By mapping the brains of seals and sea lions, researchers have uncovered specialized neural circuits that have evolved to support the control of complex vocal behavior and learning in the species. Humans are vocal learners, but they are not unique; some birds, bats and some marine mammals have demonstrated the ability to modify or acquire new vocalizations that fall outside of their inherited repertoire through experience or by mimicking novel sounds. Among marine mammals, pinnipeds, a group of mammals that includes seals (phocids) and sea lions (otariids), show clear behavioral evidence of different components of vocal flexibility, ranging from highly developed volitional breathing control to flexible vocal production learning. Harbor seals have even shown the ability to mimic human words and phrases. However, the neurobiological basis for such abilities in these animals has not been fully characterized.
Using histology and ex vivo diffusion MRI tractography (dMRI), Peter Cook and colleagues examined postmortem brains from harbor seals, elephant seals, California sea lions, and coyotes (as a non-vocal-learning carnivore relative). Cook et al. focused on the neural pathways between vocal motor cortex and phonatory brainstem nuclei, as well as known forebrain circuits associated with vocal learning. The findings revealed distinct differences in brain circuitry that appear to align with known differences in vocal flexibility among the species examined. According to the authors, seals and sea lions showed strong bilateral connections between the vocal motor cortex and the brainstem nucleus ambiguus, a pathway associated with voluntary control of vocalization, whereas no such direct connection was found in coyotes. Moreover, elephant and harbor seals showed especially strong connectivity between the anterior ventrolateral thalamus and vocal premotor cortex, forming a forebrain circuit like those involved in vocal learning and mimicry in birds. The findings suggest differences in neural connectivity among these species parallel their varying capacities for vocal flexibility and learning, with harbor seals showing particularly strong neural features associated with advanced vocal control and flexibility.
