Just as humans head indoors for warmth in the winter and shade in the summer, tree-dwelling animals use the arboreal equivalent of caves for refuge from temperature extremes.
At a time when climate change is supercharging heatwaves, holes and crevices in trees offer potentially life-saving cool spots for bats, birds and other creatures around the world, according to new research. When peak temperatures cross into heatwave territory, these holes are often more than 4°C cooler than the outside, according to a review of dozens of studies recently published in Forest Ecology and Management.
“Everywhere we looked, trees were great for buffering outside air temperature extremes,” said Joy O’Keefe the paper’s senior author and a professor at the University of Illinois Urbana-Champaign (UIUC).
There are growing reasons for animals to try to escape the heat. In recent years, scientists have documented mass die-offs of various species in the midst of heatwaves. In 2014, a heatwave led to the deaths of some 46,000 flying foxes on a single day in eastern Australia. Four years later, another scorcher claimed 23,000 gray-headed flying foxes in the Australian city of Cairns, a third of the species’ population on the continent.
For years, O’Keefe has studied roosts for bats, chiefly in the eastern United States. The work revealed that natural resting spots often provided more protection from extreme temperatures than artificial ones, such as bat boxes. Did those findings apply elsewhere and for other kinds of animals?
To find out, the team combed through the scientific literature. They found 36 studies from five continents that contained data necessary to compiling a more complete picture, including measurements of maximum temperatures both inside and outside the crevices being studied.
Their analysis revealed that these natural shelters consistently provided a buffer against both high and low temperatures. Daily maximum temperatures were around 2.7°C cooler in the refuges than outside. On hotter days, when temperatures climbed above 30°C, that cooling difference grew to 4.1°C. These abodes also offered protection against cold. Compared to the lowest temperatures outside, the resting places were 2.8°C warmer. On days when the mercury dropped below 10°C, the difference was slightly greater at 3.1°C.
The beneficiaries weren’t confined to bats. Among the various studies were places occupied by birds, insects, bats and other mammals such as squirrels.
The new findings offer yet another reason for keeping trees standing, especially ancient ones that tend to be more riddled with gaps and gashes where critters can find shelter.
“Forest managers should focus on protecting and regenerating natural habitats for animals, especially in areas where you get heat waves that can result in mass mortality events,” said co-author Reed Crawford postdoctoral researcher at UIUC.
It’s also clear that today’s artificial replacements for these trees, such as boxes, don’t offer the same benefits. If people are going to use them, the results suggest a need to rethink their design to better mimic the thermal performance that nature offers.
“We know bat boxes almost always exceed outside air temperature, and then they don’t retain that heat at night,” said Crawford. “If people are compelled to use artificial roosts in their conservation plans, we need to improve their design to better replicate what’s happening in nature and try to keep those animals safe.”
Cotten, et. al. “A global review of the capacity of tree cavities to buffer temperature extremes.” Forest Ecology and Management. Aug. 25, 2026.
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