ABOVE: Two hair types from Mus musculus showing characteristic banding indicative of an infrared function. The small hairs are so-called zigzag hairs, and the large one is the widest part of a spear-shaped guard hair. R soc open sci, 8:210740, 2021.

In addition to their acute senses of smell, hearing, and sight, small mammals may have an additional method for detecting predators: heat-sensing guard hairs capable of picking up the infrared radiation emitted by warm bodies, a study suggests. The new research, published last week (December 8) in Royal Society Open Science, builds on a body of unpublished evidence collected not by a biologist, but by a physicist, Ian Baker, who develops infrared sensors for a British defense company.

Baker has long brought his work home with him, he tells The New York Times, using infrared cameras containing his sensors to scan the fields and woods near his house in Southampton, England, for animals. for animals. A series of anecdotal observations—such as the fact that cats seem to “stack” their bodies behind their cold nose when hunting, along with a similar twisting behavior in swooping owls—led Baker to hypothesize that perhaps these predators “have to conceal their infrared to be able to catch a mouse,” in effect “hiding their heat.”

Predators such as cats and owls alter their behavior while hunting, perhaps to hide their heat signature from potential prey.
R soc open sci, 8:210740, 2021.

As part of his investigation, Baker put mouse hairs under a microscope, and what he saw looked immediately familiar, he tells the Times. The mouse’s guard hairs—the long, coarse strands that form a protective layer over an animal’s undercoat—looked similar to structures he often saw in his sensors. Specifically, the hairs contained evenly-spaced bands of pigment that, in a sensor, allow the instrument to focus onto specific wavelengths of light.

Measuring the stripes supported the idea that, just as with a thermal camera, the hairs seemingly tuned into the 10-micron wavelength of light, the heat signature given off by many living things. “That was my Eureka moment,” Baker tells the Times

Following up on his finding, Baker documented similar hairs in shrews, squirrels, rabbits, and other species, some with even more complex guard hair structures that suggest “really sophisticated optical filtering.” If such hairs can indeed detect heat, it would mean that many species have a 360-degree sensory shield.

Tim Caro, a professor of evolutionary ecology at the University of Bristol in England who was not involved in the work, tells the Times that the study paints an “intriguing picture.” Helmut Schmitz, a researcher at the University of Bonn in Germany who studies infrared-sensing capabilities insects, calls the findings “plausible.” But both researchers note that further study is needed to link the hairs’ structures to a function. One direction, Schmitz says, would be to confirm that the skin cells at the base of the hairs can in fact sense minute changes in temperature. 

Baker has continued to study behaviors in the animals near his home that might support his hypothesis. He recently developed a pair of “hot eyes” made of sensors that are meant to mimic the eyes of an owl and is using them to study how rats respond to the simulated predator’s gaze, although that work has not been published, according to the Times. In addition, Baker adds that he hopes to see others pick up the thread of his work to study the phenomenon in more detail. “Animals that operate at night have secrets,” he tells the Times. “There must be a huge amount we don’t understand.”