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In a Family Affair, Mother and Daughter Bats Share Mates

Lori Valigra
for National Geographic News
September 14, 2005
 
The idea of mother and daughter, and possibly even grandmother, chasing the same male would repulse most humans. But to female greater horseshoe bats, sharing a strong breeding partner can ensure fit offspring and strengthen the social group.

During their life span, most female greater horseshoe bats (Rhinolophus ferrumequinum) revisit and breed with a specific male, according to a new study.

That means offspring born in different breeding seasons are full siblings. In addition, the researchers believe that daughters follow their mothers to mating sites to breed with the same male.

" … sharing sexual partners strengthens social ties and promotes greater levels of cooperation within the [bat] colony," said Stephen Rossiter, a zoologist at the School of Biological Sciences, Queen Mary, University of London.

"The females are choosing their mates. We still don't know how they do it, how they pass along the information to their daughters, or how they mostly avoid inbreeding," added Rossiter. The zoologist is the lead author of the new study, which is described in tomorrow's issue of the science journal Nature.

Mate sharing among bats can make for some confusing relationships. In several cases genetic tests showed that a female bat and her maternal half-aunt also were half sisters on their father's side.

Bat Mansion

Greater horseshoe bats are named for the horseshoe-shape of the fold of skin called a nose leaf, which many bat species use for echolocation. They grow up to three inches (seven centimeters) long, weigh about an ounce (34 grams), and have a wingspan of 16 inches (40 centimeters).

Each female greater horseshoe bat can produce only one offspring at a time, said Brock Fenton, a biology professor and bat expert at the University of Western Ontario.

Female bats don't breed every year. But since the bats can live up to 30 years, it is possible for five generations of females to live under one roof along with similar families of unrelated female bats.

The group of female horseshoe bats that Rossiter studied live in the gothic Woodchester Mansion in southwest England. Builders abandoned the mansion in the mid 1870s before it was finished, allowing female bats to move in and raise pups.

The adult male bats live separately in nearby caves and are only visited by females during breeding cycles.

Between 1991 and 2002, Rossiter and his colleagues caught bats in nets and collected samples of skin from their wings. The scientists were able to analyze 19 different genes in a group of 452 bats, which included mothers, offspring, and potential fathers.

The researchers positively identified the mothers of 371 individual bats and the fathers of 232. The researchers also determined breeding pairs.

Further study showed that specific males and females paired together on multiple occasions more times than would have occurred at random. This finding suggests that mothers and their female offspring are selecting one male and returning to him for subsequent mating.

More Questions Than Answers

According to Rossiter, less-studied bats in other parts of the world might exhibit some of the same mating habits of the greater horseshoe bats.

Gary McCracken, professor of ecology and evolutionary biology at the University of Tennessee in Knoxville, agrees.

"It's safe to assume other bat species are doing this. The genetic data [for greater horseshoe bats] are great. But this research is just the tip of the iceberg."

Much remains unknown about how and why female bats choose certain males. Both McCracken and Rossiter suspect it involves smell, because the males mark their cave areas with an oily substance from their cheeks.

Fenton, the University of Western Ontario bat expert, says the study is important because of the length of time it covers and the large amount of genetic data collected.

He argues, however, that the data don't actually reveal much about mating behaviors.

"We don't know if the female is mating with several males in the same cave or not," Fenton said.

Rossiter uses a camera in the mansion to watch the females, but he has yet to install a similar device in a cave to observe the actual mating.

But the zoologist says he believes in bat fidelity because his team observed many individual females returning to the same mating site from year to year.

In addition, he says, the seminal fluid of the male coagulates to form a plug in the female that stays in place until pregnancy and likely prevents further mating.

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