Web of Life

Families can find it best to stick together even in the insect world, biologists at the universities of Nottingham (UK) and Georgia (US) have discovered. Researchers studying the unusual life-cycle of a parasitic wasp found its larvae refused to attack close relatives even under extreme starvation conditions.

Led by Dr Ian Hardy in Nottingham's School of Biosciences in the English midlands, and Professor Mike Strand at the University of Georgia, the team were attempting to solve an evolutionary problem that has perplexed scientists for decades - why do animals sometimes cooperate with their competition? They expected to find that, as food resources reduced, soldier larvae would be less discriminate about who it attacked in an effort to secure their own survival. Dr Hardy, a population biology lecturer, said: "We didn't quite anticipate this. We would expect that, under extreme conditions, the survival of the individual would outweigh family considerations. Recent scientific theory predicts that competition for resources overrides close family ties but we found, over the course of almost two years, that this wasn't so."

The study, funded by the UK's Natural Environment Research Council, looked at a parasitic wasp which can produce numerous sets of identical twins over its lifetime - polyembryonic reproduction. These wasps can have between 800 to 3,500 young, or larvae, from just one egg, making them the most polyembryonic species known to science.

The adult wasp finds a host, usually a caterpillar, in which to lay its egg. This hatches into two types of larvae: the reproductive larvae that will eventually emerge from the host as adult wasps and soldier larvae that are doomed to death after protecting the reproductive larvae long enough for them to consume their host and complete their life-cycle to adulthood.

Dr Hardy continued: "Despite the fact that they are both genetically identical, the two types of larvae look completely different - rather like the film Twins starring Danny DeVito and Arnold Schwarzenegger. We know that sterile soldier larvae with tough, fighting mandibles develop first. They distribute themselves throughout the host caterpillar protecting their reproductive relatives. Alien species and distant relatives feel their wrath; the soldiers attack those they do not recognise as their own. We found the soldier larvae are most protective towards closest relatives - in fact their aggression increases as the relationship to a relative reduces."

The team believes the larvae recognises close family by a membrane shrouding that has unique chemical properties. This membrane also protects the larvae from the host's immune response. Professor Strand's group has shown that removing the membrane and placing it on a close relative leads to soldier larvae attacking its own relatives. Altruistic behaviour is common throughout the animal kingdom, particularly in species with complex social structures. An animal is said to behave altruistically when its behaviour benefits other animals - at a cost to itself. The costs and benefits are measured in terms of "reproductive fitness" - expected number of offspring. Therefore by behaving altruistically, an organism reduces the number of offspring it is likely to produce but greatly boosts the number of offspring produced by close relatives - increasing the chance that a specific line of genes continues into another generation.

This biological notion of altruism is not identical to the everyday concept. An action would be called "altruistic" if it were done with the conscious intention of helping another. But in the biological sense there is no such requirement. Sterile soldier larvae devotes its whole life to protecting reproductive larvae. This behaviour is 100 per cent altruistic: sterile workers obviously do not leave any offspring of their own but their actions greatly assist the survival and eventual reproduction of the reproductive larvae. The research was published in the journal Nature.

London Press Service