Tuesday, October 11, 2011

Do plants quiver in fear?

Many trees have leaves that quiver in the wind. This property lends its name to the quaking aspen (below left), whose scientific name Populus tremuloides literally means "trembling poplar". In tropical Asia, the banyan tree (below right) is a commonly-encountered species with quivering leaves. They tend to have fairly broad, thin leaves and narrow, elongated petioles.

Aspen leaves gold backlight close Ficus religiosa Bo
Quaking aspen (Populus tremuloides, left) and banyan (Ficus religiosa, right)
But what exactly is quivering good for? Biophysicists have looked at the way that leaves deform in strong winds, and some have hypothesized that this helps them avoid mechanical damage. On the other hand, it may simply be an unintended consequence of adaptation for other reasons, such as having an appropriate shape and orientation for the efficient capture of sunlight.

A new paper recently available online has suggested that quivering may be a means of avoiding herbivores. The author cites other examples of plant "behavior", such as the Sensitive Plant Mimosa pudica, whose leaflets fold up when touched, startling herbivores while simultaneously exposing the thorns on its twigs. Could quivering serve a similar function?

The author suggests that shaking leaves not only make it harder for small herbivores like insects to land on leaves, they may also cause pests and parasite propagules to be literally shaken off. He also hypothesizes that cryptic herbivores, like leaf and stick insects, would stand out from a background of quivering leaves and be made more vulnerable to their own predators. 

At the moment this stands as only an interesting hypothesis. Most biologists would acknowledge that "theory is cheap, data is expensive". It's fun to think about, but it would be lovely if someone could do some experimental work on it. As a topic in sensory ecology, it reminds me of the hypothesis on why autumn leaves turn red, which was covered in this blog a few years ago. Yet I cannot help thinking that maybe this is reading too much adaptationist significance into what may turn out to be a "spandrel", a trait which is merely a by-product of other traits (a term coined by Steve Gould). 

In the meanwhile, we can look at the leaves that flutter in the breeze and wonder: are they nervously trying to shake off their enemies?


Kazuo Yamazaki (2011) Gone with the wind: Trembling leaves may deter herbivory. Biological Journal of the Linnean Society, published online before print 3 Oct 2011, doi:10.1111/j.1095-8312.2011.01776.x

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