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 primates see as red and green, even though close relatives are trichromatic. That might be expected to make it hard to pick out red fruit, in particular, from a green, leafy background.The assumption until now has been that these lemurs have been unlucky and have lost part of their colour vision by chance at some point in the past. But Rachel Jacobs of George Washington University, in America, disagrees. A paper she has published in argues that these lemurs’ loss of the ability to see red, as it were, is no accident.