Did George and Ira Gershwin have harbor seals in mind when they published their jazz masterpiece I’ve Got Rhythm in 1930?
Humans are chatty, musical animals. Rhythm and vocal production learning are building blocks of human music and speech, and evolutionary biologists think that our capacities for speech and music may be linked.
The perception of rhythmic sounds is fundamental to human speech and music; upon perceiving a beat, our motor system becomes readily synchronized to it. This ability, called beat perception and synchronization, is a human universal, but its evolutionary route is debated.
Scientists have believed that only non-human animals like songbirds can learn new vocalizations and seem to have a sense of rhythm. “We know that our closest relatives, non-human primates, need to be trained to respond to rhythm,” said Dr. Laura Verga, an expert in comparative bioacoustics who headed a team at the Max Planck Institute for Psycholinguistics. “And even when trained, primates show very different rhythmic capacities to ours.” But what about other mammals, she asked.
Verga and her team wrote an article in the journal Biology Letters entitled “Spontaneous rhythm discrimination in a mammalian vocal learner.”
Testing harbor seals for rhythm
They decided to test the rhythmic abilities of harbor seals (Phoca vitulina) – animals known to be capable of vocal learning. The team first created sequences of seal vocalizations. The sequences differed in three rhythmic properties – tempo (fast or slow, like beats per minute in music), length (short or long, like duration of musical notes) and regularity (regular or irregular, like a metronome vs. the rhythm of free jazz). Would infant seals react to these rhythmic patterns?
The team tested 20 young seals held at a rehabilitation center in the Netherlands before being released into the wild. Using a method from human infant studies, the team recorded how many times the seals turned their head to look at the sound source that was behind their backs. Such looking behavior indicates whether animals (or infants) find a stimulus interesting. If seals can discriminate between different rhythmic properties, they might look longer or more often when they hear a sequence they prefer.
The seals looked more often when vocalizations were longer, faster or rhythmically regular. This means that the one-year-old seals – without training or rewards – spontaneously discriminated between regular (metronomic) and irregular (arrhythmic) sequences, sequences with short vs. long notes and sequences with fast vs. slow-paced tempo.
“[We have found that] another mammal, apart from us, shows rhythm processing and vocalization learning," Verga said. “This is a significant advance in the debate over the evolutionary origins of human speech and musicality, which are still rather mysterious. Like human babies, the rhythm perception we find in seals arises early in life, is robust and requires neither training nor reinforcement.”
“This is a significant advance in the debate over the evolutionary origins of human speech and musicality, which are still rather mysterious. Like human babies, the rhythm perception we find in seals arises early in life, is robust and requires neither training nor reinforcement.”
Dr. Laura Verga
Next, Verga and her team want to find out whether seals perceive rhythm in vocalizations of other animals or even abstract sounds - and whether other mammals show the same skills: “Are seals special or are other mammals also capable of spontaneously perceiving rhythm?” she wondered.