Lab mice are often the first testing ground for human drugs, from vaccines to cancer treatments. But an oft-overlooked detail of their accommodations in laboratories is raising concerns about the results of some mouse studies: the mice have a chill.

“It’s still not widely appreciated that the housing temperature that you keep mice at affects the biology and the physiology of the mouse … enough that it can change the [research] outcome,” said Bonnie Hylander, a scientist at the Roswell Park Cancer Institute in Buffalo, N.Y.

The National Academy of Sciences recommends housing mice between 20 and 26 degrees Celsius — about 68 to 79 degrees Fahrenheit. But the natural comfortable temperature for mice is warmer — between 30 and 32 degrees Celsius (86 to 90 degrees Fahrenheit). And colder mice experience more stress than normal, potentially skewing the results of studies on them.

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That was something Hylander realized a few years ago, when the lab in which she was working was studying the impact of temperature on mouse physiology. One of their experiments found that tumors grow more quickly in cold mice than warm mice, and ever since then, Hylander has been looking into the issue.

This week, Hylander and Elizabeth Repasky, the primary investigator of the lab, published an opinion piece reviewing the field in Trends in Cancer.

For instance, one study found that mice genetically modified to develop obesity only gained a lot of weight at warmer temperatures but not at colder temperatures. In a study on inflammation, researchers found that mice at cooler temperature had lower insulin and higher glucose levels. Other research has revealed that temperature can impact how plaque builds up in mice’s arteries and can modify how effective some cancer therapies are.

But because room temperature is not usually reported in scientific studies, it may be invisibly skewing results and contributing to failed replication of published studies. That may, however, be starting to change. Dr. Ajay Chawla, professor of physiology and medicine at the University of California, San Francisco, said that more researchers have been reporting temperatures over the past handful of years.

Meanwhile, the humans working with the mice at those temperatures are quite comfortable, if not a little warm. Brianna Gaskill, an assistant professor of animal welfare at Purdue University, said that while researchers have known for a long time that lab mice are cold, a variety of factors, including researchers’ and technicians’ comfort at those temperatures, have prevented anyone from changing the thermostat.

“You’re working up a sweat,” Gaskill said of the people who work with mice. They have to wear protective clothing, ranging from gloves up to a full body suit with a ventilator.

Bruce O’Hara, a professor of biology at the University of Kentucky, said that institutional inertia also makes it difficult to change the thermostat.

“It is not easy for an investigator to go and say, ‘I want the room warmer,’” O’Hara said. “They can, but it will probably take multiple efforts, multiple times, and more often than not they will give up.”

O’Hara studies the relationship between temperature and sleep. Sometimes he had been successful in changing the temperature of the mice’s rooms. But if multiple researchers’ mice are housed in the same room, then changing the temperature would require the agreement of all the scientists, which might take months to secure.

Despite the ways in which scientists know temperature impacts their results in mice, they don’t know what effect it has on the translation of those results into human trials.

“We really don’t know to what degree — ha ha, no pun intended — this is actually affecting [mice as] experimental models,” Gaskill said.

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