“Infrared Antenna-Like Structures in Mammalian Fur”, Ian M. Baker2021-12-08 (, ; similar)⁠:

Many small animals, including shrews, most rodents and some marsupials, have fur composed of at least 4 types of hair, all with distinctive and complex anatomy. A ubiquitous and unexplained feature is periodic, internal banding with spacing in the 6–12 µm range that hints at an underlying infrared function.

One bristle-like form, called guard hair, has the correct shape and internal periodic patterns to function as an infrared antenna. Optical analysis of guard hair from a wide range of species shows precise tuning to the optimum wavelength for thermal imaging.

For heavily predated, nocturnal animals the ability to sense local infrared sources has a clear survival advantage. The tuned antennae, spectral filters and waveguides present in guard hair, all operating at a scale similar to the infrared wavelength, could be a rich source of bio-inspiration in the field of photonics. The tools developed in this work may enable us to understand the other hair types and their evolution.

…Potential counter-adaptations in predators of small rodents: If the infrared sensor interpretation of guard hair in small rodents is valid, there should be counter-adaptations in their common predators. A Leonardo Merlin camera was used to look for evidence of reduced infrared brightness or infrared concealment adaptations. Although only indicative at this stage there are some striking observations.

Most warm-blooded animals appear very bright in thermal cameras but snakes, small cats and owls appear to be exceptions. Snakes in vegetation are virtually invisible in the thermal infrared, even during movement, so they are very effective ambush predators. The domestic cat, when hunting, has very weak infrared emission from the cold nose region and suppressed emission in general (Figure 7a). In the stalking pose, cats project the cold nose forward effectively compensating thermal emission from the eyes (which are in any case squinted).

Figure 7: (a) Is a typical thermal image of a domestic cat with a characteristic cold nose that is projected towards the prey during the stalking pose; (b) illustrates the cold centre of the facial disc of a barn owl, Tyto alba, and low emission from the feathered areas in general; (c) is a late frame from a video showing a barn owl diving onto a mouse with suppressed infrared emission in the direction of the prey.
Figure 7: (a) Is a typical thermal image of a domestic cat with a characteristic cold nose that is projected towards the prey during the stalking pose;
(b) illustrates the cold centre of the facial disc of a barn owl (Tyto alba), and low emission from the feathered areas in general;
(c) is a late frame from a video showing a barn owl diving onto a mouse with suppressed infrared emission in the direction of the prey.