Acoustic cues to identity and predator context in meerkat barks

Abstract

Formants, the resonance frequencies of the vocal tract, are the key acoustic parameters underlying vowel identity in human speech. However, recent work on nonhuman animal communication systems has shown that formant variation provides potentially important information to receivers about static and dynamic attributes of callers. Meerkats, Suricata suricatta, produce broadband noisy bark vocalizations, lacking a clear fundamental frequency and harmonic structure, when they detect aerial or terrestrial predators. Here we investigated whether formants in meerkat barks have the potential to provide reliable information on caller identity and the predator context (aerial versus terrestrial predator) in which they are delivered. Acoustic analyses of naturally occurring barks and measurements of this species' vocal tract length were used to confirm that the six clear frequency bands below 15 kHz in meerkat barks represent formants. Discriminant function analyses subsequently demonstrated significant interindividual variation in the formant pattern of meerkat barks, suggesting that formants could be used by meerkats to identify conspecifics. In addition, mixed-effects models indicated that the frequency of the first formant was lower in barks produced in aerial versus terrestrial predation contexts. These results add to a growing body of literature on the potential function of formants in nonhuman animal vocal communication systems, and also imply that signalling external and referential information through such resonance frequencies, as in human language, might be more widespread in animals than previously thought.