Why male bats take the high road and female bats take the low road

Daubenton's bat is found throughout Eurasia, its habitat stretching from the United Kingdom to Japan. But as one northern English population reveals, these bats divide their space along strict gender lines, with males living up the bachelor existence at high altitudes while females raise their young at low altitudes... except when a few bats are able to find a middle path.

That's the finding of University of Leeds biologist John Altringham, who examined the distribution of the bat populations throughout the Yorkshire Dales. He found that females tended to make their roosts in the lowland areas of this hilly region, meaning the males have to look for homes at higher altitudes. According to Professor Altringham, the females' territoriality is probably due to the huge amount of food required to provide milk for their young.

Why male bats take the high road and female bats take the low road

The one exception is near the town of Grassington, which sits at an altitude somewhere between the lowland female roosts and the uphill male ones. There, and seemingly only there, male and female bats lives together. Professor Altringham offers one possible explanation as to why males and females might set aside their usual gender barriers, despite it seemingly being against both of their best interest:

When you look at the nursery colony in Ilkley, mothers and pups often have a lot of ectoparasites like ticks and mites. In a warm, crowded nursery, parasites can thrive, especially if there's less time for good personal hygiene. Parasites not only make life uncomfortable but can affect a bat's health. The males that live by themselves are usually very clean in their bachelor pads, so you can understand why they might not want to move in.

"Females may roost as high up the dale as Grassington because they have these warm, cuddly males to bunk up with. This way, females use less energy keeping warm and babies grow faster. In these marginal conditions, they may just tolerate a few males to keep them warm. Otherwise they kick them out. Why do the males co-habit if they are going to get parasites all over them? Well, that may be down to the usual answer: sex."

The researchers have found that the male bats cohabiting with females around Grassington enjoy a definite reproductive advantage over their bachelor counterparts. Altringham explains this phenomenon with the help of an extended clubbing metaphor — basically, the Grassington bats are settling down early and avoiding all the sweat and hard work of trying to win a mate on the bat equivalent of the dance floor, all while cutting down on their travel costs:

"In and around these caves the bats gather in huge numbers to mate, in a behavior known as swarming. This is clubbing for bats, with males displaying to females in lengthy acrobatic chases. As winter closes in, these caves will ultimately be their hibernation sites. There are nearly 2,000 cave entrances and hundreds of kilometers of cave passages in the Dales and these attract bats from all over Yorkshire, Lancashire, Cumbria and beyond for mating and hibernation. The males in Grassington may be giving themselves the opportunity to mate with the females late in the summer before they even get to the caves."

"At Grassington, most of the fathers of bats born there spent the summer with the females. If we look at pups in Addingham and Ilkley, their dads were males caught when swarming at caves. So, as well as two different mating systems, you have distinct social groupings. A bachelor from Buckden is always a bachelor from Buckden. He doesn't pop down to Grassington to visit the females in the summer. His only option seems to be to go clubbing in the autumn.

Check out the original paper at PLoS ONE.

Quotes via the University of Leeds. Image of young Daubenton's Bat by Gilles San Martin on Flickr. Map courtesy of University of Leeds.