Are bright autumn leaves a costly signal from trees to pests?
One of the most recent – and most exciting – applications of the handicap principle purports to explain the bright autumn colors displayed by many deciduous trees. Sam Brown and the late W. D. Hamilton propose that bright leaf coloration is not just a side-consequence of chlorophyll reabsorption. Rather, autumn color may be a signal from trees to aphids (and other pests) that are looking for places to lay their eggs. Their argument is presented in three parts, each with substantial empirical support. • Autumn color (red, in particular) is nutritionally costly to trees. • Aphids, a particularly damaging group of tree pests, appear to preferentially avoid trees with bright red or yellow leaves when colonizing trees in autumn. • Tree species with bright (red or yellow) autumn leaves have more specialist aphid pests than do trees lacking bright autumn coloration. The evidence is certainly suggestive of costly signalling. But what, precisely, are the trees communicating with their signals?
