Saving the injured: The value of rescued veterans in a predatory ant species

In general, we imagine ants as li>le pieces of something bigger, with the value of the individual being marginal, even sacrificing themselves if necessary for the good of the colony. Matabele ants (Megaponera analis) are a specialized termite predator of sub-Saharan Africa. In the early morning and evening hours they leave their nest in raiding parKes of up to 500 ants to kill termites at their foraging sites (where they are more vulnerable than inside their nests). Termites though developed ways to defend themselves against predators: a special caste of soldier-termites have developed large heads with strong jaws to fend off and kill predators that are out for an easy snack. Matabele ants, being specialized on overcoming this defenses, easily overwhelm the termites' defenses and aRer 10-15 minutes have killed up to 1000 termite workers which they then carry back to their nest. These ba>les come at a cost though; some of the ants loose legs and antenna or have termite soldiers clinging to them aRer the fight. Remarkably we found that aRer the hunt nestmates take care of their injured by carrying them back to the nest. We now wondered if this behaviour was truly focused on injured individuals? If so what was the mechanism regulaKng it and what was the value of these injured ants for the colony? To answer the first quesKon we presented the ants during their return journey with arKficially injured ants (by cuXng off two legs of healthy ants). These ants were always helped, in contrast to dead or healthy ants. Being now certain that this behaviour was focused on injured ants we could move on to more detailed observaKons. Through experiments with different ant glands, we found out that injured ants gave out a "call for help" aRer the fight with pheromones in their mandibular gland. This a>racted their nestmates, which started to antennate (invesKgate) the injured individual. The injured ant then assumed a fetal posiKon (pulling in its legs) facilitaKng thereby its transportaKon and got carried back to the nest. InteresKngly the help didn't stop there. If the injured ant sKll had some termites clinging on to its legs they got removed inside the safe confounds of the nest, thereby making her again a fully funcKoning member of the colony. What happens though with the ants that lost one or two legs? These ants carry permanent injuries, which should be a major handicap. Directly aRer the injury, these ants move roughly 30% slower than healthy ants, but aRer only 24 hours they adapt their locomoKon with the remaining legs so that they reach running speeds similar to healthy ants, making them again useful members for the colony. But why do these injured ants not simply return by themselves to the nest? The reason is simple: due to their injuries they cannot keep up with the returning raid, thus becoming isolated from their nestmates and thereby easy prey for spiders or other predatory ants. Hence roughly a third of the injured ants would die during the return journey if not helped. All of these injured veterans (if helped) are observed to parKcipate again in future hunts against termites, contribuKng again fully to the colony. This allows the colony to be up to 28% larger than a colony that would not help their injured. This study was the first detailed observaKon of a 1:Department of Animal Ecology and Tropical Biology, Biocentre, University of Würzburg, Am Hubland, D-97074 Würzburg, Germany.