When Texas Bat “Cities” Look Like Thunderstorms

For at least 8,500 years, the community inside Texas’ Bracken Cave has swelled exponentially every spring as up to 10 million free-tailed bats fly 700 miles north from various smaller roosts in and around central Mexico. After mother bats give birth to their pups, the population will double again to nearly 20 million—the largest congregation of nonhuman mammals on Earth. By midsummer, the millions of hungry pups grow restless as food demand reaches its zenith.

The result is a nightly commute of epic proportions, as legions of nocturnal hunters, young and old, flood into the surrounding fields—an event known simply as “the emergence.” At its peak, Bracken’s population is so dense that it takes over three and a half hours for every bat to hurtle out of the cave’s bottleneck entrance, creating a plume of tiny flapping wings that resembles a serpent extending into the evening sky. Soaring up to 10,000 feet, the adults and pups fan outward, traveling between 60 and 100 miles a night and collectively consuming more than 150 tons of moths, beetles, and other insects, before returning to their hideaway home just before daybreak. As I watched this mesmerizing scene play out in person from the rim of the cave, one of the cave docents held up his phone to show me the live Doppler radar feed.

The nearby New Braunfels radar station, which continuously scans the horizon for approaching thunderheads, soon picked up a massive blob larger than metropolitan San Antonio. Weather apps generally modify their default settings to remove such scenes from view, the docent said. Otherwise, even on a clear night, people watching the radar feed would break out their umbrellas, believing they were about to get caught in a torrential downpour.

Detecting Animal Metropolises Within Humanity’s Urban Sprawl There is no universal criteria for what defines a “city” beyond the dictionary definition of an inhabited place of comparatively greater size, population, or importance than a town or village. Generally speaking, it’s understood to be an area in which a large number of individuals gather or live in close physical proximity, usually to gain certain community advantages. In practice, cities also often serve as centralized hubs where cultural activities are carried out. (In the United Kingdom, for instance, a city’s status was once tied to whether or not it had a cathedral.) It’s where the action happens.

A space bound by a collective spirit that gives a city its beating heart. Animal cities are not measured by their concrete, glass, or steel, but by the community itself and the tangible advantages that large-scale cohabitation provides. Safety in numbers, knowledge sharing, services and trade, and environmental changes tied to a population’s sizable footprint are just a few possible reasons for coming together en masse rather than going it alone.

As a general rule, the larger an animal community is, the more information is being passed around, and the better its residents can collectively sense the nuances of the landscape. The increased swirling of knowledge around a community can, in turn, contribute to a sort of collective intelligence. Some fruit bats, for instance, maintain a mental map of all the trees that could potentially produce fruit, but they don’t always know which ones will bloom next.