How many ants does it take to fill an apartment? Tenants!

We all know how hard it can be to get your group of friends to decide on a restaurant, or for your family to decide on at which hotel to stay; there are just too many opinions, options and qualities to take into account! Our behavior lab is interested in how social insects make these kinds of decisions about food and housing resources. Over the last few months, we have been particularly interested in how the turtle ant Cephalotes varians makes the hard choice of which home(s), or nest(s), to move into. These ants are especially interesting because one colony will live in multiple nests at once AND these nests need to have some very special characteristics. This is because this species of ant defends its nests with a special subset of soldier ants, which have big, round head plates that they use to physically block the entrances to their nest (Powell 2008). They also live in pre-made nest cavities so they must find, assess and choose nests wisely so they can easily defend themselves (Powell, 2009). However, previous research has suggested that it is the much more numerous, but smaller, workers that do the investigating and decision-making (Powell, 2009). We thus set out to discover how these ants are investigating and making decisions on which nests to occupy.

Video of two ants in colony D under high threat communicating in box 1 then one enters the large entrance nest.

In this experiment, we looked at three different colonies. Colony C was the largest with close to 200 ants and colonies A and D were smaller with less than 100 ants each. (If you’re wondering what happened to colony B, they died in the first experiment and thus could not be used in further analysis.) We destroyed the original nests of these colonies and forced them to investigate and move into new nests under high threat (with another species of ant present) and low threat (without another species of ant present). We then videotaped their investigation and movement into new nests. So far we have only analyzed the first 8.5 hours of video of colony D under high threat, 8.5 hours of A under high threat, 8.5 hours of A under low threat, and 2.5 hours of colony C under low threat. However, the early investigation patterns were pretty interesting in themselves.

We found that there were bursts of investigation, or “runs,” throughout the investigation time, especially in colony A under high threat and colony C under low threat (Fig. 1). This suggests that the ants are not just randomly investigating the nests, but rather they may be communicating to coordinate their investigation. We were also able to observe a colony’s actions once they had made a decision to occupy a nest. Colony A under high threat chose to move into a nest, as evidenced by their placement of a baby ant (larva) in that nest. After this choice occurred, around 8 hours into the experiment, there was greatly increased activity in the colony overall and that activity centered on that chosen nest. This further suggests that there is communication between ants to increase activity at certain nest sites.

Fig. 1: Different colonies and conditions had varying patterns of investigative effort over time. The cumulative activity at (ants entering or looking into) any nest over the first few hours of the experiment is displayed for colony A under low threat (AL), C under low threat (CL), A under high threat (AH), and D under high threat (DH).

We also wanted to see if individual ants were directly comparing multiple nests, or if they were sharing information between individuals who had visited different nests to make a decision between multiple nests. We found that most ants only investigated one nest, but there were a few that visited more than one nest (Fig. 2). However, Colony D under high threat had a few individuals visit as many as 5 out of the 6 nests! This was high compared to colony A under both threat conditions and colony C under low threat, which had few ants visit multiple nests, and those ants only visited only 2-3 different nests. Colony D was also the smallest colony. This indicates that there may be colony differences, like individual differences between people, or that colony size may affect the colony’s investigation strategies.

One may think that the strategy would be the same over time, for example in a sports team individual players have assigned roles that are consistent game after game. However, we found that the ants that investigated more than one nest in colony A under high threat were not the same as those that investigated more than one nest under low threat. This may mean that the individuals do not have fixed roles for the number of nest comparisons they make.

Fig. 2: In all colonies, ants visiting only one nest seems to be most common, except for colony D under high threat. Each graph shows the number of different nests visited versus the total number of visits that ant made for each investigating ant. The relative sizes of the points on all graphs correspond to the number of individuals at each point. AL shows colony A under low threat, CL shows colony C under low threat, AH shows colony A under high threat, and DH shows colony D under high threat.

By characterizing these ants’ investigation patterns and strategies, we can better understand what factors affect their nest choice. Because good nest choice is crucial to this species’ survival, having a good system for investigation and choice could mean the difference between life and death for a colony. I know restaurant and hotel choice don’t seem like big decisions, but they can tell others a lot about how you work through different situations. Similarly, nest choice can give us insights into how adaptable this species and others like it are to higher threat conditions, such as threat from invasive or non-native species. It can also help us understand how other systems, like the human immune system, allocate resources.

Further Reading:

To find out how some other ants navigate nest choice: 
https://www.insidescience.org/news/when-ants-get-together-make-decision

https://blogs.scientificamerican.com/not-bad-science/how-is-past-experience-biasing-our-decision-making-insights-from-rock-ants/

https://www.sciencenews.org/article/rock-ant-decisions-swayed-six-legged-social-media

References:

Powell, S. (2008) Ecological specialization and the evolution of a specialized caste in Cephalotes ants. Funct. Ecol. 22, 902–911.

Powell, S. (2009) How ecology shapes caste evolution: linking resource use, morphology, performance and fitness in a superorganism. J. Evol. Biol. 22, 1004–1013