According to Guinness World Records, the largest known colony of bats can be found at Bracken Cave, near San Antonio in Texas.
These are Helen (Tadarida brasiliensis), a small bat about the weight of a walnut, so-called because its tail protrudes freely beyond the membranous patch of skin that stretches between its hind legs. In the summer, an estimated 15 million bats form a massive roost inside the cave, making it one of the largest gatherings of mammals on Earth.
Every evening, they spiral into the sky at dusk, creating their very own “batnado,” as they head off to hunt insects. It’s a spectacular site that can be watched on live cameras at the Bat Conservation International website, or in person via organised bookings.
This is a maternity colony. In March and April, pregnant females return to Bracken after overwintering in Mexico, Central and South America. Over a two week period towards the end of June, each female gives birth to a single pup. The cave’s population booms.
The new mothers then park their pups on the cave wall in clusters called creches. With up to 4000 pups per square metre, the hairless youngsters are densely packed. This helps them to stay warm, when their mothers fly off to forage.
In the past, it was assumed that when the mothers returned, they made no effort to identify their pup and instead nursed whatever youngster they found. Then research in the eighties and nineties showed this was not the case. Mothers can discriminate the scent of their own pup, and make directed calls towards them. Each mum has her own unique call, which she makes as she searches for her youngster, and each pup has their own unique call, which they make in response. Along with an excellent spatial memory, this helps the mother to find and then feed her pup.
The babies grow rapidly and start learning to fly at around four weeks of age. If everything goes well, a young bat drops into the darkness, flies at speeds of up to six metres per second and turns an almost complete somersault to land back on the cave wall, a few seconds later.
Collisions can be fatal, so each bat must dodge the many thousands of other nearby learners, whist at the same time road-testing an echolocation system they are yet to become familiar with. Unscheduled landings are certain death, as the cave floor is teeming with millions of carnivorous beetles which can devour a young bat in seconds. Fatalities are high, and only half of the new arrivals make it past their first birthday.
Then, no sooner have they found their wings, than they spread them. Brazilian free-tailed bats are migratory, and whilst some do overwinter in Texas, most head south in search of warmer weather. The world’s biggest bat colony shrinks, until the cycle begins again the following year.
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