How ants evolve different ways of defending themselves

Did an ant sting you? Did it run away from you? Did an entire ant colony swarm in large numbers to defend its nest from you? If so, then you have witnessed firsthand the importance of defensive traits in ants.

Ants use a remarkable array of defenses to repel or avoid attackers, ranging from painful stings to the recruitment of a soldier caste. But despite this variety of traits, very little work has addressed the role of defensive traits in ant evolution or ecology.

I have been investigating the evolution of ant defenses for my Ph.D. dissertation in evolutionary biology, under the guidance of Corrie Moreau, my PhD advisor at the University of Chicago and the Field Museum of Natural History. In the first chapter, which was recently published in the journal Evolution, I asked two primary questions: Do defensive traits promote the evolution of increased species diversity in ants, and do these defensive traits exhibit an evolutionary trade-off?

But what is an "evolutionary trade-off"? Different scientists may give you slightly different answers, but in my paper, I define an "evolutionary trade-off" as a negative correlation across species between different traits that serve similar functions. For example, I hypothesized that the presence of a chemical sting may "trade-off" with sharp spines on the exoskeleton. Both spines and a sting typically serve defensive functions, so using both traits may be redundant and therefore a waste of energy. Thus, over evolutionary time, it is possible that species usually evolve either spines or a sting, but not both.

To conduct my study, I first compiled a large trait database for all 326 currently identified ant genera, and included five traits that often serve defensive functions: Large colony size, large eye size, worker polymorphism (i.e. having a "worker" caste and a "soldier" caste), exoskeletal spines, and chemical sting. I also reconstructed a new, expanded phylogeny ("tree of life") for the ants. This took quite a long time, but with these data, I could address my questions.

Using various statistical analyses, I found that several defensive traits are very important in ant evolution. In particular, spines, large eyes, and large colony size all promote increases in species diversity in ants. Furthermore, the chemical sting appears to exhibit an evolutionary trade-off with all of the other defensive traits I investigated, possibly due to the high cost of a sting with its chemicals and complicated structure.

The mechanisms for the patterns I detect are still uncertain though, due to surprisingly little work on defensive traits in ants. My results show that studies of these important traits are likely to be an exciting avenue of research, because such work will help us understand the causes of widespread ecological success in one of the most dominant and conspicuous group of insects on Earth.

So, the next time you see an ant defend itself by using a sting or swarming in large numbers, you're not just witnessing a split decision. They may be traits that have driven the evolution of these little insects over the course of millions of years.