The discrimination of baboon grunt calls and human vowel sounds by baboons

The ability of baboons to discriminate changes in the formant structures of a synthetic baboon grunt call and an acoustically similar human vowel (/ε/) was examined to determine how comparable baboons are to humans in discriminating small changes in vowel sounds, and whether or not any species-specific advantage in discriminability might exist when baboons discriminate their own vocalizations. Baboons were trained to press and hold down a lever to produce a pulsed train of a standard sound (e.g., /ε/ or a baboon grunt call), and to release the lever only when a variant of the sound occurred. Synthetic variants of each sound had the same first and third through fifth formants (F₁ and F_3-5), but varied in the location of the second formant (F₂). Thresholds for F₂ frequency changes were 55 and 67 Hz for the grunt and vowel stimuli, respectively, and were not statistically different from one another. Baboons discriminated changes in vowel formant structures comparable to those discriminated by humans. No distinct advantages in discrimination performances were observed when the baboons discriminated these synthetic grunt vocalizations.