The infrared world of mammals and birds – an infrared engineers view
“It will take many years and the contribution of specialists from many disciplines to fully understand the infrared world of animals. This website is just a start, using infrared science to launch the debate on the many infrared adaptations in nature. “
Visible and infrared adaptations
Nature abounds in adaptations to manipulate light. Many insects have iridescence as a result of complicated 3D sculpturing in the exoskeleton or wings. The rods and cones of mammal retina are tuned antennas to focus and concentrate the electromagnetic field to enhance chemical stimulation. In all cases the dimensions match or are slightly less than the wavelength of visible light. The graph below shows the solar spectra which originates in the chromosphere an outer layer of the sun that averages about 6000 degrees. Strangely the sun is a near-perfect blackbody and at this temperature the radiation peaks at 0.5 micron which we perceive as green. Most light adaptations, such as the carapace of this iridescent beetle by Adrian Thysse, are in the range 0.3 to 0.5 microns. The bottom-left scanning electron microscope photograph, by Shinya Yoshioka of Osaka University, shows the complicated topology of a morph butterfly wing with features at 0.3 microns. Centre-left shows a photograph by Scott Mittman and David R. Copenhagen of rods and cones that are typically 0.3 microns in diameter. At normal ambient temperature blackbody radiation is shifted well into the infrared with a peak at 10 micron. By analogy to the visible we expect infrared adaptations to be in the range 7 to 10 microns. The hair anatomy of the mouse is a good example of infrared engineering.
Known infrared adaptations in nature
We need to distinguish between near-infrared and thermal infrared. Mammal vision is mainly in the wavelength range 0.42 to 0.65 microns. Nocturnal animals are able to sense much lower light levels than humans but in the pitch dark they are blind. Thermal infrared is the natural radiation produced by all objects around us as the result of a continuous interchange of energy between vibrating atoms and vibrating electro-magnetic waves. At normal temperatures the radiation has a broad spectrum between 3 and over 100 microns and peaks around 10 microns. Water vapour in the atmosphere absorbs much of the infrared spectrum but there is a window between 8 and 14 microns where the most energy is concentrated.
Small mammals, such as rodents, can be very active and bright in the infrared so it is not surprising that some snakes, such as: pit vipers, boas and pythons, have heat sensing organs for exploiting this. Snakes have enervated membranes within a pit (red arrows) as shown in these images for a python and pit viper (photo citation needed). The sensors are bolometers, so that they respond to a change in temperature, exactly how modern uncooled infrared cameras function. Heat sensing is known in many other animals, especially insects, with the most well verified example: the terminal guidance of a mosquito.
Mammals have been evolving as night specialists for over 200 million years and the dark-adapted vision of nocturnal animals is celebrated (a barn owl sees a full-moon night of 0.25 lux as room lighting of 200 lux due to eye anatomy alone). However there should be overwhelming evolutionary pressure for thermal sensing to detect the heat signature of potential prey or to detect the approach of predators. The laws of physics do not prevent warm-blooded animals from having infrared sensors that can detect heat from other warm blooded animals.
Infrared sensors in the fur of small mammals
The larger hair type in the mouse hair image below is called a guard hair. Most small mammals have guard hair and each hair has a characteristic distal, spear-shaped structure known as a shield. The regular spacing of air cavities within the shield is consistent across a wide range of species and is tightly tuned to a wavelength of 10 microns, the optimum wavelength for thermal imaging. We can interpret guard hair as a probable infrared sensor or at least the antenna part of an infrared sensor.
In fast breeding animals such as mice and rabbits the sensors are very sophisticated and have optical filtering in the hair shaft to tighten up the angular resolution. This is a rich area for bio-inspiration. The sensors comprise 1-3% of the hairs and protrude from the fur to provide a 360 degree infrared threat warning system. The sensors only need a sensitivity of a degree centigrade or so to detect the infrared emission from predators. So we know that in the Rodentia order the following families have species with sensors: New World rats, mice, rats and voles (Cricetidae with 580 species), Old World rats, mice and gerbils (Muridae with 710 species), squirrels and chipmunks (Sciuridae with 307 species), pocket gophers (Geomyidae with 35 species), Kangaroo rats (Heteromyidae with 59 species) and chinchillas (Chinchilllidae). In the order Insectivora, shrews have sensors (family Soricidae with 385 species) and in the order Lagomorpha there are 96 species of rabbits and pikas. Potentially over 2180 species could have infrared sensors, although this conclusion is based on only a dozen or so records. They are not found in predators, bats and tellingly moles and dormice that have few predators.
The details are described in the Infrared sensors in mammals and predator adaptations.
