Leopard seal (Hydrurga_leptonyx). Credit: Andrew Shiva / Wikipedia / CC BY-SA 4.0
Leopard seals have massive heads, powerful jaws and distinctive spotted coats, earning them a fierce reputation as one of the top predators stalking the Antarctic.
Did you know they’re also incredibly talented singers?
A new study published in Scientific Reports shows these “songbirds of the Southern Ocean” belt out tunes which share remarkable structural similarities with human nursery rhymes.
“Nursery rhymes are simple, repetitive and easy to remember – that’s what we see in the leopard seal songs,” Lucinda Chambers, a PhD candidate at Australia’s University of New South Wales and lead author of the study.
“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.”
Leopard seals (Hydrurga leptonyx) live and hunt alone. But in the southern hemisphere’s spring, males put on performances from late October to early January to entice females to their sides.
In Eastern Antarctica, they busk for hours each day. It’s done in cycles, with 2 minutes spent on underwater opera and 2 minutes catching their breath on free-floating sea ice, also known as pack ice.
“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,” says co-author Professor Tracey Rogers from UNSW.
The songs aren’t random riffs. They’re made up of 5 distinct calls – high double trills, medium single trills, low descending trills, low double trills and a hoot with a low single trill – which are arranged in unique sequences.
“You can’t tell them apart by how the call sounds,” explains Rogers. “It’s the order and pattern that matters.”
The researchers examined the estimated “entropy” – a measurement of how predictable or random a vocal sequence is – in 26 individual male leopard seal songs.
“A sequence with higher entropy is more random, or less predictable, than a sequence with lower entropy,” they write in the study.
The team compared this estimated entropy across songs of other mammals, including humpback whales, bottlenose dolphins and squirrel monkeys.
They also did this for human music, specifically the progression of musical notes in the melodies of nursery rhymes, songs by the Beatles and music by Classical, Baroque and Romantic composers.
“The entropy estimates of the leopard seals being higher than those of the humpback whale suggests that there is comparatively less predictability within the leopard seal sequences, and as such more information is encoded in the structure of the sequences,” the authors write.
“The estimated information entropy of the leopard seal songs is comparable to nursery rhymes but unsurprisingly, lower than contemporary, classical and baroque music.”
“They’ve stylised it to an almost boring degree, which we think is a deliberate strategy, so their call carries a long distance across the ice,” says Rogers.
Getting the message out loud and clear is important when the females of such a widely dispersed species are in heat for only 4 or 5 days each year.
“We think it’s a bit like each seal having its own name,” says Chambers. “They’re all using the same alphabet of 5 sounds – but the way they combine them creates a pattern that’s individually distinctive.
“The greater structure in their songs helps ensure that distant listeners can accurately receive the message and identify who is singing.
“It’s a bit of a dual message. 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 the next stage of research will mathematically analyse 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 and if patterns evolve from generation to generation,” she says. “We’d love to investigate whether their ‘alphabet’ of 5 sounds has changed over time.”