Sept. 20, 2005 — Female bats share mates with their mothers, grandmothers, and even aunts, according to a new genetic study.

The idea of mother, daughter and grandmother chasing the same male can sound disturbing.

But to female greater horseshoe bats, the behavior helps strengthening family ties, making possible greater levels of cooperation within the colony, Stephen Rossiter, of Queen Mary, University of London, and colleagues report in the current issue of Nature.

Amazingly, amidst the wild partnering, the bats avoid mating with their blood relatives.

"Our study shows the astonishing complexity that can underpin breeding behavior in wild animals. In these female bats, mating behavior has evolved to increase levels of relatedness in the colony without incurring any inbreeding," Rossiter told Animal Planet News.

"This happens by occasionally switching mates. A young female bat will not mate with her father. Only when her mother has switched partner will she share her mother's new mate," he said.

Getting their name from the distinctive horseshoe-shaped flap near the nose, greater horseshoe bats (Rhinolophus ferrumequinum), can live up to 30 years.

Females stay segregated in single-sex colonies, raising the pups together, and only visit males living in nearby caves for the mating season.

Rossiter and colleagues examined a bat colony living in the attics of the Gothic Woodchester Mansion, in southwest England.

Abandoned by its builders in the mid 1870s before it was completed, the building has turned out to be the ideal shelter for greater horseshoes, one of Britain's rarest bats.

The researchers studied summer colonies, each comprising about 40 bats, over 10 years. DNA tests drew up the family trees for 452 bats.

Each female greater horseshoe bat can produce only one offspring at a time, so each bat represents the outcome of a separate mating.

Overall, Rossiter's team detected 20 groups of related females sharing mates, with two to five individuals per group, representing eight matriarchal lines.

It emerged that 11 pairs of mothers and daughters shared mates at least once, along with seven pairs of grandmothers and granddaughters.

Yet there was only one case of close inbreeding, with a female mating with her father. Six cases of moderate inbreeding were also recorded (three maternal half-siblings, two paternal half-siblings, one granddaughter-maternal grandfather).

It's not clear why female bats choose certain males and how they can find them time and again.

"One possibility is that the female relatives are sharing information and following each other to suitable mating sites, where the males live," Rossiter said.

"This is a phenomenal study. The depth of lineage information is beyond anything yet seen for wild bats. I suspect females choose mates on the basis of some cues that indicate high fitness, maybe odors," Gary McCracken, professor of ecology and evolutionary biology at the University of Tennessee in Knoxville, told Animal Planet News.