Parent-offspring vocal recognition in the Mexican free-tailed bat, Tadarida brasiliensis mexicana

Maternity colonies formed by Mexican free-tailed bats (Tadarida brasiliensis mexicana) are thought to be the densest, most populous aggregations of a single mammal species on Earth. Each mother bat bears a single pup per year, and nurses it at least twice per day. How mother-pup reunions are achieved in these dark, noisy, crowded colonies has been the subject of several previous studies. The possible role of vocalizations in mother-pup recognition forms the basis of the present study. The ability of Mexican free-tailed bat mothers and pups to recognize vocalizations of their presumptive kin (pups and mothers, respectively) was tested using playbacks of recorded calls. Mothers were tested with pup isolation calls, which pups produce when separated from their mothers; pups were tested with mother echolocation calls. Captive mothers were presented with isolation calls of two pups, one the presumptive kin (i.e., found nursing from the mother in the maternity colony) and the other a stranger, each from opposite sides of a circular wire arena. Response was determined by the amount of time spent in contact with a cloth bat model in front of each speaker. Mothers showed a significant preference for the calls of their presumptive pup. When tested with the echolocation calls of their presumptive mother and those of a strange mother, pups were attracted to these calls but did not show preference for calls of different mothers. The ages of pups appeared to have no effect on the responsiveness of either pups or mothers to these playbacks. Mother Mexican free-tailed bats were found to produce "directive calls" while searching for pups inside of cave maternity roosts. These calls consisted of highly repetitive pulses of sound uttered in rapid sequence. Directive calls were stereotyped within individuals, and statistically discriminable among individuals, and pulses within a call had rapid onset and broad frequency bandwidths. These calls were sufficiently intense to be easily perceptible to the human ear, above the substantial background noise within roosts at distances of at least 1 m. These characteristics are expected for vocalizations that function for mother-pup reunions in these roosts, and are shared with directive calls described previously in other bats representing two different families. Playback experiments using recordings made inside the cave colony showed that pups perceive directive calls and are strongly attracted to them. This supports the hypothesis that pups recognize the directive calls of their mother. In captivity, females would not elicit directive calls, and playbacks of the calls of known mothers to pups could not be conducted. However, in-cave video recordings corroborate the suggestion that female directive calls are important to mother-pup reunions. Since traits used for individual recognition should remain stable over time, and since T. b. mexicana isolation calls and echolocation calls differ substantially in character, the prediction was tested that isolation calls are not developmentally continuous with adult echolocation. This prediction was confirmed by recordings of seven different pups, each producing both isolation and echolocation calls simultaneously. It is suggested that the notion of ontogenetic continuity between isolation and echolocation calls is unlikely for any bat species even in the absence of individual recognition. The structural and functional similarities between pup isolation calls and adult directive calls suggest that these calls may be developmentally continuous with one another.