When primates are hunting for food, only the ripest and juiciest fruits will do. Most primates, even humans, consume enormous quantities of fruit, and it's thought that primates broadened their visual spectrum from two to three colour receptors, to improve their chances of locating a luscious lunch. But no one had put primates to the test. Would trichromatic animals fare better in the forest than their dichromatic group mates (p.3159)?

Andrew Smith and his colleagues travelled to Peru where they monitored wild tamarins' fruit preferences, collecting spectral data from discarded food morsels to find out which colours were most attractive to the animals. Returning to the UK to work with animals in captivity, the team built a simulated forest, where they could tempt tamarins to choose between ripe-fruit-coloured boxes that held a candy treat, and `unripe' boxes with no reward. The trichromatic tamarins were much quicker at learning that the`ripe' boxes contained a treat, than the dichromatic tamarins.

But Smith adds that this doesn't mean that colour vision evolved solely for everyday foraging. Colour discrimination may also be important for visual communication, or even spotting lurking predators, but it's sure that three photoreceptors give primates the evolutionary edge in fruit foraging.

Smith, A. C., Buchanan-Smith, H. M., Surridge, A. K., Osorio, D. and Mundy, N. I. (
). The effect of colour vision status on the detection and selection of fruits by tamarins (Saguinus spp.).
J. Exp. Biol.