Complex vocal production in a Californian sea lion

Understanding the evolutionary origins of the unique articulatory abilities enabling human speech is a key objective in comparative vocal communication studies. Here, we combine behavioral and acoustic analyses to examine these abilities in California sea lions, a pinniped species that vocalizes both underwater and in air, known for its diverse vocal repertoire and ability to learn to produce specific call types on command in captivity. We report that the larynx of male and female California sea lions appears positioned in a “descended” state, posterior to a large, fully ossified hyoid apparatus. Observations and audio recordings of a trained female reveal that sea lions may further lower their larynx (∼5 cm) when producing roars, reducing formant frequencies in a manner akin to size-exaggerating vocalizations in terrestrial mammals. The vocal tract extends from ∼20 to ∼25 cm during these calls. Additionally, when producing low-frequency, pulsed creaks on command, this female spontaneously achieves discrete, stable vocal tract configurations through abrupt horizontal tongue movements. These movements, resembling those in human speech articulation, affect the second formant (F2), creating F1/F2 contrasts analogous to back versus front vowels in human speech.