Humans shift their tone of voice constantly, whether to indicate surprise or concern, but it’s a relatively rare trait among other mammals. As such, researchers interested in vocal learning are very interested in the few animals that can do this, one of which is the harbor seal. In a new paper, scientists explored whether young pups could adapt their voices to a noisy environment and found that even at an early age these animals are capable of shifting their tone of voice to be heard.
“Without our developed control for sound production, spoken language — including learning new sounds over infancy — would be impossible. Can other species do this?” Andrea Ravignani, group leader of Comparative Bioacoustics at the Max Planck Institute for Psycholinguistics, told IFLScience.
“In the '80s, a seal named Hoover showed vocal imitation, parroting human speech (you can find audio recordings on YouTube). Seals seem to be one of the few mammals, apart from humans, with advanced vocal control and plasticity. But Hoover was one case and an adult. We asked: can several individuals do the same at only a few weeks of age?”
The study, published in the journal Philosophical Transactions of the Royal Society B, looked at eight harbor seal (Phoca vitulina) pups aged one to three weeks old that were housed in the Sealcentre Pieterburen in the Netherlands. The pups were destined to return to the ocean but were enrolled in the study for a quick bit of science before their departure.
The team behind the study exposed the seals to sounds recorded from the nearby Wadden Sea, regaling the pups with sounds of the ocean that varied from silent to 65 decibels. The tone height of the sounds matched that of the pups’ calls, so the researchers were looking to see if the seals altered their tone in response, something they termed their “fundamental frequency”.
Their results showed that the pups modified their vocalizations by lowering their fundamental frequency in response to a noisy “ocean”. The changes they exhibited correlated to the particular noise conditions they were exposed to. According to Ravignani, this adaptation may benefit the animals in helping their mothers to hear them in a crowded nursery.
“If baby seals acted as most animals, we would just expect them to increase the intensity of their voices as noise increased,” said Ravignani. “However, what seals did was lowering the pitch of their voices to escape the frequency range of noise, something that only animals with good control of their larynx (including humans but potentially excluding most mammals) can do."
“This shows vocal plasticity in seal pups from an early age, suggesting that seals may be one of the very few mammals which, like humans when singing or speaking a tonal language, can flexibly modulate the pitch of their voices.”
The team behind the research hope that by studying another mammal capable of manipulating its voice in a way similar to humans, we can begin to build an evolutionary tree laying out the emergence of building blocks of speech, demonstrating that the voice control of humans might not be as unique as previously thought. The next step is to look for plasticity in other aspects of seal voices, such as formants (which allows humans to produce a variety of vowel sounds) and voice rhythm.