Brazilian free-tailed bat  (Tadarida brasiliensis)

A particularly well-studied species, the Brazilian free-tailed bat exhibits some spectacular behaviour. It forms the largest warm-blooded colonies in the world, emerging to feed at dusk in huge columns of several million individuals. Their flapping wings create a sound equivalent to a white-water river and their numbers are great enough to be detected by airport and weather radars (7). Feeding for longer each night than any other bat species, it travels as far as 31 miles from the roost to the feeding grounds and uses echolocation to find its prey. The Brazilian free-tailed bat flies at up to 47 mile per hour in open spaces, foraging with fast, straight flight (6). Each bat consumes between 200 and 600 insects a night, selecting mainly moths, but also eating beetles, flying ants and leafhoppers (3). The Brazilian free-tailed bats of Texas are estimated to consume from 6,000 to 18,000 metric tons of insects each year, many of which are agricultural pests (2). At dawn, they return to their roosts where they swarm before re-entering. Thought to be a predator-avoidance tactic, the bats gather into groups at a great height above the cave, before closing their wings and dropping rapidly in one continuous stream. Predators waiting at the mouth of the cave to catch emerging bats include red-tailed hawks, owls, raccoons, opossums, skunks and snakes (7).

Mating takes place in March and shortly afterwards the females migrate to female-only maternity roosts. Most adult males do not leave the tropical and subtropical part of the range and therefore contribute nothing to rearing the young. Gestation lasts for 90 days (2) and females give birth within around 15 days of each other to a single young, known as a pup (7). The female clings to the roost with both thumbs and one or both feet to give birth, and remains attached to the pup via the umbilical cord for up to an hour while she cleans and nurses her offspring. In the first hour the young bat learns to cling to the roost wall and other bats with its hands, feet and teeth. Once stable, the female pulls away from the pup, dislodging the placenta, which hangs from the pup until it dries out and falls off several days later. During this time the number of bats in the roost doubles, and a female must locate her own pup by listening for its calls. She may land several times, eventually finding her pup by scent. Once reunited she touches the top of its head with her muzzle to confirm the bond (7).

A consequence of the enormous number of bats in a single cave is the build up of guano, or bat droppings. This nutrient-rich mixture was once commercially extracted from caves on a large scale, to be sold as fertiliser. In the early 1900s it was the largest mineral export from Texas after oil, and it continues to be sold commercially although to a lesser degree. Bat caves are widely known to contain noxious gases, but this is actually a result of carpet beetles (Dermestidae) that feed on guano and fallen bats. These beetles multiply so rapidly as a result of such a constant food supply that the whole floor of a cave may be ‘carpeted' with them, hence their common name. They produce waste that combines with water vapour to make ammonium hydroxide which is poisonous to most animals. Bats have adapted to this potent atmosphere by lowering their metabolic rate, which causes the level of carbon dioxide dissolved in their blood to rise, thus neutralising the ammonia. Their fur, however, may become bleached to a reddish-brown colour. The first flight of the five-week-old bats is fraught with danger as they become used to their wings and echolocation system. Collisions, failed flights, and unsteady landings can result in bats falling to the cave floor, where they are stripped to the bone in minutes by the beetles (7).