They were picky about what type of coral they were rubbing, said wildlife biologist Angela Ziltener, a visiting researcher at the University of Zurich, who spent the next 13 years trying to unravel the confusing behavior.
The results of her extensive study on the 360-dolphin community were released Thursday.
Observing dolphins and studying the properties of corals, Ziltener and her colleagues found that dolphins appear to use the reef as a medical box: Bioactive compounds in the mucus released from two different types of coral and sea sponge may help dolphins protect their skin.
This is the first time this type of behavior has been observed in cetaceans – the scientific order of marine mammals, which includes dolphins, whales and guinea pigs – the study said. However, some birds, mammals, insects and reptiles have previously been observed using parts of plants or other substances to fight pathogens or parasites.
Building trust
It took Ziltener years to dive with the local dolphin population to gain their trust. “You have to be adopted by dolphins. It took time to see all their secrets, “she said.
Dolphins rubbed only in Gorgon coral, known as Rumphella aggregata, leather coral Sarcophyton sp. and the sea fungus Ircinia sp., observed by Ziltener. Moreover, they have used organisms in different ways.
With leather corals and mushrooms – which are more compact and harder in texture than the soft branches of gorgonian coral – dolphins tend to press on an isolated part of the body and twist it, the study found. In contrast, they slid their entire body into the Gorgonian coral several times, rubbing several body parts at once.
Gorgoning friction behavior of dolphins, called gorgoning, and Ziltener’s research were first revealed in 2017 in the BBC documentary Blue Planet II and a number of other nature documentaries. However, this is the first time a detailed study of behavior has been published in a scientific journal.
When in groups, dolphins often line up and take turns rubbing against Gorgonian coral. Interacting with leather coral did not seem like a group activity.
With leather coral, a dolphin sometimes uproots it from the ground and carries it in its mouth for a few minutes, twisting it around, an action that causes compounds to leak out of the coral and spread around the dolphin’s head, turning it yellow and green.
Coral samples
Because the reef is protected, the team was allowed to take small samples – just an inch – of coral and sea sponge used by dolphins. The study found that these organisms contained 17 bioactive compounds with various properties, such as antibacterial, antioxidant or hormonal properties, said co-author Gertrude Morlock, an analytical chemist and professor of food science at the Justus Liebig University in Giessen in Germany.
The three different organisms showed similar and some different effects, Morlock said.
“The common denominator was that all three had plenty of antibacterial and antimicrobial action. And what was special about leather coral, for example, was that it contained estrogen-like compounds, while the other two didn’t.”
“We were surprised to find that there are so many (compounds),” she said. “We think that (dolphins) choose these substrates very clearly and we have proven that they have bioactive compounds and when they rub on it (coral), their skin is in direct contact with these molecules.
Skin treatment
The purpose of the behavior is likely to regulate and protect the skin microbiome – a bit like how people can use skin cream, Morlock explained. She said the research team had no conclusive evidence that dolphins used coral as a form of medicine, although dolphins regularly suffer from fungal infections and skin rashes.
Not every dolphin in the pod rubs against coral. Young calves under 1 year old are just watching, Ziltener said. This has led researchers to believe that behavior is learned, not innate.
“Initially, this behavior could have occurred as a result of aspiration or instinct, or just by accident. Maybe a dolphin with irritated skin, rubbed on any coral that releases chemicals to heal the skin. The dolphin remembered the behavior with relief and repeated it, then taught it to others, as in the case of the Australian bottlenose dolphin population, “said Diana Barrett, a professor of biology at the University of Massachusetts Dartmouth who is not participated in the study.
“This ability to remember the behavior and its effects and then repeat the behavior to treat future skin problems adds to the wealth of evidence that dolphins are intelligent,” she said.
Dolphins have long been considered extremely intelligent animals, able to communicate and use tools such as shells to help them hunt. Ziltener said it is possible for other marine animals to use corals in this way, but it is difficult to monitor underwater animals systematically.
Ziltener said dolphins often wake up from a nap to perform coral friction.
“It’s almost like bathing, cleaning up before going to sleep or getting up for the day,” she said.
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