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In a in Scientific Reports, UNSW Sydney researchers found that the underwater songs of leopard seals (Hydrurga leptonyx) in Antarctica share structural similarities with the nursery rhymes often sung by humans to their young.

"Leopard seal songs have a surprisingly structured temporal pattern," says Lucinda Chambers, a UNSW Ph.D. 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."

The authors found it was the male seals in particular spending their days singing—but not to lull their babies to sleep.

The 'songbirds' of the Southern Ocean

The leopard seal is a solitary apex predator that lives in Antarctic waters. Here, they rely on floating sea ice to rest upon.

Every spring—from late October to early January—the male leopard seals enter a demanding routine.

Across the Eastern Antarctic pack ice, they can be found performing underwater solos for up to 13 hours a day. They dive in and out of the sea, singing in two-minute cycles: two minutes submerged, two minutes up for air—over and over again, day after day.

"They're incredibly committed," says co-author Professor Tracey Rogers, also from UNSW. She began collecting leopard seal sound recordings as part of her own Ph.D. research in the 1990s.

"It's big business for them," Prof. Rogers says. "They're like the songbirds of the Southern Ocean. During the breeding season, if you drop a hydrophone into the water anywhere in the region, you'll hear them singing."

And the "songs" aren't random but composed of five key "notes"—calls that are common among the population.

"You can't tell them apart by how the call sounds," Prof. Rogers says.

"It's the order and pattern that matters. 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."

A warble to woo and warn

Leopard seals are not only solitary but widely dispersed, staking out their own territories across shifting sea ice. But their songs travel underwater across vast stretches of ocean.

A female leopard seal is in heat for four or five days each year—and while they themselves sing for a few hours across these days, they're most likely bombarded by the sounds of males singing from all directions, all breeding season.

"The greater structure in their songs helps ensure that distant listeners can accurately receive the message and identify who is singing," Chambers says.

The authors say the songs likely serve multiple purposes. While linked to mating—the singing coincides with elevated hormone levels for both sexes—the songs could also signal dominance or fitness from the males.

"It's a bit of a dual message," says Prof. 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."

Chambers says, "It's like they're saying, 'I'm the biggest and the strongest, look how long and how loud I can sing.'"

Melodies of land and sea

To understand how leopard seal songs compare to other species, Chambers and her co-authors looked at entropy—the measurement of how predictable or random a vocal sequence is. The lower the entropy, the more structured and repetitive the song, like in nursery rhymes.

They compared the songs of 26 individual male leopard seals with sequences produced by humpback whales, bottlenose dolphins, squirrel monkeys and several styles of human music: baroque, classical, romantic, contemporary and Beatles songs.

Chambers says what stood out was the similarity between leopard seal songs and the predictability of human nursery rhymes.

"Nursery rhymes are simple, repetitive and easy to remember—that's what we see in the leopard seal songs," Chambers says.

"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."

Voices from the past

The data used in the study came from analog sound recordings made in the 1990s, when Prof. Rogers would bike out across Antarctica to the edge of the sea and mark individual seals with dye before capturing their songs.

"They sing at night, so I would mark them during the day and go back out at night to visit each of the seals to get recordings from different males," Prof. Rogers says.

While the calls themselves are shared, each male arranges them into their own unique sequence, creating a sonic signature that sets one individual apart from another.

"We think it's a bit like each seal having its own name," says Chambers. "They're all using the same alphabet of five sounds—but the way they combine them creates a pattern that's individually distinctive."

This sequencing—rather than differences in frequency or pitch—is what gives each seal its song. Unlike humans, whose voices vary by tone and accent, leopard seals otherwise all sound identical.

Today, with decades of technological advancement, Chambers says she hopes to revisit these Antarctic singers with fresh ears and new tools.

She says the next stage of research will mathematically analyze whether leopard seals use their songs to express individual identity, like bottlenose dolphins do with their own unique 'signature whistle."

"We want to know if new call types have emerged in the population," Chambers says. "And if patterns evolve from generation to generation.

"We'd love to investigate whether their 'alphabet' of five sounds has changed over time."