In many parts of the world, urban development is providing new roosting sites for bats. Concrete road bridges seem to be a favourite place that bats congregate and rear their young. You might expect that what with toxic fumes, heavy-metal run-off and noise pollution, bats living in these sites would do less well than their relatives living in more traditional sites like caves. That was certainly the prediction of researchers from Boston and Knoxville, who studied birth size and postnatal growth in Brazilian free-tailed bat pups, and are just about to publish their findings in the Journal of Zoology.
As its name suggests, the Brazilian free-tailed bat (Tadarida brasiliensis) is found throughout much of South America, but its northern range is the southern states of the USA. The study was carried out south-central Texas – the bats migrate north from Mexico each spring, then rear their young. The bats can roost in groups of over a million. Females bear a single pup in May/June; pups can apparently be fed by any lactating female and can fly within 8 weeks. Adults are around 10 cm long and can weigh up to 14g. The bat is called “free-tailed” because it has a tail that projects beyond the hind-most membrane.
Around 600,000 bats live under the McNeil Bridge in Williamson County. Every night they come flying out of the bridge to hunt for insects, as seen in this picture:
To their surprise, the researchers found that pups born under the bridge were heavier and larger at birth, and grew faster, than pups born in a cave. Although a number of factors may have affected the result (in particular, larger females may have been roosting under the bridge, and larger females have larger offspring), they conclude that temperature conditions under the bridge and proximity to foraging areas may account for the apparent success of those bats that took advantage of the man-made structures.
L. C. Allen, C. S. Richardson, G. F. McCracken & T. H. Kunz (2009) Birth size and postnatal growth in cave- and bridge-roosting Brazilian free-tailed bats. Journal of Zoology. (Early View, not yet published).
Abstract available here, subscription needed to read full text.