One of the most striking features of nature is the autumnal colours of leaves. Why leaves change pigment so dramatically is a mystery.
The conventional explanation is that the green pigment used to assist photosynthesis degenerates at this time of year. But this theory broke down with the discovery ten years ago that losing the green pigment chlorophyll merely renders leaves colourless, while trees seem to manufacture new pigments that produce red and yellow.
Why should leaves go to this trouble when they are about to die?
Marco Archetti, an evolutionary biologist at the University of Perugia in Italy, has come up with an explanation, which is published in the Journal of Theoretical Biology.
Archetti knew that aphids use colour to select a leafy tree in autumn on which to lay eggs.
These tiny insects cause substantial damage to host trees. Archetti believes that the aphids probably use leaf colour as a guide to which trees are the least able to mobilise defences.
Brightly coloured leaves could therefore be acting as a deterrent signal to aphids, as the ability to produce new pigments so near winter is a sign of the tree's vigour.
Also, as aphids are green, they are more likely to show up to predators against a red or yellow background.
According to Archetti, each tree is competing to produce the brightest leaves to avoid attack - hence the riot of colour.