One skill in the study of physics is being able to select appropriate sensors for measuring quantities. Here, we dive into natural sensors. Although we can't ask an owl for a numerical value of sound intensity, having an understanding of sensors in the environment makes more obvious what can be measured.

 

Have you ever wondered about the degree to which physics has played a part in the evolution of how we perceive the world around us? Certainly, we humans along with all of our fellow mammals and other fauna (and flora), in general, must in some part look the way we do due to conditions on the planet we call home: its gravity, atmosphere and so on. But to what degree has the way we observe and react to our surroundings evolved in parallel with our survival needs?

As a starting point, take for example the mammalian eye. This sophisticated organ has seen most of its development relatively recently (albeit over millions of years) in terms of its evolution from the earliest primitive eyespots of our ancestors (merely patches of light-sensitive skin that allowed monitoring of the cycle of light and dark, important to our circadian rhythms). The eyes of most animals have evolved to have a combination of rods and cones, photoreceptors sensitive to particular wavelengths of light. Rods are good at detecting low light levels but not picking up colour. Cones, on the other hand, which are efficient at detecting bright light, are differentiated according to their sensitivity to different wavelengths of light. Nocturnal animals tend to have more rods than cones and better night vision, whereas diurnal creatures such as ourselves see colour well. 

Photoreceptors are essentially photon counters, which respectively detect red, green and blue light in much the same way as a television works. The colour we perceive, as interpreted by the brain, results from the weighted relative strengths of the photon counts from each type of cone. For example, equal responses of red and green light will be interpreted as yellow light, which is the region of the spectrum we are most sensitive to and which is also the approximate colour temperature of the Sun, the star which illuminates our world.

Of course, not all creatures “see” the world in the same way. Bees can see in the ultraviolet part of the electromagnetic spectrum. An interesting example of how independent organisms can exploit the physics of the environment to mutual advantage is the fact that the flowers from which bees take nectar produce patterns visible in the UV, which act as targets, or landing lines, for bees in search of nectar. Bees don’t see the colours of the petals as perceived by us.

Certain species of snakes, in particular boas, pythons and pit vipers, can sense the infrared region of the spectrum, which means that they can detect heat from both prey and predators in the dark. They have vascular glands on their faces, which detect thermal radiation, information of which is carried by nerves to the optic centre of their brains, where the low-resolution infrared image is superimposed onto the optical image (which essentially enables them to see other warm-blooded creatures in the dark).

Other animals, including bats, which are nocturnal, and whales, who spend much of their lives in the murkiness of the deep ocean, have evolved to use biosonar or echolocation as a means of detecting prey as well as mapping their surroundings. This is done by sending out pulses of sound, usually squeaks or clicks, and computing distances by measuring the time delay before the echo is received. Bats use a combination of frequency-modulated sounds (which are high in resolution, although with low range) and constant frequency sounds (that exploit the Doppler effect) to determine the radial speed of their prey. So sophisticated is this system that they change the amplitude and frequency of the emissions in order both to prevent deafening themselves and being detected by their prey. Incidentally, owls aren't echolocators but do have good hearing for locating prey in the dark. Certain species have concave faces, which direct sounds towards their often asymmetric acoustic locating ears, like satellite dishes.

Finally, there is strong evidence that certain species can detect the Earth's magnetic field and use it to navigate. Examples are birds and amphibians (turtles in particular). The mechanism involved remains inconclusive, however, there is evidence that some animals experience quantum magnetic chemical changes in their eyes. Also, species like homing pigeons have clusters of ferromagnetic material in their beaks which may act like iron compasses. Pertinent evidence implies that, whilst birds can detect the direction of magnetic field lines, they don't necessarily pick up on their polarity - so perhaps quantum chemistry is the answer. It is thought that some birds can detect polarised light which could be useful for orientation in the sky at certain times of day.

In some ways, all the above poses more questions than answers. However, asking questions is part and parcel of the scientific process: the means to understanding our natural existence in the context of the physical world.