An evolutionary hypothesis for fall color and a search for it in Houston

Even though we always say we would be better scientists if we could recapture the curiosity and willingness to question everything that we had as 5 year olds, we don’t usually mean it. Surely we think more clearly now that we have all this education? Aren’t we now better at both choosing our questions and answering them? Well, I think we really have lost something, and we should try to recapture it with adventure science. No one I know did this more effectively than Bill Hamilton. Usually he was brilliant, with kin selection, the organizing theory for social evolution, to his credit. Sometimes he was not, and died at 63 in 2000 shortly after a trip to Congo chasing down a hypothesis for HIV/AIDS that didn’t pan out. I was lucky enough to get lots of advice from him on my own wasp work (the stinging kind). We first met in my early years as a graduate student in Austin, and renewed our conversations on my many trips to Ann Arbor in the late 1970s. Bill always had time for talking about wasps. At one point he gave me some of his own field notes from his early work on Brazilian wasps. I’ll have to post those on the web soon.

So, what does this have to do with fall color? Well, why do leaves change color in the fall? I think we were told as kids that the green just drained out of the dying leaves, leaving that brilliant color behind. Of course, that doesn’t actually make any sense. The trees are talking, and they actively make those colors in an expensive process before they die. The brightest ones are anthocyanins. Why? What Bill Hamilton and Sam Brown proposed in a paper in Proc B from the Royal Society is that the red was a signal to insects, particularly aphids. And that signal says: “Don’t come here, I’m strong and healthy and will kill you. Go find a weak tree.”

The study by Hamilton and Brown has been cited 84 times in the last 9 years, according to Web of Science. There has been support for the hypothesis, in the form of finding brighter leaves are more symmetrical, something that caught Carl Zimmer‘s attention in the Loom. Perhaps most importantly, why trees change color has become a field of study. People look at apple trees and see how they do. They look at leaf symmetry as a function of color. More symmetrical leaves are from healthier trees. They compare the yellow leaves of Europe to the red ones of the US and Canada. It has become a field that the tools of educated minds can study, like Marco Archetti. To begin it we needed an educated mind that retained its sense of wonder. And a collaborator, Sam Brown, whose wife comes from Houston.

Houston is not known for great fall color, but if you look carefully, you can find red sweet gums, and tallow, and yellow elms and pecans. They are a brilliant window into a challenging biological question. It took an open mind to see what is before us all.

I’m guessing St. Louis will have some great fall color, at least compared to Houston.


Sweetgum is our best bet for red leaves.


Even in late December the leaves have not all changed.


Sycamores turn more brown than red.


Chinese tallow are a non-native invasive, destroying our prairies. They have the reddest leaves.


Color comes to Houston trees in other ways.


The bougainvillea are still blooming!


Who wouldn’t miss the year-round green of our wonderful live oaks?


About Joan E. Strassmann

Evolutionary biologist, studies social behavior in insects & microbes, interested in education, travel, birds, tropics, nature, food; biology professor at Washington University in St. Louis
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