The study, published in this week's Nature, shows that groups of related females from different generations tighten their family bonds by sharing the same mates, although they nearly always avoid mating with their blood relatives.

Greater horseshoe bats (Rhinolophus ferrumequinum) tend to live in segregated sexual groups, with females raising their offspring together.  But when mating season comes around, groups of females search for individual males and form temporary mating groups consisting of several individuals.  Females tend to seek out the same male year after year, ensuring that their offspring are always full siblings and promoting cooperation among roostmates. 

However, when the researchers used genetic techniques to compile the family trees of some 452 bats at Woodchester Mansion in Gloucestershire, UK, they found that many females share mates with their mothers and grandmothers.

Why might these breeding patterns evolve? "One possibility is that by increasing kinship, sharing sexual partners strengthens social ties and promotes greater levels of cooperation within the colony," said Dr Stephen Rossiter of Queen Mary, University of London, who led the study.  "In fact, the study also found another way in which these bats strengthen levels of kinship, with most females returning to the same male over many years."

The study is part of a long-term collaboration between scientists at Queen Mary, University of London and the University of Bristol. 

Professor Gareth Jones of Bristol's School of Biological Sciences said: "This is one of the longest-running and most detailed mammal studies in the world.  My colleague, Roger Ransome, has been working on this particular population of bats for more than forty years.  Long term studies of this kind are important as bats are long-lived - they can survive for up to thirty years in the wild. 

"These new findings illustrate the hidden complexity that can underlie animal mating patterns in natural conditions, and could have important implications for conservation strategies in a range of important mammalian species."

Stephen J. Rossiter, Roger D. Ransome, Christopher G. Faulkes, Steven C. Le Comber and Gareth Jones:  'Mate fidelity and intra-lineage polygyny in greater horseshoe bats'   Nature  Vol 437 15 September 2005 doi:10.1038/nature03965