"When you see them emerge as a group at dusk, it feels like a river, a flow," said Aya Goldshtein of the University of Konstanz, Germany. She began a study on this phenomenon while at Tel Aviv University in Israel. Such a density of bats might suggest that accidents are common. "Actually the risk of collision is very low," said the researcher, who revealed in the PNAS journal on March 31 part of the flying mammals' strategy to avoid them.

Bats use echolocation to navigate and recognize their peers, prey and obstacles in the dark: They emit ultrasounds and detect how these sounds bounce off objects to orient themselves in space. But what happens when a large number of them are chirping at the same time? How do they overcome what researchers have called "the cocktail party nightmare," when everyone talks simultaneously and no one understands what others are saying?

To find out, Goldshtein and her colleagues focused on a colony of greater mouse-tailed bats in the Hula Valley, Israel. It numbers 2,000 individuals that, as night falls, hunt insects around the cave they live in. At this rush hour, no fewer than 25 of these animals, each weighing about 30 grams, exit every second at 50 kilometers per hour through a 3-square meter opening. Dozens of them were equipped with movement-tracking devices, and a smaller number carried microphones on their backs, capable of capturing their ultrasonic signals and those of their peers.

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