Rubbing against coral seems to be a popular skin treatment in dolphins

Bottlenose dolphins appear to seek out certain corals and sponges that produce compounds with antibacterial or hormone-like properties

By Alex Wilkins

Dolphins may be treating their own wounds and infections by rubbing themselves against corals and sea sponges that have antibacterial properties.

Many marine mammals, such as orcas and beluga whales, rub their bodies against underwater materials like sand, pebbles or limestone, perhaps to help shed the outer layers of their skin in the summer. But similar behaviour has been less well studied in dolphins.

Angela Ziltener at the University of Zurich, Switzerland, and her colleagues filmed Indo-Pacific bottlenose dolphins (Tursiops aduncus) in the Red Sea off Egypt queueing up to rub against specific corals and sea sponges.

They noticed that the dolphins would repeatedly rub certain body parts on the corals and sponges and that some of the corals released mucus as the dolphins swam past, so they suspected the animals might be self-medicating.

“They always come back to the same organism and they are really rubbing different body parts on them,” says Ziltener. “It’s not observed in sand or in seagrass, for example; it’s a different behaviour. [They’re] queueing up and waiting for their turn.”

The researchers took samples of gorgonian coral (Rumphella aggregata), leather coral (Sarcophyton sp.) and a sea sponge (Ircinia sp.), then analysed them using a high-resolution spectrometer in the lab to identify chemicals in the samples. They found 17 bioactive compounds with antibacterial, antioxidant or hormone-like properties.

Ziltener and her team think the dolphins may seek out these specific corals and sponges to help keep their skin healthy and perhaps treat bacterial infections. However, we can’t yet be sure whether the behaviour has a medicinal purpose.

Jason Bruck at Stephen F. Austin State University in Texas suggests the next step should be to look more closely at the dolphins’ behaviour to see whether they really are selective about which body parts they rub on which materials.

“I also would encourage work looking at how these compounds affect the very unique skin structure of dolphins,” he says. “Dolphins replace their outermost skin layer, at most, every two hours – a rate nine times faster than that of humans. This unique dermal anatomy and physiology means we cannot assume the medicinal effects of the corals would be the same for dolphins as they would be for us.”

Journal reference: iScience, DOI: 10.1016/j.isci.2022.104271

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