Dolphins use coral as skin medicine, study finds

In the Red Sea off the coast of Egypt, bottlenose dolphins were spotted in 2009 doing something unusual. They lined up to rub their bodies against the coral.

They were selective about the type of corals they rubbed against, noted wildlife biologist Angela Ziltener, a visiting researcher at the University of Zurich who spent the next 13 years trying to unravel the puzzling behavior.

The results of his extensive research into the community of 360 dolphins were released on Thursday.

By observing dolphins and studying the properties of the coral, Ziltener and his colleagues discovered that the dolphins seemed to use the reef like a medicine chest: bioactive compounds in the mucus released by two different types of coral and a sea sponge help probably the dolphins to protect their skin.

This is the first time this type of behavior has been observed in cetaceans – the scientific order of marine mammals that includes dolphins, whales and porpoises – according to the study. However, some birds, mammals, insects and reptiles have been observed using plant parts or other substances to fight pathogens or parasites.


It took Ziltener years of scuba diving with the local dolphin population to earn their trust. “You have to be kind of adopted by the dolphins. It took time to see all their secrets,” she said.

The dolphins only rubbed shoulders with a gorgonian coral known as Rumphella aggregata, leather coral Sarcophyton sp. and the sea sponge Ircinia sp., observed Ziltener. Moreover, they used organisms in different ways.

With leather corals and sponges – which are more compact and harder in texture than soft gorgonian coral branches – dolphins tended to push an isolated body part and twist it, according to the study. Instead, they slid their entire bodies into the gorgonian coral several times, rubbing multiple body parts at once.

The behavior of dolphins rubbing against gorgonian coral, nicknamed gorgonians, and Ziltener’s research were first revealed in 2017 in the BBC documentary “Blue Planet II” and a number of other documentaries on the nature. However, this is the first time that a detailed study of the behavior has been published in a scientific journal.

When in groups, dolphins often line up and take turns brushing against gorgonian coral. Interacting with the Leather Coral didn’t seem like a group activity.

With the leather coral, a dolphin would sometimes uproot it from the ground and carry it in its mouth for a few minutes, rocking it – an action that caused the compounds in the coral to leak out and spread around the dolphin’s head, staining it. yellow and green.


Because the reef is protected, the team got permission to take small samples – just one centimeter – of the corals and sea sponges used by the dolphins. Analysis of the study revealed that these organisms contained 17 bioactive compounds, with different properties, such as antibacterial, antioxidant or hormonal attributes, said co-author Gertrud Morlock, an analytical chemist and professor of food science at the Justus Liebig University of Giessen in Germany.

The three different organisms showed similar and some different effects, Morlock said.

“The commonality was that all three had a wealth of antibacterial and antimicrobial activities. And what was special about the leather corral, for example, it contained estrogen-like compounds while the other two did not.”

“We were surprised to find that there were so many (compounds),” she said. “We think (the dolphins) very clearly select for these substrates, and we’ve proven that they have bioactive compounds and when they rub on it (the coral), their skin is in direct contact with these molecules.”


The purpose of the behavior is likely to regulate and protect the skin’s microbiome — much like how humans might use skin cream, Morlock explained. She said the research team had no definitive evidence that dolphins used coral as medicine, although dolphins regularly suffer from fungal infections and rashes.

Not all pod dolphins rub against coral. Young calves less than a year old are just watching, Ziltener said. This has led researchers to believe that behavior is learned rather than innate.

“Initially, this behavior could have arisen on impulse or instinct, or simply by chance. Perhaps a dolphin with irritated skin rubbed itself along a random coral that “released skin-healing chemicals. The relieved dolphin remembered the behaviors and repeated them, then taught those behaviors to others, as in the case of the Australian people wearing a big-nosed sponge,” said Diana Barrett, a lecturer in the biology department at the University of Massachusetts Dartmouth, who was not involved in the research.

“This ability to remember behaviors and their resulting effects and then repeat those behaviors to treat future skin problems adds (to) the wealth of evidence that dolphins are intelligent,” she said.

Dolphins have long been considered highly intelligent animals that can communicate and use tools, such as seashells, to help them hunt. Ziltener said it’s possible that other marine animals use corals in this way, but observing underwater animals systemically is difficult.

Ziltener said dolphins often wake up after a nap to perform coral rubbing behavior.

“It’s almost like they’re showering, cleaning up before going to sleep or getting up for the day,” she said.