Bats born under bridges are larger and grow faster than those born in caves
Concrete bridges could make better roosts for certain species of bat than natural caves, according to research.
Scientists in the US monitored the birth weight and growth of bat pups that were born under bridges over busy freeways, and pups born in caves.
They found that, rather than the relatively noisy man-made roosts being detrimental to the animals, pups born in bridges were larger and grew faster.
The findings are reported in the Journal of Zoology.
Lead researcher Louise Allen told BBC News that, because the Brazilian free-tailed bats she studied live in such large colonies, it is very difficult to repeatedly recapture and study them.
"You can't really do whole-life reproductive studies on them," she said. "So as a [measure] of reproductive success, I looked at birth size and how fast they grew."
When she carried out the study, Dr Allen, now based at Wake Forest University in North Carolina, was a member of Professor Thomas Kunz's research team at the University of Boston.
Home to roost: man-made bridges could help the bats expand their range
She and her team captured almost 200 bat pups at two sites in Texas - a natural cave roost and a bridge over a large highway.
They used foot-warming pads to prevent the tiny newborn bats from getting cold while they were being weighed.
To monitor their growth, Dr Allen recaptured the bats up to six times and measured the length of each animal's forearm bone - the long bone on the wing.
"Not only were the bats in the bridges larger, they also grew faster," she told BBC News.
"They reached the size where they could go and start foraging on their own up to five days earlier [than the bats born in the cave]."
This finding was contrary to what the scientists expected.
They initially thought the environment - with traffic noise and pollution - would be "stressful" for the animals.
"In a stressful environment, animals often put reproduction to the side and concentrate on survival."
But when it comes to living above a busy road, it seems the benefits for these bats outweighed the disadvantages.
Bridge roosts are warmer than caves
"The [bridges] are often closer to foraging sites and they are warmer," explained Dr Allen.
"So they can spend less time on thermal regulation and put more of that time into fat stores and bone growth."
Dr Allen explained that such man-made structures could provide "corridors" for these important creatures to expand their ranges if, as scientists predict, the changing climate moves US agriculture northwards.
"These bats rely heavily on agricultural pests... They feed on some of the most devastating agricultural pests in the US," she said.
She explained that when they are feeding their young, the bats can consume more than half their own body weight in insects each night.
She concluded: "Not every species [of bat] is going to respond the same to man-made habitat changes, but here we have an example of a species where the animals are doing just fine in a human environment."
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