Ants are renowned for being able to carry objects much bigger than themselves, but when they cooperate as a group, they can move even larger chunks of food back to their nest. This collective carrying requires quite a sophisticated level of coordination, and is rarely seen in animals other than humans and ants. Using longhorn crazy ants as their subjects, researchers decided to find out how the insects manage this.

The feat requires that all the individual ants lift and move the food in unison in order to avoid a tug-of-war situation, but this then raises the issue of how they decide what direction to go in and how they respond to obstacles. It turns out that individual “scout” ants can join in the lifting, and direct the group by subtly pulling in the right direction, and rather than resisting, all the other ants follow suit.     

“The individual ant has the idea of how to pass an obstacle but lacks the muscle power to move the load,” explained Ofer Feinerman, senior author of the study published in Nature Communications, to Agence France-Presse. “The group is there to amplify the leader's strength so that she can actually implement her idea.”

For many animals living in groups, the ability to synchronize their movements to all move in unity is obviously highly advantageous. But there are moments when this ability to move as one is actually a hindrance, as it reduces their flexibility to react to a changing environment. This, it appears, is where the scout ants come into play.

^{Longhorn crazy ants moving food are influenced by a scout to go towards the nest (to the left). Credit: Ehud Fonio and Ofer Feinerman/NPG Press/YouTube}

“As far as we can tell, the scout is no different than the other ants,” Feinerman, who is from the Weizmann Institute of Science in Israel, said by email. “No one designates the leader, she – not he – designates herself because she has current knowledge about the correct direction.”

But the scout doesn’t stay leader for long. If another one comes along and tries to change the direction of the group, the first scout will willingly go with the second and do as she suggests. The researchers also report that little communication is actually needed to decide who the leader is; all the scout has to do is turn up and join in, and the rest follow.   

In addition to this, they found that there was an optimum size for the object that would allow the group to follow the lead of the scout. With large objects, the influence of the single ant that knows the direction was pretty much nonexistent. But they found that Cheerio cereal that had been left in cat food over night (one of the researchers found that kitty food was particularly attractive to the ants) was around the perfect size for the lead ants to alter the direction of the group. The researchers suspect that this is because the object more closely matches the size of the entrance to the ants' nest.