Male bats range from devoted fathers to independent bachelors along a 50 kilometer stretch of the River Wharfe in Yorkshire, England.
A team led by Professor John Altringham from the University of Leeds’ School of Biology has studied a population of several hundred of the tiny Daubenton’s bats for more than a decade.
The researchers report that all Daubenton’s bats in nursery roosts in lowland areas of Wharfedale during the spring and summer were females and their offspring.
Male bats were mostly restricted to a windier, Heathcliff-like existence in roosts at the top of the Dales.
But the researchers were surprised to find a small oasis of cohabitation in Grassington, sandwiched between the bustle of the female-only childrearing in the lowlands and the more relaxed lives of the bachelors in the highlands.
“Low down the dale, the females appear not to tolerate males and we assume they won’t let them in the roost. They don’t want anything to do with them,” says Altringham.
“High in the dales, all the roosts are bachelor pads. But in the middle, at Grassington, males and females live together—the social structure changes with the environment.
Why not stay?
“One possible reason for not finding males low down the valley could be that the mothers just want to avoid competing with males for food. It takes a lot of insects to make the milk needed to feed their young,” Altringham says.
“But it is also possible that the males choose not to roost with the females. 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,” he adds.
At Grassington, which is deep in the Yorkshire Dales National Park but not as high as Buckden and Kettlewell, the bats have a completely different social structure. Both male and female bats live with the young throughout the spring and summer in roosts in the stonework of the old Dales bridges and in holes in ash trees.
“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,” Altringham says.
“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.”
Clubbing for bats
Although male and female Daubenton’s bats usually live apart throughout the spring and summer, they meet when they begin flying to caves in late summer.
“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,” says Altringham.
“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.”
The researchers have built up a detailed picture of social and sexual behaviour by genotyping hundreds of individuals. The evidence gathered from this supports the theory that the Grassington males enjoy an advantage in mating.
“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,” Altringham says.
One pup a year
The Daubenton’s bat, named after the 18th century French naturalist Louis-Jean-Marie Daubenton, is widespread across the United Kingdom and specializes in hunting insects over water. Full-grown adults weigh only 7 to 12 grams, but they can live for 20 years or more.
“These bats are the size of a shrew but have a very different lifecycle. A shrew typically spends its entire life in a few meters of hedgerow, eats and breeds with a ferocious intensity, for a year if it is lucky, and then dies.
“In contrast, these bats lead a complex life over a huge area and females produce only one pup a year,” Altringham says. “This makes bats particularly vulnerable to the problems of habitat fragmentation and climate change.”
Ruth Angell of the University of Leeds and Professor Roger Butlin of Sheffield University are co-authors of the paper, which is published in PLOS ONE.
The Natural Environment Research Council (NERC) and the NERC Biomolecular Analysis Facility at Sheffield supported the research.
Source: University of Leeds