A property of light that is invisible to humans is routine to help animals get about. Birds are known for remarkable quality of vision. They have larger eyes, as a proportion to body size, than other animals, and the eyes are adapted, for a wide field of view range, for grazers, or sharp focus, for birds of prey. The colour-sensitive nerve endings too, in the eyes of birds, fish and reptiles, are specialized and of four kinds, against single kind that we have, and this helps some of them see in the ultra-violet too. Insects have compound eyes, which help them cover a very wide angle of view by each eye and detect very fast movement. And then, some snakes have ‘pits’, which are like eyes and can detect the warmth of prey in pitch darkness.
But a most remarkable feature of birds and insects is that they can sense a property of light called polarization, which helps them deduce the position of the sun, even if it has just set or is covered by clouds. In fact, some birds can even tell the position of the moon, when it is hidden by clouds, with the help of this property of light, in moonlight! But the best that human eyes can do is to tell whether the light is bright or dim, and the only use we make of the property of polarization of light is in special sun-glasses, to cut glare due to reflected light.
The journal Nature Communications carried a report by Stefan Greif, Ivailo Borissov, Yossi Yovel and Richard A. Holland, scientists at Belfast, Seewiesen, Germany and Tel Aviv, that they have found that bats too, can make out the polarization of light, and this helps the calibrate their magnetic compass, so that they can navigate at night (1). Bats use sound waves to detect and pinpoint prey, but they need other means to know their way about and get back to their roosting place after a night out in search of food. That bats make use of the polarization of light is a new discovery, as the ability has not been noted in mammals so far.