The dramatic colors of trees in autumn have long been considered a pointless beauty, merely a byproduct of how leaves age. But now researchers propose a new hypothesis: The bright colors warn insects of the trees' chemical defenses. The idea, if correct, would identify a spectacular example of signaling between plants and insects.
The notion comes courtesy of the late William Hamilton, renowned for his contributions to evolutionary biology, such as demonstrating how altruistic behavior can be adaptive. In an obituary last year, his former student Marlene Zuk of the University of California, Riverside, wrote that Hamilton "was not afraid of saying outrageous things. ... Most of them were wrong, and some were even ridiculous, but the ones that were right were gems." Hamilton's latest outrageous idea was published posthumously with colleague Sam Brown of the University of Montpellier, France, in the 22 July issue of the Proceedings of the Royal Society of London B.
Hamilton and Brown propose that trees displaying bright red and yellow leaves are advertising defensive chemicals, in essence telling insects to go pick on someone else. To address this idea, the authors compiled published data on fall color intensity for each of 262 tree species in Europe and North America and looked at the number of aphid species that colonize the trees in the autumn. After controlling for factors such as climate and geographic range, they found a correlation that matched their prediction: Tree species with more aphids show brighter red and yellow fall colors, possibly because species suffering greater attacks should invest more energy in chemical defenses and signaling.
Biologists applaud the originality of the proposal while pointing out numerous problems. Some question whether aphids can see red or would avoid bright yellow. Others aren't sure whether fall color production is biologically costly for the plant, an assumption of the signaling thesis. But the paper will likely spur reexamination of fall foliage, says entomologist Ronald Prokopy of the University of Massachusetts, Amherst. "And sometimes being a foundation for future experiments is more important than whether you're right or wrong."