Small mammals, such as shrews and rodents, have a unique protective fur that includes specialized “guard hairs.” These hairs, which make up only 1-3% of the fur, have a distinctive banding pattern and protrude from the rest of the coat. Researchers have discovered that these guard hairs act as finely tuned infrared sensors, allowing small mammals to detect warm heat sources, like predators, approaching from behind.
The periodic banding patterns in the guard hairs align with the wavelengths of infrared radiation, suggesting an unrecognized function as infrared sensors. This ability would provide a significant survival advantage for nocturnal and heavily predated animals, as it would allow them to detect predators without the need for eyes on the back of their heads or constantly looking over their shoulders.
Researchers focused their study on three species: the house mouse, a mouse-like marsupial, and the common shrew. Despite the differences among the species, the guard hairs share similar microscopic structures that are tuned to wavelengths between 8-12 um, which is perfect for thermal imaging.
The structure of a guard hair from the house mouse has a sophisticated design. The wider sections of the guard hair, referred to as the “shield,” act as infrared absorbers and have a structure similar to a Fiber Bragg Grating (FBG). The hairs have a membrane that separates two tubes with air cavities spaced at periodic intervals of approximately 10 um. Towards the base of the shield area, the hair narrows, and the hair features the characteristic dark bands with similar spacing. It’s believed the narrower sections help focus absorbed infrared energy into the base of the hair.
The “zipper” section of the hair acts as a “spectral filter” that radiates away wavelengths outside the 8-12 um band. Calculations suggest that the zipper filter means that infrared energy in that critical wavelength range makes up 72% of the “signal” reaching the base of the hair, rather than just 33% otherwise.
At the base of the hair, the animal has Merkel cells, which are believed to be responsible for the actual infrared sense, with the hairs themselves acting as antennas to focus the infrared energy to them.
This research could have implications in the field of photonics, as the guard hairs’ ability to act as finely tuned infrared antennas could potentially inspire new optical devices or improve current technologies. Additionally, this research could provide new insights into the ancient origins of mammalian and marsupial hair, as the guard hairs’ resilience over millions of years suggests they played a crucial role in early mammalian survival, possibly dating back to the Triassic period.
In conclusion, small mammals have a hidden thermal sense that allows them to detect predators approaching from behind. This ability could have evolved as a means of survival for nocturnal and heavily predated animals, and it could inspire new technologies in the field of photonics.
