How Hibernating Squirrels Retain Sense of Touch in the Cold

If you forget your gloves on a chilly winter’s day, your fingers soon become numb and stiff. For most mammals, cold suppresses neural activity and diminishes the sense of touch.

But the thirteen-lined ground squirrel can detect touch in conditions as low as 4 degrees Celsius, or about as cold as the inside of a refrigerator, finds a new Yale study published in Current Biology.

For many mammals, including humans, prolonged exposure to the cold can permanently damage nerves. But a team of Yale School of Medicine researchers found that ground squirrel mechanosensory neurons—the nerve cells that convert touch sensation into electrical signals—are adapted to survive in the low temperatures and retain their signaling capabilities.

“This was the first time we’ve demonstrated that any mammalian species can detect touch at this low of a temperature,” says Elena Gracheva, PhD, professor of neuroscience and of cellular and molecular physiology and the study’s co-principal investigator.

Gracheva and Slav Bagriantsev, PhD, professor of cellular and molecular physiology, have been investigating how sensory systems differ across species. In 2017, they discovered that thirteen-lined ground squirrels, as well as some other hibernating animals, don’t perceive cold in the same way as other mammals. This is due to an adaptation in which the nerve cells that detect cold are less sensitive to low temperatures.

In the latest study, the researchers turned their attention from cold receptors to mechanoreceptors. These specialized nerve cells have long protrusions that penetrate the skin; if something comes in contact with those protrusions, they generate an electrical signal that informs the brain of a physical contact.

The team investigated whether mechanosensory neurons found in thirteen-lined ground squirrels had adaptations that enabled them to operate in the cold. After incubating the neurons at 10 degrees Celsius, the scientists stimulated them and measured the electrical signals the cells produced in response.

Exposure to this temperature would typically permanently damage or kill mechanosensory neurons from other mammals. But the researchers observed that the nerve cells from squirrels produced a robust response to the stimulation.

Based on these findings, the researchers predicted that hibernating squirrels would respond to touch even at low body temperatures. To test this, they gently touched animals hibernating at 4 degrees Celsius with a paint brush. The stimulation was enough to awaken each one.

“Even though they are completely out, they still sense touch so acutely that one single poke triggered arousal,” says Bagriantsev, the study’s co-principal investigator.

The researchers propose that this adaptation might have evolved in thirteen-lined ground squirrels to alert them of potential dangers while hibernating, such as a nearby predator. “They don’t feel cold, but they maintain this sensory modality that is essential for species survival,” Gracheva says.

Studying the different adaptations of hibernating species that enable their long winter slumber also has direct clinical applications, the researchers say. Certain clinical procedures require placing patients in a hibernation-like state. Heart surgeries, for example, can involve cooling down the body to induce a temporary cardiac arrest as it is more challenging to operate on a beating heart. But these procedures themselves pose their own risks.

Understanding examples of how animals naturally achieve this physiological state could help researchers learn how to make these procedures safer for patients.

The researchers are especially interested in adaptations related to touch because it is the most important sense we have. It seems infeasible to imagine a world in which we lack this sense, they say, because it is the only one that is impossible to live without.

“We tend to think of mammals as having a sense of touch that only operates in a very, very narrow window of temperatures,” Bagriantsev says. “But in fact, our findings exemplify the breadth of plasticity and adaptability of the mammalian mechanosensory system.”