Leopard seals may be one of Antarctica’s most fearsome predators, but these vocalizers sing with the structured charm of a child’s nursery rhyme. In a surprising new study, researchers found that the underwater vocal patterns of these solitary marine mammals are so predictable and orderly, they closely resemble the rhythmic repetition found in human song.
Researchers from The University of NSW analyzed 1,900 vocal elements taken from underwater leopard seal (Hydrurga leptonyx) recordings, using computational linguistics to examine whether there were distinct patterns to the noise. They compared the samples to the kind of structure and pattern found in English speech, birdsong, human nursery rhymes and random noise.
What they found was that their vocalization had "low entropy" – meaning their songs were highly structured and predictable in pattern. And this predictability most closely resembled that of nursery rhymes. Their songs included repeated sequences, consistent transitions and ordered arrangements of elements – which were more pronounced during mating season.
What's more, each individual had its own signature in its song, even if those tunes followed a rough template across all seals.
“Leopard seal songs have a surprisingly structured temporal pattern,” says Lucinda Chambers, a UNSW PhD candidate and lead author of the study. “When we compared their songs to other studies of vocal animals and of human music, we found their information entropy – a measure of how predictable or random a sequence is – was remarkably close to our own nursery rhymes.”
From late October to early January, the male seals belt out their solos for up to 13 hours each day, singing in two-minute bursts underwater before taking a quick breather on the ice above.
“They’re incredibly committed,” said co-author Professor Tracey Rogers, who has been sampling leopard seals since the 1990s. “It’s big business for them.
“They’re like the songbirds of the Southern Ocean," she added. "During the breeding season, if you drop a hydrophone into the water anywhere in the region, you’ll hear them singing.”
It's a big commitment but it's also a high stakes game. Females are in heat for only four to five days a year. However, because the animals are solitary, the males ultimately sing into the void for months on end, throughout the entire breeding season.
“The greater structure in their songs helps ensure that distant listeners can accurately receive the message and identify who is singing,” Chambers explained.
And the researchers believe the personal touches in delivery of their songs could also help them show off their vocal capacity to a distant female, hoping to also impress her with their stamina.
“It’s a bit of a dual message,” said Rogers. “It could be a ‘this is my patch’ to other males and also a ‘look how strong and lovely I am’ to the females.
“It’s the order and pattern that matters," she added. "They’ve stylized it to an almost boring degree, which we think is a deliberate strategy, so their call carries a long distance across the ice.”
The team also compared the tunes of 26 individual seals to other vocal species including humpback whales, bottlenose dolphins and squirrel monkeys, as well as different styles of music – baroque, classical, romantic, contemporary and Beatles songs.
And, ultimately, the researchers found that the leopard seals had a species-specific way of serenading underwater, and it all came back to having most in common with nursery rhymes.
“Nursery rhymes are simple, repetitive and easy to remember – that’s what we see in the leopard seal songs,” Chambers said. “They’re not as complex as human music but they aren’t random either. They sit in this sweet spot that allows them to be both unique and highly structured.”
And each male relies on his own specific sequence of "notes," believed to be a tool to make them stand out from the crowd.
“We think it’s a bit like each seal having its own name,” Chambers added. “They’re all using the same alphabet of five sounds – but the way they combine them creates a pattern that’s individually distinctive.”
The researchers now plan to mathematically analyze the songs in an effort to tease out more nuanced aspects to their sounds and determine just how individualistic they may or may not be.
“We want to know if new call types have emerged in the population,” said Chambers. “And if patterns evolve from generation to generation."
While the study expands our understanding of elusive leopard seal communication, it also reveals new insight into their cognitive abilities and could potentially be used as a tool in non-invasive tracking of individuals and populations. Considering a key non-invasive research method is collecting seal scat (feces), studying their songs seems like more enjoyable work.
Take a listen in the following video:
The research was published in the journal Scientific Reports.
Source: The University of NSW
