A hair-raising hypothesis about rodent hair

It’s tough out there for a mouse. Outdoors, its enemies lurk on all sides: owls above, snakes below, weasels around the bend. Indoors, a mouse may find itself targeted by broom-wielding humans or bored cats.

Mice compensate with sharp senses of sight, hearing and smell. But they may have another set of tools we’ve overlooked. A paper published last week in Royal Society Open Science details striking similarities between the internal structures of certain small mammal and marsupial hairs and those of human-made optical instruments.

In this paper as well as other unpublished experiments, the author, Ian Baker, a physicist who works in private industry, posits that these hairs may act as heat-sensing “infrared antennae” — further cluing the animals into the presence of warm-blooded predators.

Although much more work is necessary to connect the structure of these hairs to this potential function, the study paints an “intriguing picture,” said Tim Caro, a professor of evolutionary ecology at the University of Bristol in England who was not involved.

Baker has spent decades working with thermal imaging cameras, which visualize infrared radiation produced by heat. For his employer, British defense company Leonardo UK Ltd., he researches and designs infrared sensors.

But in his spare time, he often takes the cameras to fields and forests near his home in Southampton, England, to film wildlife. Over the years, he has developed an appreciation for “how comfortable animals are in complete darkness,” he said. That led him to wonder about the extent of their sensory powers.

Observations of predator behavior further piqued his interest. While filming and playing back his videos, he noted how cats stack their bodies behind their faces when they’re hunting. He interprets this, he said, as cats “trying to hide their heat” with their cold noses. He has also observed barn owls twisting as they swoop down, perhaps to shield their warmer parts — legs and wingpits — with cooler ones.

Maybe, he thought, “predators have to conceal their infrared to be able to catch a mouse.”

Eventually, these and other musings led Baker to place mouse hairs under a microscope. As the hairs came into view, he felt a strong sense of familiarity. The guard hair in particular — the bristliest type of mouse hair — contained evenly spaced bands of pigment that, to Baker, closely resembled structures that allow optical sensors to tune into specific wavelengths of light.

Thermal cameras, for instance, focus specifically on 10-micron radiation: the slice of the spectrum that most closely corresponds with heat released by living things. By measuring the stripes, Baker found they were tuned to 10 microns as well — apparently homed in on life’s most common heat signature. “That was my Eureka moment,” he said.

He found the same spacing in the equivalent hairs of a number of other species, including shrews, squirrels, rabbits and a small mousy marsupial called the agile antechinus. The antechinus hair in particular suggested “some really sophisticated optical filtering,” starting with a less sensitive absorber at the top of the hair and ending with patterns at the base that eliminated noise, he said.

As these hairs are distributed evenly around the body, their potential infrared-sensing powers could help a mouse “spot” a cat or owl in any direction, Baker said.

Baker’s hunch that these hairs help small mammals perceive predators is “plausible,” said Helmut Schmitz, a researcher at the University of Bonn in Germany who has investigated infrared-detecting mechanisms in fire beetles. (These beetles use organs in their exoskeletons to sense the radiation, which leads them to the recently burned forests where they lay their eggs.)

But jumping straight from structural properties to a biological function is risky, he said. To show that the hairs serve this purpose, it is necessary to prove that the skin cells they are attached to are able to recognize very small differences in temperature — something that has not been observed, even though these cells have been heavily studied, Schmitz said.

Baker has continued to look into this question, designing his own observational tests. (A recent endeavor involves filming how rats respond to “Hot Eyes,” an infrared emitter he built that mimics the eyes of a barn owl.) As these experiments were not controlled, they were not included in the published paper. But now that he has lit this metaphorical torch, Baker hopes to pass it to others who can look deeper into these anatomical questions and design more rigorous experiments.

“Animals that operate at night have secrets,” he said. “There must be a huge amount we don’t understand.”

This article originally appeared in The New York Times.

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