International Journal of Comparative Psychology

The present study aimed to investigate the effects of modifying human vocal discriminative stimuli (hereafter referred to as vocal cues) on the behavioral responses and discrimination performance of two adult Steller sea lions (Eumetopias jubatus), a female (Hama) and a male (Shunta), housed under managed care. Both subjects could already discriminate standard human vocal cues. To determine if they could spontaneously respond to modified versions of these cues without prior training, we evaluated three types of modifications: presenting only the first or second half of the vocal cue (Test 1), altering a single phoneme within the vocal cue (Test 2), and changing the accent of the vocal cue (Test 3). During testing, the modified cues were presented randomly mixed with the original vocal cues that the subjects had previously learned to associate with specific behaviors. We analyzed the subjects’ accuracy rates, based on a binomial distribution, and reaction times across the three tests. Accuracy rates were significantly lower for modified cues across all test types (p < .05), with accent alterations having the greatest negative impact on performance. A detailed analysis of behavioral responses suggested that the subjects did not use a single strategy for all vocal cues, such as always focusing on a specific phonetic position. Instead, they developed individual strategies to find a unique discriminative point for each cue. Hama, who possessed a large repertoire of learned vocal cues, relied on the vowels produced by the trainer (vowel dominance) during her discrimination process. Frame-by-frame video analysis (30 fps) also revealed that certain cue modifications significantly influenced the reaction times of correct responses for both subjects. These findings indicate that Steller sea lions possess the cognitive flexibility to spontaneously associate modified vocal cues with their original counterparts, even when the acoustic properties deviate somewhat from familiar forms. These results also suggest that Steller sea lions, like other animals (e.g., primates, dogs, and rats), do not use the phoneme-based strategy that humans apply to large vocabularies. Instead, Steller sea lions rely on markers specific to each cue.