If you ever watched Predator, you’ve noticed the tactical advantage his heat vision gives the alien warrior. Despite the otherwise solid camouflage, Dutch and his squad ended up being heavily pursued.
And yet, it actually seems that the prey could be one that has the ability to sense in the infrared spectrum. Research has now revealed that the hairs on the backs of some of the smallest mammals are responsible for this unique ability.
Not By My Hair Backy-Back-Back
Small mammals such as shrews and rodents have fur that combines several types of fur into a dense protective mantle. The coat must keep the animal warm, relatively dry and protect it from the elements. But what if it can also protect against predators? For among this fur lie special “guard hairs,” which scientists now believe function as finely tuned infrared sensors.
Guard hairs make up only about 1-3% of the coat. They usually stick straight out, protruding somewhat from the rest of the coat. They also tend to feature a fairly distinct striped pattern not seen on other types of mammalian hair.
The regular banding observed in the hair of small mammals has puzzled scientists for years. These hairs exhibit internal striations spaced at 6-12 micrometer intervals. If you’ve been staring at a map of the electromagnetic spectrum recently, you might notice that it closely matches infrared wavelengths. This indicated an unknown function – perhaps the ability to perceive infrared light. Indeed, these wavelengths cover the same part of the infrared spectrum used by heat-seeking missiles and thermal imaging devices.
This was a trait with clear survival advantages in nocturnal and heavily predated animals. It could give the tiny creature the ability to detect warm heat sources — like predators — approaching from behind. There is no need to keep your eyes in the back of your head, or to always look over your shoulder. If your thermal sense detects something warm approaching from behind, it might be worth taking action.
The researchers focused their research on hairs on three species: Mus musculus, the house mouse Antechinus agilis, mouse-like marsupial a Sorex araneus, common shrew. Despite the many differences between species, guard hairs share some similarities. Their findings showed that despite evolutionary differences spanning millions upon millions of years, these species share very similar microscopic hair structures that appear to be tuned to wavelengths between 8-12 µm – ideal for thermal imaging.
Taking the house mouse as a prime example, hairs have a sophisticated structure. The wider sections of the guard hairs, referred to as the “shield”, are thought to act as infrared absorbers. They practically have two tubes connected by a membrane with air cavities spaced at periodic intervals of about 10 µm. Towards the base of the shield area, the coat tapers and instead of air cavities, the hairs are characterized by similarly spaced dark bands. The narrower sections are believed to help focus the absorbed infrared energy into the root of the hair. This is followed by a rather variable “zipper” part with dark hemispheres arranged around the axis of the hair. It is believed to act as a “spectral filter” that emits wavelengths outside the 8-12 µm band. Calculations suggest that the zip filter means that infrared energy in this critical wavelength range makes up 72% of the “signal” that reaches the root of the hair, rather than just 33% otherwise. The last part of the shaft has a finer banding at a distance of only 6 µm.
The hairs are thought to act as infrared antennae – their rigid, straight alignment and periodic striations allow them to function as heat detectors. The bands themselves appear to be made of infrared-transparent biological material with varying refractive indices. Scientists liken it to a man-made invention called a Bragg grating, or FBG. This device uses periodic changes in the refractive index of an optical fiber to create a filter for a particular wavelength. Guard hairs could use a biological version of the same mechanism to filter out the desired infrared wavelength.
Meanwhile, to pick up the signal, the animal needed some kind of hair-based sensor cell. Indeed, scientists have discovered that the house mouse actually has Merkel cells uniquely located at the base of guard hairs, arranged around the shaft. It is thought that these cells may be responsible for the true infrared sense, with the hair itself merely acting as antennae to focus infrared energy onto it.
The researchers also branched out and looked at whether predators might have adapted to it. Remarkably, they found that cold-blooded snakes are virtually invisible in the thermal infrared region. Similarly, cats have relatively low thermal emissions from a frontal perspective. Both would thus have an advantage when hunting against mammals with a thermal sensing defense mechanism. And indeed, these creatures are particularly adept at hunting mice and other small mammals, as you’d expect!
The research is still in its early stages. Work remains to confirm the true purpose of these protective hairs. Regardless, their intricate microstructures provide compelling evidence that they do indeed function as antennas to capture infrared radiation for sensory purposes.
The discovery of these natural infrared sensors is not just a biological curiosity. It could also serve useful in the field of photonics. The ability of hairs to act as finely tuned infrared antennas could potentially inspire new optical devices or improve current technologies.
In addition, this research could impact evolutionary biology and provide new insights into the ancient origins of mammalian and marsupial hair. The durability of guard hairs over millions of years suggests that they played a key role in the early survival of mammals, probably dating back to the Triassic period.
In any case, certain small mammals have always been wildly capable of escaping predators sneaking up from behind. Now we may have a sneak peek at this little party trick. Maybe it was never their eyes or keen sense of vibration, but a hidden thermal sense that had been hiding in the fur all along.
Featured image: “Lab mouse mg 3263” by [Rama]