Need to signal the Caped Crusader? Try an ultrasonic chirp, not a spotlight. The same sonar calls that bats use to navigate at night may also help them to tell friends from strangers, according to scientists. Lesser bulldog bats (Noctilio albiventris) roost with hundreds of other bats but bond and hunt side by side with only a handful. In a study published  in the this month's issue of Animal Behaviour, researchers watched how these bats responded to a suite of sonar call recordings. Individuals grew antsy after hearing bats from different social groups versus hearing bats they knew well; they brayed with low vocalizations called “honks” (which sound like clicks  to our ears) or stretched their wings more. This ultrasonic ID may help bats to spot their chums in a chaotic roost and, like a walkie-talkie, may also help groups to communicate on hunting expeditions, researchers say.