The short, pointed face of a weasel allows both binocular vision forward and a wide arc of monocular vision on each side. The eye of a weasel, like that of a cat, is constructed so that it can see well in both bright and dim light. Since the requirements for good vision by day and by night are not the same, the eyes of an animal active at both times have to achieve something of a compromise. The retina of a mammalian eye has three kinds of receptor cells, of which two, called (from their appearance in histological sections) cones and rods, are part of the visual imaging system. The cones are used for perception of bright light and colors, and are found in diurnal species. The rods are particularly sensitive to low-intensity light and are most numerous in the eyes of nocturnal animals. The balance of rods and cones gives a fair idea of how sensitive and acute an eye is, and whether or not it can see in color. The third type registers only the intensity of light, and its function is to direct a mammal's internal clock (Hattar et al. 2002).
All species of weasels have duplex retinas (including both rods and cones), suggesting some degree of color vision. Unfortunately, although Herter (1939) and Gewalt (1959) set out to test the color discrimination abilities of common weasels, neither got past preliminary training before their animals died. Nonetheless, the structure of the retinas in common weasels is histologically very similar to that in stoats, which have been shown in behavioral tests to be able to see at least red, and perhaps also yellow, green, and blue (Gewalt 1959).
On the other hand, the presence of cones need only show that the eye is adapted for use in bright light. Since all weasels and most of their prey are shades of brown, white, or grey and important markings on them are usually black or white, weasels may not need color perception for identification of food, potential rivals, or mates. Even if they can see it, color may not mean much to weasels, anyway.
In most carnivores, the sensitivity of eyes in dim light is increased by the tapetum lucidum, a reflective layer behind the retina. The eyes of an animal that has a tapetum, such as a cat, have very obvious "eyeshine" when caught in a flashlight, compared with the dull red glow of human eyes. Weasels have a vivid green eyeshine, a fact well known to naturalists: Wood (1946) wrote of "their eyes . .. glowing with a strange green fire."
Eyes sensitive enough for night hunting must also be well protected if their owners are to be about during the day. The brown iris of the eye, which automatically closes in bright light, protects the retina from damage: The more sensitive the retina is, the more protection it needs. A slit pupil can close more tightly than a round pupil, so that even very sensitive eyes are not blinded in sunshine. Weasels have slit pupils, but the slits are horizontal rather than vertical as in cats.
Eyes that are good at seeing in dim light often achieve this ability at the expense of sharp acuity in bright light. As every photographer knows, photos taken on film of 400 ASA, or digital images saved to too few pixels, should not be too greatly enlarged because they lack fine detail. Most carnivores key into movements rather than entire pictures, but that does not mean they cannot be sharp-eyed when something catches their attention. A long series of patient, form-discrimination tests with one common weasel in Germany showed that this individual could distinguish quite minor variations in shapes presented as cues for food rewards (Herter 1939). In fact, this animal eventually learned to distinguish seven letters, offered in various combinations. The letters were attached to a pair of boxes, in one of which was the reward (a mealworm). By the end of the series, the weasel could "read" the label on the box containing the reward, which was WURM (worm), and preferred this box consistently to the other one, which was labelled LEER (empty). Learning the location of a reward, and detours to it, are easy tasks for weasels, and they easily remember pathways and places around their home ranges.
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