For elephants in Africa an old social truth lives on: it is the grandmothers who keep the family going. British researchers combed 30 years of elephant family history in Amboseli national park, Kenya, to discover that the most successful families were led by the oldest matriarchs.

They taped and then broadcast elephant recognition calls to prove the point. When the elephants heard a call from a group they did not know well, they went into a defensive huddle. When they heard calls from a more familiar herd, they carried on grazing.

But in either case, the older the matriarch, the more sure the group's behaviour. The research could have implications for conservation.

Elephants live almost human lifespans, cover huge distances and spend all day feeding. Females stay in family groups, led by the oldest female, to protect and rear their calves. Each group encounters around 25 other groups a year. Some groups - often related - see each other frequently, others meet occasionally. And occasionally, groups get into conflict and threaten each other.

Karen McComb of Sussex University and colleagues report in Science today that they recorded the low, rumbling elephant identity calls and played them from the back of a Land Rover, to watch the behaviour of elephant groups.

"What we found was that it all boiled down to what age the matriarch was. Families with older matriarchs were massively better at picking out genuine strangers. When faced with more familiar individuals, they would remain relaxed. But they pinpointed the ones that would have constituted a genuine threat."

The payoff was that the older the matriarch, the greater the number of calves produced by the family each female reproductive year. "We believe this to be the first statistical link between social knowledge and reproductive success in any species," Dr McComb said.

"The results highlight the disproportionate effect the hunting and poaching of mature animals might have. Other large mammals, such as whales, dolphins and chimpanzees, also live in fluid social systems where an ability to recognise friends among many acquaintances might be expected to have an impact on reproductive success."

But the oldest female in a herd will have the biggest tusks and be most likely to face danger, making her most vulnerable to poachers.