: "Roosting bats may face lethally high body temperatures during extremely hot days.

"And bat boxes are often designed to retain heat to keep bats cozy, which could place the animals at even greater risk during heatwaves, depending on the box’s position in the sun."

Doctoral candidate de Mel and his colleagues at the University of New England, Australia, wondered how much of a risk poorly positioned bat boxes pose to roosting big brown bats during a heatwave.

Their findings, published in the Journal of Experimental Biology, showed that bat boxes situated in full sun on a hot day could warm to temperatures exceeding 50 degrees Celsius (122 F) while the bats are asleep and unable to move, causing them to die from extreme dehydration.

To verify the risk to sleeping bats, the research team traveled to Lillooet in Canada where they collected 22 bats from the surrounding woods and forests and measured the amount of carbon dioxide the mammals exhaled.

That allowing them to calculate the bats’ metabolic rates – and the amount of water that they lost from their bodies by evaporation at temperatures ranging from 28C to 48C, before returning them to their homes.

Over the same month, the team recorded the temperatures in four man-made roosts.

More from this section

+3

Bat boxes potential ‘death traps’ for winged mammals during heatwaves

+4

Ultra-strong magic material may mean end of potholes

+4

Famous ‘Rat Hole’ of Chicago was actually left by a squirrel

Two of the roosts were in the roof of a building at nearby Kwotlenemo Lake, Canada. One facing east reached 38.5C (101.3F),but the second facing west only reached 32C (89.6F).

But the other two boxes located in direct sunlight in Lillooet both exceeded 40C (104F).

Back in Australia, the researchers used their measurements of the water losses experienced by the warm bats to calculate how much water bats inside the roosts would have lost on the hottest day in 2023.

The temperatures that year didn’t pose a serious threat to the bats, which would have lost between 2.5% and 6.2% of their body mass in water in the Kwotlenemo Lake roosts and 10.8% to 15.3% of their body mass in the relatively exposed bat boxes in Lillooet.

But calculations based on the extreme heatwave of June 2021 showed that, on the hottest day, the temperature in the east-facing Kwotlenemo Lake roost would have rocketed above 50C (122F) for six hours, peaking at 55.5C (132F).

The researchers say that, potentially, the bats could have lost more than 50% of their body mass, which would have killed them.

The bats in the Lillooet boxes would have lost 25.5% and 36.7% of their body mass, which would also have been fatal.

de Mel said: "Inappropriately placed artificial roosts could function as death-traps for bats due to overheating."

He says that conservationists should provide bats with a choice of roosts in the same location, some constructed from insulating materials to maintain a stable temperature while others could cool and warm relatively quickly.

de Mel added: "Some roosts could be located in full sun to warm quickly in early Spring while others are placed in shade for protection during the summer.

"But the message is clear: think hard about where you locate your bat box, or it could become a bat sarcophagus during a heatwave."