Bat boxes built to shelter the animals can become deadly during heat waves, scientists say, as climate change brings more frequent and intense extreme heat. Conservationists say the answer is to add more boxes with varied designs, not to remove them.
Bats are already under pressure from disease and habitat loss, which is why experts say the boxes still matter. Of the world's nearly 1,400 known bat species, 25 are considered critically endangered, 87 endangered and 110 vulnerable, according to the International Union for Conservation of Nature.
What happened in 2018
At ƛ̓éxətəm Regional Park in Port Coquitlam, B.C., roughly 800 female bats use a group of tall wooden boxes each summer to give birth and raise their young. John Saremba, a bat enthusiast who has helped care for the colony for more than a decade, arrived for a routine check in July 2018 and found 65 dead bats.
"We came out here and I was just devastated," he said.
The colony had survived a heat wave a month earlier. Leah Rensel, a wildlife biology PhD candidate at the University of Montana who was studying the bats at the time, said temperatures inside the boxes had exceeded what she believes was 44 C, which is lethal for the species.
Rensel suspects the bats stayed because the colony was likely giving birth, and the mothers did not want to leave their small pups.
How the boxes heat up
Pregnant and nursing bats seek warm roosts so they can spend energy on birth, nursing and hunting instead of regulating body temperature. Rensel said the ideal range for little brown bats and Yuma myotis bats is between 35 C and 41 C.
Wooden boxes can heat up quickly, particularly when they are painted black and placed in direct sunlight, as the Port Coquitlam boxes were. Stuffy conditions also add humidity.
"When you bring temperature and humidity together, that's heat stress," said Cori Lausen, director of bat conservation with the Wildlife Conservation Society Canada and Rensel's thesis adviser. She noted that bats do not sweat, which makes cooling down harder.
Proposed fixes
Lausen said boxes should be added, not taken away. She recommends placing them less than 100 metres apart so bats can switch roosts to warm up or cool down, with an absolute minimum of three per property. She suggests a mix of sizes, styles, venting and paint colours, which affect how much heat the bats absorb.
She also pointed to "bat condos," structures with different spaces that let bats move between microclimates without leaving the roost, much like an old attic.
Saremba added shading to the old boxes using white blankets and built emergency exits from PVC pipes. Bat deaths stopped after those changes, he said. He has also built a mini condo meant to mimic an attic, with a space for young bats to practise flying.
Bats also suppress insect populations that can damage forests and crops, according to a 2019 paper in the journal Biodiversity and Conservation, and they disperse seeds in tropical and subtropical forests.
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