How animals could extort relatives into helping with the kids

In a Darwinian sense, family members are valuable as vehicles for shared genes. Whether helping evolves depends on a simple cost-benefit calculation known as 'Hamilton's rule': help if it will lead to a net increase in copies of your genes in the population.

Dr Patrick Kennedy, lead author and research associate in the School of Biological Sciences, said: "We explored a devious form of manipulation. An individual can threaten to harm its own survival or reproductive success if relatives withhold help."

Andy Radford, Professor of Behavioural Ecology and co-author, explained: "We considered an overlooked aspect of Hamilton's rule. Animals might increase their value to kin by acting in a way that puts shared genes in jeopardy if relatives fail to help."

The authors used simple kin-selection models to extend the theory of 'blackmail' -first suggested four decades ago by Israeli biologist Amotz Zahavi in relation to parental care -- to the evolution of altruism between any relatives.

"The logic is similar to the 'Doomsday device' satirised in the film 'Dr Strangelove', added Dr Kennedy. A Doomsday device is a mechanism that will trigger a disastrous nuclear strike if a rival makes an unwelcome move. "If animals can tie more of their own survival or reproductive success to a partner's behaviour, they can make a threat of self-sabotage credible."

Professor Radford suggested: "Shrewd use of resources may be a promising focus to uncover blackmail among animals. For instance, a mother wasp could lay a large clutch and all but exhaust her energy reserves; unless relatives' step in to help, the babies may not leave the nest alive."

Dr Kennedy added: "Our paper looked at whether blackmail between kin is theoretically possible. We showed that, under the right conditions, it can indeed evolve. Underlying the illusion of harmonious co-operation, some animals could be making their families an offer you can't refuse."