Zoologists have discovered a mammal whose queen uses sheer force of personality to suppress the fertility of most of her subjects. The queen of an African mole rat colony spends her time pushing and shoving other rats down their tunnels, biting them and terrifying them regularly into paralysis.

Zoologists now believe that this is what causes all the other female rats - and most of the male rats - to be infertile. Researchers from the Institute of Zoology in London and the School of Biological Sciences at the University of Manchester have been studying the mole rats because they have a social system that is more sophisticated than any non-human mammal, including primates. They are "the mammalian equivalent of a social insect", they say.

The pink, hairless mole rats live in tunnels burrowed underground, complete with nest and toilet areas, in arid parts of Kenya, Ethiopia and Somalia. In extremely arid conditions with dry soil they form colonies that are completely isolated from each other and they inbreed. They need each other in order to burrow for their food - roots and tubers - in an organised way. "If they tried to do this in groups of two or three, burrowing randomly, they would starve," says Frank Clarke, of the Institute of Zoology.

The colony is ruled by a queen, who is the only female who gets to mate, with a select group of non-working males. A colony would typically be one queen, 14 non-workers and 90 workers.

ÁñÁ«ÊÓƵ

The zoologists initially believed that the mass infertility was caused by pheromones given off, perhaps by the queen, but could not detect any such chemicals. Now they have evidence that the queen's stressful presence represses the secretion of a hormone that usually starts the cascade of hormone releases which lead to fertility: gonadotrophin-releasing hormone produced in the hypothalamus of the brain. In females this suppresses ovulation and in males it reduces testosterone and sperm concentrations.

The infertility is reversible after a few years isolated from the queen.

ÁñÁ«ÊÓƵ

Using the theory of "inclusive fitness", the zoologists have shown that it is probably in the worker rats' genetic interests not to reproduce. They are almost identical genetically, because they do not mix with other colonies. So the best way for them to pass their genes on may be to co-operate with a system which increases the chances that their genes will survive, rather than try to reproduce themselves.

Mr Clarke said that the work supports the theory that sociality has evolved in mammals as a reaction to living in extreme environments.