Evolutionary zinger: Ants and acacia trees

I treasure the science education I obtained at Oregon State University. One standout class was General Biology, a portion of which was taught by professor Fred Rickson. He did not like people slipping in late, so he gave short teasers promptly at the start of the hour, which he entitled evolutionary zingers, hoping that his students would enjoy the stories sufficiently to show up on time. It definitely worked for me. I was not in the habit of attending a lot of lectures, but the intricate stories of complex life told by professor Rickson were as attractive to me as nectar “fountains” are to acacia ants, the subject of one of his zingers.

Ants in the acacia tree

Professor Rickson described one morning some of his own research, which involved the study of mutualism between trees and ants. In particular, he had done a good deal of field work with African savannah acacia trees and the ants which lived in those trees. He described the first day of observation of an acacia tree that was known to provide food and shelter for a colony of acacia ants. The ants had a ferocious sting, so the observation team was cautious in approaching the first tree of the day. They carefully observed the tree from a few meters outside of the canopy perimeter, and after a few hours, had observed no ants on the ground.

This was puzzling, so professor Rickson approached the canopy perimeter, and passed under the shade line. Not long afterwards, he felt a sharp sting, and retreated rapidly. He had indirectly proven that the ants were there, so . . . where were they? Where were they coming from? The team continued to observe, and finally saw an ant appear on the ground, as if out of nowhere, at the canopy perimeter. The ant moved on a direct line back to the tree trunk, and climbed up the trunk. Soon, another ant was spotted, at a different compass position on the canopy perimeter, and it also moved back to the trunk and climbed up.

Further observation revealed that ants regularly and apparently randomly walked to the end of a branch and dropped to the ground, then walked in a straight line right back to the tree and climbed back up. They deviated if there were some debris immediately in their way, in which case they stopped, picked up the debris, took it out and dropped it well beyond the canopy perimeter, then returned and climbed the tree.

This was interpreted as both a sentinel activity (thus the professor’s sting) and a fire prevention measure. Large animals are discouraged from nibbling at the leaves of the trees by the nasty stings they would receive from the ants. Also, these acacia trees are situated in the dry grasslands, which are prone to flash fires; the area under and immediately surrounding the trees are kept free of combustible material by the ants, protecting both the tree and their colony from the flash fires, which would burn around the tree without consuming it. In return, the tree provided shelter for the ants, who lived in hollow thorns that the tree produced, and the ants are provided nourishment in the form of the aforementioned “nectar” fountains, or nectaries, and other more complex protein-rich “seeds”. All in all, a rather neat example of mutualism in biology.

The story is, of course, much more complex than that, with ants defending the tree against other herbivores, including other insects, competing ant species, ants removing competing nearby plants, and so on, but it served well for a five minute biological vignette. See related articles for more details.