Female greater horseshoe bats share male mates with their mothers and grandmothers, Nature magazine reports.
The tactic helps bind families without the dangers of inbreeding
This serves to bind families together, but avoids the dangers of inbreeding.
The females live together in groups segregated from the opposite sex, but gang together to prowl for males once the mating season arrives.
Scientists from the University of Bristol and Queen Mary in London made the discovery using genetic techniques to construct family trees for the bats.
Most female greater horseshoe bats (Rhinolophus ferrumequinum) seek out the same male to mate with year after year.
The bats produce only one offspring each year, so each animal represents the outcome of a separate mating.
Stephen Rossiter, of Queen Mary, University of London, and colleagues used genetic typing to compile the family trees of some 452 bats at Woodchester Mansion in Gloucestershire, UK.
The researchers mapped male partners on to pedigrees of female bats to examine patterns of pairings down the years.
They found that relatives on the maternal line shared male partners more often than would be expected by chance.
In all, they detected 20 groups of related females sharing mates, with two to five individuals in each of these groups.
Dr Rossiter believes that by sharing sexual partners, the greater horseshoe bat "strengthens social ties and promotes greater levels of cooperation within the colony".
It pays to share
Kinship between individual animals is extremely important for cooperation and, therefore, social cohesion, says Dr Rossiter and his team.
The tendency for females to return to the same males each year also strengthens this kinship.
Any behaviour, such as this, which increases the levels of relatedness within social groups while dodging the costs of inbreeding is likely to be favoured by natural selection, the researchers write.
In the UK, greater horseshoe bats are thought to have declined by 90% over the past 100 years.
This may be due to the disturbance of roosts and changing farming practices, since the use of pesticides has caused a declined in the insects they prey on.