Why have some lemurs lost their colour vision?

PRIMATES' TRICHROMATIC colour vision, with its red-, blue- and green-sensitive cone cells in the retina of the eye, is better than that of most mammals, which have to limp along dichromatically. It is thought to have evolved because primates are generally arboreal frugivores, and fruit are often brightly coloured. Some lemurs, however, are exceptions. They do indeed live in trees and consume fruit. But they have only two sorts of cone cell and are therefore unable to distinguish what other prima...