Identity cues in animal calls are essential for conspecific vocal individual recognition. Some acoustically active species mainly show reliable identity cues in their vocalizations because of variation in anatomy and life history. Long and strenuous-to-produce vocalizations may be particularly effective for showing identity cues because sustaining such calls may reveal individual anatomical differences in sound production. It is largely unknown whether reptiles possess acoustic individuality despite some groups being vocal. We analysed 814 bellows from 47 American alligators, Alligator mississippiensis, extracting spectral characteristics and manually corrected contours of the fundamental frequency. Recognition was up to 66% correct with a supervised classifier (random forest) and 61% with unsupervised clustering (chance = 2.1%), indicating that individual alligators have highly distinct bellows. Alligators were distinguished primarily based on the call spectrum, fundamental frequency contour and amplitude modulation, which also provided information about the animal's size. Neither manual supervision of acoustic analyses nor supervised training on labelled data was necessary to achieve reasonable accuracy, which has promising potential for identification of individuals via passive acoustic monitoring for research and conservation purposes. Additionally, our results highlight the importance of studying the utilization of acoustic individuality in the social lives of crocodylians.
