Pinnipeds – animals like seals and walruses that belong to the Pinnipedia order – are great models for vocal learning because they are closer to humans than other species in terms of evolutionary development and diversification, the authors write.
The authors then tested eight wild-born, healthy and unrelated seals, between 1 and 3 weeks old, from the Seal Rehabilitation and Research Center in the Netherlands. This center eventually releases seals back into the wild.
When the seals heard a loudspeaker play a 45-minute recording with high noise, low noise, or no playback at all, they called out spontaneously. When the pups heard the louder sounds of the sea, they lowered their voices. At higher noise levels, the pups used a more even pitch – and a seal raised its voice. This behavior, known as the Lombard Effect, is typical of human speech, when people speak louder to be more understandable, the authors said.
The seals may have lowered their pitch as low-frequency sounds continue to migrate in windy environments, such as those projected by the recorded audio, said Caroline Casey, research scientist and associate professor at the University of California, Santa Cruz’s Institute of Marine Sciences and Ocean Natural Sciences Department. Casey reviewed the study independently, but was not involved in the research.
The results showed that “seal pups have better control over their vocalizations than previously thought,” Ravignani said in a statement. “This control seems to be in place as early as a few weeks of age. This is astonishing as few other mammals are able to do it.”
Humans were once thought to be the only mammals with direct neural connections between the cortex – the outer layer of the brain – and the larynx, which we use to make the voice, he added. “These results show that seals are the most promising species for finding these direct connections and unraveling the mystery of language.”
When examining what neural networks or social conditions must be in place for language to develop, birds were the best animals we could compare ourselves to, Casey said. This is because, unlike most mammals, birds can be raised in captivity in environmental and vocal or plasticity studies and naturally mature faster due to their short lifespan.
“However, there are many differences, particularly in terms of sociology and life history, between humans and birds,” she added. “We are always looking for mammals that would be good models for studying voice learning.”
There hasn’t been a lot of research into what plastic cub calls can be like. In most studies, especially in the wild, sometimes you don’t really know the age or sex of the animals you are taking in, so it is very difficult to get good quality images of animals from different ages, ”added Casey. “To be able to experience that in this very young age group is unique.”
Future research, the authors wrote, could further investigate what other factors might be important in seal vocal plasticity.