Clunk, roll. It isn’t loud enough to wake me up, but it can be loud enough to keep me from falling asleep, as the acorns fall. If my car is parked under a live oak, an acorn will clank down on the metal before I can even start the engine. Why so many acorns? I don’t remember it being like this last year!
The live oaks are masting in Houston. I’m so glad we’re still here for this mast, because one of the first (unpublished) papers I wrote in graduate school was on masting, but in bamboo. The mast is an old-fashioned word for the crop of a nut tree, or something like that. And the crops don’t come evenly every year. Why is really cool.
Suppose you are a little squirrel and you are counting on having enough food to get you a great female mate if you are or a male, or if you are a female, to get you through the winter with enough fat to give birth to healthy babies in early spring and again in the summer. Acorns are one of your favorite foods, and you are great at finding, hoarding, and eating them. If this is the only food squirrels ate, then you could imagine that acorn numbers would be very important in determining the numbers of squirrels in the population. For example, my block in Bellaire would support the number of squirrels that this number of acorns could feed.
But guess what? Feeding squirrels is not high on a live oak’s list of things to do with its acorns. In fact, it’s not on the list at all. Those acorns are the live oak’s babies, and they are only for making more oak trees. So there is conflict between the squirrel’s interests and the oak tree’s interests, and this can lead to a leafy, chattery arms race. Of course they don’t consciously know they are fighting each other, but any gene that favors the bearers interests will become more common in the next generation, and become fixed over the many generations of evolutionary time.
So, what’s an oak tree to do? How can the genes of a ponderous old oak fool a skittery squirrel? The answer is in the mast. If oak trees concentrate their acorn production at irregular intervals, then the squirrels will have a hard time keeping up. They will starve, or not reproduce in years of few acorns, and in years of plenty, have more than they can efficiently turn into more squirrels. The longer the time between masts, the fewer squirrels will be around to eat the nuts. Of course, if the oak trees wait too long, they may lose out in other ways. One of the problems of organisms is that they evolve in an environment where many different things are going on, and most of them can hurt reproduction.
Masts tend to occur in intervals of prime numbers, 3, 5, 7,11, 13, 17 years apart, or even more. These are numbers that are harder to track by squirrel populations. Evolutionary adaptation is fascinating. One of my favorite masting organism is the cicada Magicicada. Some of them stay underground for 13 or 17 years, only hatching to adulthood, mating, and reproducing, on these prime years when they can escape the predators by saturating them. But there are also annual cicadas that fight the evolutionary battle in different ways. I love cicadas because I came by them young, introduced by my first mentor, Dick Alexander, at the University of Michigan.
There are lots of challenges to understanding masting. One is how the different trees synchronize. It won’t help to produce nuts only every few years if you don’t do it in synchrony with your neighbors. How is masting synchronized? This is harder than you might realize, because the trees are also competing with each other to make more oak trees. See why evolutionary biology is so much fun?
There are lots of articles out there on these topics, and still lots of things to learn. Me? I’m just happy when I hear another acorn fall (just heard one out the window now) and can think of Houston’s beautiful live oaks.
Acorns from the live oak behind the biology building at Rice.
A lovely live oak at Brazos Bend State Park, the one Jeremy Field set up his tea under.
Neighbors a few blocks away.
Dick and Laurie Alexander. He is the best University of Michigan professor, mentor, brilliant thinker.