Nepotism in ants: ant workers can regognize their kin

Darwin in his time wondered about the existence of ants – how can natural selection as a process based on individual reproductive success give rise to sterile individuals such as ant workers? The solution comes from kin selection theory, which holds that an individual’s reproductive success can also be measured in the number of collateral kin produced. This is how the ants have solved the problem. In an ant colony with only one queen all the workers are her offspring and will in practice help to raise their siblings, which are genetically as valuable to them as own offspring.

However, colonies with multiple reproductively active queens occur commonly in ants. As a result workers will also be raising the more distantly related offspring, produced by queens other than their own mother. This dilutes the genetic returns from the help the workers provide. By selectively favouring offspring of the queen of closest kin, the workers can, however, partly mitigate this loss and more efficiently propagate their genes to the next generation. Because it is the workers who raise the brood they also have the opportunity to favour closer kin and disfavour (e.g. eliminate) more distant kin. Such nepotism har not been demonstrated before in ants.

Professor Liselotte Sundström and scientist Minttumaaria Hannonen from the University of Helsinki studied black ants and found out that kin does matter, and relatedness is a matter of discrimination. Their article is published in Nature, 27th of February, 2003.

In their study of the black ant (Formica fusca) the scientists show that worker ants are able to discriminate between closer and more distant kin, and also use this ability to raise offspring of higher genetic value. They constructed laboratory nests with two queens and took samples from a single offspring cohort both at the egg- and the adult stage. With the aid of genetic marker genes the scientists assessed both the reproductive shares each of the two queens obtained, and the relatedness between the workers and each of the two queens. If the workers are more closely related to one of the two queens, kin selection theory predicts that the reproductive share of this queen should increase during brood development. Conversely, if the workers are equally related to both queens no changes in brood composition are expected between the egg and the adult stage. The results show that this was the case – the closer the relatedness between the workers and one of the queens, the greater was the increase in her reproductive share during brood development.

The results thus show that worker ants can have very accurate kin discrimination abilities and that they also capitalize on this ability to pursue their selfish genetic interests. Nepotism has been difficult to demonstrate and has therefore remained a controversial issue. Most likely the tendency towards nepotism is a universal phenomenon, but the expression of this behaviour is likely to be modulated by costs due to recognition errors. When errors are made owing to informational constraints the costs will be paid by both the nepotistic individual and the entire colony. Under such circumstances natural selection will tend to eradicate nepotism from the population.