Leopard Seals Sing Songs In Search of Love And It Resembles ‘Human Nursery Rhymes’
Leopard seals are solitary predators. They patrol the edges of Antarctica’s sea ice, waiting for fish, and mostly, a penguin to jump down. Once the prey is caught in its mouth, it repetitively thrashes, smashes, and flips the penguin on the ice until it is dead and ready to be eaten. From spring to summer, the seal remains solitary, just hunting, eating, and resting. But as autumn ushers in, the lonely seal starts yearning for a mate. To fulfil this biological longing, the males sing “love songs” that resemble “human nursery rhymes,” recently documented in Scientific Reports.
The dramatic episode unfolds every season, mostly from October to January. With the onset of the breeding season, the leopard seal starts hankering for a mate. But since it has remained solitary through the spring and summer, it becomes challenging to find one in the vast stretch of ice. To overcome the challenge, male leopard seals spend days and weeks perfecting their underwater solos. And when the time comes, they dip into the water and start singing songs through the night.
While gliding through the waters and clobbering through the sea ice, they make deep guttural sounds like blowing bubbles, as a mechanism to signal their territory and attract a female for mating. These high-pitched love songs are supposed to act like “advertisements” of their individual identity and fitness, which will help them attract a female. The romance continues throughout the breeding season, lasting from two to four months. After the end of the season, males cease their mating calls due to the drop in their reproductive hormones until the next breeding cycle.
For this study, researcher Tracey Rogers from UNSW Sydney and her colleagues hopped on their quad bikes to ride to the coastal region of Davis Sea in Eastern Antarctica. Stopping by the edge of the sea, they tracked 26 individual male leopard seals as they slept and marked their bodies with dye and oil-based paints to prevent identity mix-ups. As it turned out, it wasn’t just one particular sound or song, but an entire cacophony made of multiple sound patterns and sequences. They noticed that while female seals sang for a few hours during the day, the males kept on singing throughout the night. At times, their solo performances continued for up to 12 hours a night.
To deconstruct the mystery of these love songs, the first parameter Rogers and her team employed was to test the “entropy.” In information theory, “entropy” is the information contained within a sequence of sounds. Investigation revealed five key notes, including low double trill, hoot with low single trill, high double trill, low descending single trill, and medium single trill. Then, they compared the songs of the male leopard seals with human-related styles like baroque, classical, romantic, contemporary, songs of The Beatles, and others. In the end, they found that the songs were similar to “human nursery rhymes” more than anything else.
Like these rhymes, the songs were “simple, repetitive and easy to remember,” as Rogers wrote in The Conversation. The range of the songs’ entropy was found to be similar to that of 39 nursery rhymes from the Golden Song Book, including Twinkle, Twinkle, Little Star, Ring Around a Rosy, Baa, Baa Black Sheep, Humpty Dumpty, and Rockabye Baby. This is to say that, no matter how ferociously predatory these leopard seals are during the summer, love for them, is a lullaby.
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