19.2 THIS IS HOW WE DO IT: Do thorns really protect plants from being eaten?

It might seem obvious why an organism has a particular feature; a quick observation or two and some intuition can seem sufficient to draw a conclusion. But the power of scientific thinking is that we can go beyond our intuition or hunches. And in doing so, we can increase our confidence that what we believe is true actually is true. Here’s a case in point. Many plant species have spines and thorns. What is their function?

Are they defensive structures? That is, do they deter animals from eating the plants? This may be true. But it also might be true that thorns help plants in other, maybe less obvious, ways. Consider just a few alternatives: Perhaps spines and thorns help a plant regulate its temperature, possibly increasing physiological efficiency. Or they may serve as a defense against vines that use trees for vertical support, giving the vines access to light for photosynthesis without the need for producing more rigid stems or trunks. Vines can harm their host trees in numerous ways, including by reducing the soil resources available, causing abrasion, and increasing the risk of mortality from falling. Or thorns may help plants conserve water, increasing their drought resistance, perhaps by shading sensitive tissue.

A common and effective approach to “doing science” is to focus on just one clear question at a time. So let’s consider the question asked in the title of this section: How could we investigate experimentally the hypothesis that thorns really protect plants from being eaten? And just as it is helpful to focus on a single question when approaching a problem scientifically, the simplest methods are often the best place to start. On the topic of thorns, some researchers did exactly that.

Acacias are common on the savannas of Africa and, despite their thorny branches, are eaten by several species of animals, including giraffes. A group of researchers selected 50 individual acacia trees. On each tree, they chose two pairs of branches at the same height from the ground. Randomly selecting one of the branches in each pair, they removed all of the thorns from that branch.

Over the next three months, they surveyed all of the branches for signs of being eaten by giraffes. Their prediction was that if thorns protect a plant from being eaten, the experimental branches (with the thorns removed) would be eaten more than the control branches.

The researchers found that giraffes showed a very strong preference for the branches with thorns removed, and they ate more from each of those branches:

Plant growth suffered significantly as a result of giraffe feeding:

In the course of their experiments, the researchers noticed something else suggesting that the acacias’ thorns function to provide protection from being eaten by giraffes. When they measured the average leaf and thorn size at different heights above the ground, here is what they found:

The acacia plants produced dramatically bigger leaves at heights above the highest levels the giraffes could reach. They also produced much shorter thorns on those branches, and at a significantly lower density. (Interestingly, the researchers found that on fallen trees, giraffes quickly and completely ate the leaves from what were the highest branches.)

All of the observations above are strong evidence that, at least in acacia trees, thorns protect them from being eaten. But does that mean alternative explanations for the function of thorns are not valid? Not necessarily. Remember, a structure can have more than one function. However, the data from the comparison of plant structures on branches above the height at which giraffes could eat leaves do have some bearing on those alternative hypotheses. Why? And what do the data suggest?