Mother Mexican free-tailed bats, Tadarida brasiliensis mexicana, produce 'directive' calls while searching for pups inside cave maternity roosts. These calls consist of highly repetitive pulses of sound uttered in rapid sequence. Calls are sufficiently intense that they are perceptible above the substantial background noise within roosts at distances of at least 1m. Calls are stereotyped within individuals, and statistically discriminable between individuals. These characteristics are expected for vocalizations that function for mother-pup reunions, and are shared with directive calls described previously in other bats. Mother T. b. mexicana directive calls are statistically no less discriminable than are the isolation calls of pups. Playback experiments, using recordings made inside the cave colony, show that pups perceive directive calls and are strongly attracted to them.
Balcombe, J.P., & McCracken, G.F. (1992). Vocal recognition in Mexican free-tailed bats: do pups recognize mothers? Animal Behavior, 43(1), 79-87.