The eyes of monitor lizards are very well developed, and capable of seeing slight movement over distances of at least 50 metres. Footprints of perenties that suddenly break into a run and change direction suggest that they can see a moving human from over 200m away in open areas. Sight is important to monitors both for finding food and escaping from predators. Some monitors may lie in wait for tiny prey such as crickets and spring on them as they pass by. However they do not appear to be able to see still objects clearly and will sometimes ignore stationary people for hours at a time, but take flight at the first sign of movement The eyes are situated on the sides of the head and the eyeball cannot rotate in the socket. This means that the animals must move the entire head in order to look round . Unlike snakes, the eyes of lizards have lids, but they also possess a thin protective membrane which is moved horizontally across the eye with such speed it is rarely seen in life but often appears on photographs. I do not know whether monitor lizards are able to see in colour. Only one species, the Bengal monitor has been subjected to a visual discrimination test, involving two choices; light and dark . After two days they were scoring 85%, and after being left for 18 days they still managed to achieve the same score (Loop 1974). Monitor lizards appear to communicate with each other by posture. They adopt highly stereotyped stances when approac.:hed by other lizards which seem to be indicative of their social status. Some are said to be able to recognise individual people (Proctor 1929 for the Komodo dragon, Andres 1904 for the desert monitor), but whether this identification is based on visual or olfactory stimuli, or a combination of the two, has not been established.
Monitors cannot see in the dark, and so almost all activity is restricted to the daylight hours. However individuals may occasionally move about on warm, moonlit nights (Auffenberg 1981), can forage at night around artificial lights (Fyfe 1979) and one species is reported to be most active at twilight (Christian 1977).
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