Top predators have reacted to the threat warning systems of prey by lowered their thermal presence. A common example is the domestic cat which has suppressed infrared emission from its whole body and counteracts the high emission from the eye area by very low emission from the cold nose. In the stalking pose it flattens its head and projects its nose forward to defeat the resolution of threat warning sensors. Another example is the barn owl which has very low thermal emission from its feathered areas except the armpits. The thickly haired region above the beak serves an analogous function to the cats nose and has lower emission than the background. During an attack the facial disc seems to cool, the eyes squint and the feet are projected forward but have no emission. The hot armpits are concealed behind the body. The infrared disguise is so effective that along with silent flight the target prey would find it difficult to detect an attack.
The details are described in the Infrared sensors in mammals and predator adaptations.
First clues for infrared thermoregulation in mammals
Thermal images of small active mammals, such as bats, can prove challenging to explain in a physics sense. Formally it was thought bats shed heat through the wings but recent images show that the wings and tail membranes are cool. This is possibly an adaptation to prevent freezing on a cold night. The fur of bats is very thick for the same reason and it is easy to show it is an impenetrable shield for visible and infrared radiation. A small 10 gram bat generates about a Watt of heat and without any obvious cooling mechanism should rapidly overheat especially on a hot tropical night. The only clue we have is the bright fur of bats in the infrared, as shown in the authors image below of a brown long-eared bat. Other small mammals often appear very bright and almost glowing in the thermal infrared. Thermal radiation is bound by strong laws and small animals cannot radiate enough energy to make a difference. The strange infrared characteristics of the fur of small mammals focused attention on the hair anatomy.
Complete video in Merlin videos
The evidence for infrared thermoregulation
Thermal images of animals highlight that we are lacking a thermoregulation mechanism in small, energetic animals.
Almost all small, furry, energetic mammals (possibly over 3400 species) have hair with dark bands. The dark bands have a higher refractive index and these structures are well understood in the field of fibre optics and are known as Fibre Bragg Gratings, FBGs. They are used to manipulate light within a fibre. By analogy we can propose that mammal hair is for manipulating infrared within the hair shaft.
Below is a scanning electron microscope photo of coronal hair found in the fossilised stomach contents of bat Palaeochiropteryx tupaiodon (presumably due to grooming). The fossil was found in the Messel pit near Darmstadt in Germany and dated at 49 million years. It shows that infrared fur anatomy was already well developed in ancestral bats. The adaptation has been inherited and sustained in many species which hints that it is part of a vital thermo-regulation mechanism.
Photo by E Haupt (Brussels Museum of Natural History)
The internal bands have dimensions in the infrared wavelength range, typically 7.3 microns for small mammals, rising to 9.0 microns in larger mammals (Hausman 1920). Bats have external features with a typical scale of 11.3 microns (Sessions, 2014). By measuring the variation of the banding along the hair we can identify patterns that suggest these are radiating antenna. Rabbits and moles are distantly related but the band spacing oscillates and chirps to exactly the same template, suggesting that these hairs have evolved as part of a common infrared cooling mechanism. The house mouse, Mus musculus, and common shrew, Sorex araneus, have a zigzag hair forms that are very complex comprising an apparent transfer section leading to a radiating awn (tapered air-filled cylinders with internal gratings). It is possible to ascribe a far-infrared wavelength of 16 microns to the antennas.
There is no known source for 16 micron infrared radiation and crucially we do not expect that the hair shaft will be transparent. However there are intriguing pieces of evidence supporting a source within the follicle. Images of hair follicles in small mammals “look” optical with optical structures surrounding the dermal papilla. Melanosomes are laid down at the end of the growth to protect the papilla from UV indicating a continuing function. The intertwining arteries and veins servicing the hair follicles is evidence that these hairs act as cooling agents. Some thermal images of animals show stripey patterns and this could be zonal moulting. The African mole rat is the only small mammal with no hair and it cannot thermoregulate.
In heavily furred animals there is a need for a highly controllable cooling mechanism and analysis of the infrared engineering in underfur strongly suggests that this is an infrared radiating antenna. The complementary part of the model is the unknown infrared source. There is enough evidence here to justify measuring the infrared emission spectrum of energetic animals such as bats.
Hints of more widespread heat sensing than just snakes
The house mouse has an extraordinary adaptation to reduce its infrared brightness. There is a risk that radiating at a specific wavelength will allow predators, such as snakes to tune to the wavelength. The mouse has a diffraction grating at the end of each hair that radiates the infrared back into the hair mass. There it is absorbed and re-radiated at broadband thermal radiation which effectively reduces its brightness by two orders of magnitude. The number of snakes known to have infrared organs is small and these snakes do not overlap in distribution to the house mouse. For such a sophisticated structure to have evolved implies constant predator pressure over a long period of time. It is worth keeping an open mind on hot-spot detection in other common predators.
It is very unlikely a keratin hair evolved into an infrared organ. It is more likely the embryonic structures for radiating chemi-luminescent infrared from a capillary bundle evolved into radiating antennas. By implication the origin of mammal hair was probably not for keeping animals warm. It probably evolved from the beginning as part of the cardio-vascular system for radiating infrared to keep animals cool. In the case of the infrared sensors it probably evolved independently as part of the sensory system.
Last updated – 12th March 2018