CassandraSm/ShutterstockLate in the evening, the Antarctic sky flushes pink. The male leopard seal wakes and slips from the ice into the water. There, he’ll spend the night singing underwater amongst the floating ice floes. For the next two months he sings every night. He will sing so loudly, the ice around him vibrates. Each song is a sequence of trills and hoots, performed in a particular pattern. In a world first, we analysed leopard seal songs and found the predictability of their patterns was remarkably similar to the nursery rhymes humans sing.We think this is a deliberate strategy. While leopard seals are solitary animals, the males need their call to carry clearly across vast stretches of icy ocean, to woo a mate. Solitary leopard seals want their call to carry. Ozge Elif Kizil/Anadolu Agency via Getty Images A season of underwater solosLeopard seals (Hydrurga leptonyx) are named after their spotted coats. They live on ice and surrounding waters in Antarctica. Leopard seals are especially vocal during breeding season, which lasts from late October to early January. A female leopard seal sings for a few hours on the days she is in heat. But the males are the real showstoppers.Each night, the males perform underwater solos for up to 13 hours. They dive into the sea, singing underwater for about two minutes before returning to the water’s surface to breathe and rest. This demanding routine continues for weeks.A male leopard seal weighs about 320 kilograms, but produces surprisingly high-pitched trills, similar to those of a tiny cricket.Within a leopard seal population, the sounds themselves don’t vary much in pitch or duration. But the order and pattern in which the sounds are produced varies considerably between individuals.Our research examined these individual songs. We compared them to that of other vocal animals, and to human music.Listening to songs from the seaThe data used in the study was collected by one author of this article, Tracey Rogers, in the 1990s. Rogers rode her quad bike across the Antarctic ice to the edge of the sea and marked 26 individual male seals with dye as they slept. Then she returned to record their songs at night. The new research involved analysing these recordings, to better understand their structure and patterns. We did this by measuring the “entropy” of their sequences. Entropy measures how predictable or random a sequence is.We found the songs are composed of five key “notes” or call types. Listen to each one below. A low double trill. Tracey Rogers UNSW Sydney, CC BY-SA28.5 KB (download) A hoot with low single trill. Tracey Rogers UNSW Sydney, CC BY-SA53.8 KB (download) High double trill. Tracey Rogers UNSW Sydney, CC BY-SA29.7 KB (download) Low descending single trill. Tracey Rogers UNSW Sydney, CC BY-SA49 KB (download) Medium single trill. Tracey Rogers UNSW Sydney, CC BY-SA22.7 KB (download) A remarkably predictable patternWe then compared the songs of the male leopard seals with several styles of human music: baroque, classical, romantic and contemporary, as well as songs by The Beatles and nursery rhymes. What stood out was the similarity between the predictability of human nursery rhymes and leopard seal calls. Nursery rhymes are simple, repetitive and easy to remember — and that’s what we heard in the leopard seal songs.The range of “entropy” was similar to the 39 nursery rhymes from the Golden Song Book, a collection of words and sheet music for classic children’s songs, which was first published in 1945. It includes classics such:Twinkle, Twinkle, Little StarFrère JacquesRing Around a RosyBaa, Baa, Black SheepHumpty DumptyThree Blind MiceRockabye Baby.For humans, the predictable structure of a nursery rhyme melody helps make it simple enough for a child to learn. For a leopard seal, this predictability may enable the individual to learn its song and keep singing it over multiple days. This consistency is important, because changes in pitch or frequency can create miscommunication.Like sperm whales, leopard seals may also use song to set themselves apart from others and signal their fitness to reproduce. The greater structure in the songs helps ensure listeners accurately receive the message and identify who is singing. Male leopard seals produce high-pitched cricket-like trills. An evolving song?Leopard seals sound very different to humans. But our research shows the complexity and structure of their songs is remarkably similar to our own nursery rhymes. Communication through song is a very common animal behaviour. However, structure and predictability in mammal song has only been studied in a handful of species. We know very little about what drives it.Understanding animal communication is important. It can improve conservation efforts and animal welfare, and provide important information about animal cognition and evolution. Technology has advanced rapidly since our recordings were made in the 1990s. In future, we hope to revisit Antarctica to record and study further, to better understand if new call types have emerged, and if patterns of leopard seal song evolve from generation to generation.Tracey Rogers receives funding from ARC. Lucinda Chambers does not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.