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New Paper Friday

Social learning strategies in house-hunting ants

Patricia Jones

Ants leading their nest-mates to a new colony using "tandem runs". Photo by Thomas O'Shea-Wheller. 

Ants leading their nest-mates to a new colony using "tandem runs". Photo by Thomas O'Shea-Wheller. 

A new social learning strategy paper is always exciting! Especially when it uses a less common system such as the ant, Temnothorax albipennis. This week's paper comes from Nigel Frank's lab at the University of Bristol, lead-authored by Nathalie Stroeymeyt, and is in Scientific Reports

The "teaching" ant

Temnothorax albipennis is a tiny European ant species that live in cracks in rocks, or even can make homes in hollow acorns (how adorable!). The latin name, albipennis, means "white feathers", I assume because the ants' bodies have a smattering of white hairs.  This ant species has become well known because of it's extraordinary behavior when searching for and moving to a new nest. Their fragile acorn homes are frequently destroyed, forcing a colony including its brood (little larval ants that can't do much on their own) to move to a new nest. Scout ants go out and search their environment for a new nest. When they find a possible new home they return to their old nest (or the ruins of it) and attempt to "recruit" other scouts to come and visit the nest they have found. They do this through "tandem running" which involves one ant leading the way while the other ant follows right on its heels (so to speak). The ant that leads goes slowly, and only proceeds when the following ant taps its back legs with their antennae. Ants that have followed in a tandem run then are able to recruit other ants to the same location. This behavior is so cool because it is one of very few examples of teaching in animals. In order for a behavior to be designated teaching in a strict sense, the "teacher" individual must modify their behavior at some cost to themselves. They cannot be simply doing whatever they like while the student watches. Because leader ants in tandem runs travel distances more slowly and only proceed when tapped by following ants, this behavior qualifies as teaching. 

Once an ant has visited a new home it then uses tandem running to bring other scout to the new home. When the number of ants in the new home (there through individual exploration or recruitment) reaches a threshold number (this is a form of quorum sensing) , then the ants go back to their old nest and start carrying other nest mates and brood over to the new nest. Because ants only recruit other ants if they find the new potential nest satisfactory, this quorum sensing is a means for insect groups to make a group decision based on a conglomeration of the available data each ant has collected. It is also used by honeybees when deciding where to make a new nest (although it is slightly different in honeybees). 

Social learning strategies

Social learning strategies are the strategies that animals employ when deciding whether to use social information (information acquired from others) versus private information (information acquired through personal experience). I often use an airport food court as an analogy. Let's say you walk into the food court section of an airport in a foreign country where you do not speak the language. There are lots of options of where to eat, but you cannot read the signs or the menus, so how do you decide? What if most of the restaurants are empty, but one has 20 people eating there? I would go an check out the place where there are other people eating, with the assumption that these people have information I do not about the superior quality of that restaurant. This risks the danger of an "information cascade" where everybody is relying on other people in making their decisions and therefore all eating crappy food, but when you have no personal information the presence of other people may be your best indicator of food quality. In contrast if you walk into a familiar airport where you have eaten in some of the restaurants before, you may ignore the other people (or even avoid them! because they are competition after all!) and eat at the place you know that you like. This social learning strategy is called "copy when uncertain" because you only use the social information when your personal information is "uncertain". 

This paper

This week's paper examined whether ants use the social learning strategy of "copy when uncertain" when deciding to move to a new home. They set up the ant colony in an arena so while still living in their old home they could explore a potential new home ("new home 1"=NH1). When the researchers destroyed the old home, forcing the ants to move, they added in a second potential new home (NH2) that the ants had not had the opportunity to explore. They then recorded the proportion of ants visiting each of the new homes that were "transporting" other ants as a function of how many ants had been in the NH the last time they were there. Ants that had been to NH1 many times before quickly started transporting nest mates to NH1 regardless of how many other scout ants they saw at NH1. In contrast, ants that had made only a few previous visits to NH1 waited until there were more other ants present at NH1 before beginning to transport ants. These ants were dependent on social information (the presence of other ants) because they had less personal information. Similarly, ants visiting NH2 did not start transporting other ants until there was a quorum of other ants present at NH2. Essentially the number of visits an ant has previously made to a nest decreases the quorum sizes (number of ants present) necessary for that ant to start carrying its nest mates to the new nest. If an ant has been to a nest many times before, there do not have to be a lot of other ants there for it to decide to start moving its nest mates, but if it has never been before, or only a few times, there have to be more ants there for it to decide to start moving. A "copy when uncertain" strategy!