Ant and termite tandem runs
Ants and termites show a behaviour called a “tandem run”. It’s where two of the insects venture out, one leading, one following. If you just glimpsed them doing this, you’d conclude that it was the same behaviour carried out for the same reason. But actually ants do it multiple times to teach nest-mates the way to new locations, while termites do it just once after mating to set up a new nest. And for the termites, it’s a case of follow my leader; but for the ants, it’s different: the leader determines the direction, but the follower sets the pace so she can learn the way. To discover this, Gabriele Valentini made video recordings to watch the animals running around in petri dishes and then extracted the information about which insect was calling the shots using image tracking software, as he explained to Chris Smith…
Gabriele - This behaviour is actually widespread in many insects like ants and termites. They use a very similar communication mechanism. So we have the follower touching the rear of the leader to communicate its presence, and the leader instead emitting volatile pheromones that can be perceived by the follower. We also know that, in ants, the leader is making sure that the follower is learning a new path for the environment, while in termites they are just navigating an unknown environment. And so the function of the tandem run in termites is simply to ???
Chris - So how did you actually do the study and what emerged as you did this?
Gabriele - The way we approach this question has been by looking at how leader and follower share information with each other. We performed several experiments where we tracked leaders and followers over time and what we did was to look at how the trajectory of the leader and that of the follower are caused by each other, by looking at how different types of information flows between leader and follower.
Chris - Can you just describe the experiments you actually did?
Gabriele - We did experiments with ants and termites. The setup is different between the two species. Ants are looking for a nest. We are actually starting a new immigration and having them explore the environment, look for a new home. And once they found this new home, they will start to recruit there using tandem rounds. And this is the part of the experiment we're very interested in. So we repeat several of these experiments, making this colony immigrate several times. And for each immigration we then recorded the tandem ants and the trajectory of leaders and followers. The situation is a bit different with termites. This is because they are not recruiting to a known position in the environment, but they use this behaviour only after mating. So we had to collect unmated termites, allow them to mate, and after that we put them in a petri dish where there was no nest they could find. We recorded the entire experiment using a video camera placed on top of this arena. And from the video recording, then we were able to use tracking software to extract the trajectory of the leader and followers.
Chris - And once you piece together, all these videos and watched all these ants and termites running, what did you learn?
Gabriele - There are at least two types of information that are being exchanged between leader and follower. One is information about the direction of motion, whether to turn left or right. And another important information is instead about the pace of their run - when to stop and want to resume motion. Although ants and termites can use the same communication mechanism, what we discovered is that in the case of termites, the leader is controlling both the direction of motion and the speed. And this all makes sense because we know they are searching at random and so the input of the follower to that behaviour is likely to be less important. The interesting discovery was for the ants, where we saw that the leader is deciding about the direction of motion, whether to turn left or whether to turn right, while the follower is actually controlling the speed of the run - when to stop and want to resume. And this was a very important finding because you provide the support for a hypothesis behind the tandem behaviour of ants, that is that the follower is acquiring information from the environment in order to guide a future journey for the environment. So in this case, we have that the leader is travelling through the environment, choosing the direction of motion, in order to show this part of the environment to the follower.
Chris - And why do they therefore need to slow down? What's the benefit of the changed rate from the follower?
Gabriele - We haven't proved it yet, but our main assumption, our main hypothesis, is that the follower is looking around the environment, looking for landmarks, that she can memorise and use as an information to guide a future journey. While they are moving they have a relatively higher speed and the follower is very much focused on not to lose the leader. But from time to time, she takes a break. And during this break, the follower detaches from the leader and walks around a few centimetre apart from the leader, usually making a sort of loop, before coming back to the leader, touching and restarting the journey. So these longer pauses are actually a mechanism, but provide the follower with the time to acquire visual information from the environment. And only when the follower has enough confidence in that information, she will then prompt to resume the journey.