Bats split on family living
Daubenton's bat. Credit: Wikipedia |
For the
tiny Daubenton's bat, the attractions of family life seem to vary more with
altitude than with the allure of the opposite sex. For more than a decade, a
team led by Professor John Altringham from the University of Leeds' School of
Biology has studied a population of several hundred bats along a 50-km stretch
of the River Wharfe. They monitored roosts in Ilkley and Addingham, upstream in
the market town of Grassington and higher still in the villages of Kettlewell
and Buckden.
The
researchers found 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 women-only childrearing in
the lowlands and the more relaxed lives of the bachelors in the highlands.
Professor
Altringham said: "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. 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"
"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," Professor Altringham
said.
"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 added.
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," Professor Altringham said.
"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."
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.
Professor
Altringham said: "In and around these caves the bats gather in huge
numbers to mate, in a behaviour 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 kilometres 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," Professor
Altringham said.
The
Daubenton's bat, named after the 18th Century French naturalist
Louis-Jean-Marie Daubenton, is widespread across the United Kingdom and
specialises 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 metres 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," Professor Altringham said. "This makes bats
particularly vulnerable to the problems of habitat fragmentation and climate
change."
The paper, which is published in PLOS ONE, was co-authored by Dr Ruth
Angell and Professor John Altringham at The University of Leeds and by
Professor Roger Butlin at Sheffield University. It was funded by a Natural
Environment Research Council (NERC) PhD studentship to Ruth Angell, with
additional support from the NERC Biomolecular Analysis Facility at Sheffield.
Source: University of Leeds
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