Ant collective behavior is heritable and shaped by selection (2020) Walsh et al., The American Naturalist, https://doi.org/10.1086/710709

Image Credit: Землеройкин, CC BY-SA 4.0

The Crux

Working together to achieve a common goal is nothing new to us. We as humans are famously social organisms that not only crave interactions with others, but quite often succeed due to the way that we work together. Interestingly, we tend to work well when we have some form of organizations or leadership, but there are other animals that do not require such leadership. This so-called “collective behavior” is the behavior of a group that emerges without a form of central control. Think of a large school of fish avoiding a predator at the same time, or birds flocking together and flying through the sky. All of this happens as a result of those animals interacting with one another, not because there is some boss animal telling them to do it.

Not surprisingly, groups of animals will vary in their exact method of collective behavior. It’s assumed that this variation is largely dependent on natural selection, but there isn’t actually much that is known about it. For this variation in behavior to have been the result of natural selection, the variation itself has to be advantageous and heritable, meaning that it is better to have the variation and you can then pass it on to your offspring. Today’s authors wanted to measure just that.

What They Did

Using pharoah ants (Monomorium pharaonis), the authors set up replicate colonies to conduct behavioral assays on the collective behavior of different colony behaviors. They first tested the exploratory behavior of A) a group of five workers from a given colony and B) the entire colony itself. They measured how much of the testing arena the two groups covered (exploratory rate). For the next test, food was placed in a testing area with a colony. The authors monitored the foraging rate, or how many ants visited the food over an hour. Finally, the productivity of each colony was measured by doing weekly counts of the number of workers and brood within colony.

The authors then tested for the heritability of and correlations between the measured behaviors of the different colonies. Heritability was measured by comparing the phenotypes of the ants to the known genetic information of each colony. This would allow them to test for if and to what degree these behaviors are passed down to future generations. To test for selection on these behaviors, the authors defined colony fitness as its productivity. This is fairly straightforward, as fitness is defined by how many offspring that a given organism’s offspring produce. Higher productivity rates associated with a given behavior(s) would indicate that those behaviors are selected for.

Did You Know: Superorganisms

Ant colonies are pretty unique in the animal kingdom in that they are what is known as a “superorganism”. Although each ant is an independent and separate animal from the other ants in the colony, their collective behavior means that they can work together as a colony to act as a single organism. That is what is so interesting (and cool!) about collective behavior, many different organisms working together to achieve common goals, such that the colony itself can be thought of as a single unit, not a collection of many organisms.

Another way to think about this is organs in the body. Taken at their base level, each organ performs a unique function and serves a given purpose within the body. But, all of the organs work together to perform the functions that a human body needs in order to survive. That is how a superorganism works, many smaller (and separate) parts working together such that they can be thought of as a single organism.

What They Found

The collective behavior of each colony was repeatable over time, meaning that the foraging and exploratory behavior of individual groups or whole colonies was not a one-off, as each colony would repeatedly behave in a similar way. Not surprisingly, foraging rate and exploratory rate were positively correlated: the more a given group/colony explored, the more they would forage for resources. Interestingly, they found that higher foraging rates were selected for, but that exploratory rate was selected against. The authors also found moderate values of heritability. This means that the collective behavior traits of the ant colonies would be able to respond to selection.

Problems?

This was a really interesting study on collective behavior, but I had one main issue with it: how much variation in the results the given tests explained. For example, foraging rate (which was found to be selected for) only explained ~1-2% of the variation in colony productivity. The results of an experiment have a given amount of variation, and relating that variation to the experimental factors helps scientists to explain how certain patterns are likely to occur. For example, if one were to test for a relationship between toilet paper shortages in the US and fears over a coronavirus lockdown, I would bet that news reports over alleged “shortages” would explain most of the variation in how many stores were out of toilet paper. Bringing us back to this study, ~1-2% is a very low value, which despite its statistical significance points to a number of other factors that are likely influencing selection more than what the authors tested.

So What?

This study is one of the first to investigate the genetics of collective behavior, while also testing for selection on various phenotypic traits. By investigating the genetic makeup of collective behaviors, relating that various phenotypic traits, and measuring both selection on and heritability of those traits, the authors were able to show that collective behavior is the result of natural selection on colonial ants. Though I think that there is a lot left to learn about collective behavior, this study is a great first step in uncovering some of the dynamics of how selection shapes it.

Adam Hasik is an evolutionary ecologist interested in the ecological and evolutionary dynamics of host-parasite interactions. You can read more about his research and his work for Ecology for the Masses here, see his personal website here, or follow him on Twitter here.