Bottlenose dolphins glow-up with coral body scrubs

To keep skin healthy, dolphins will even wait in line for these coral and sea sponge spas.
three dolphins near the sea floor rub themselves against coral
These swimmers have a clever skin routine to avoid infections. Here, a dolphin mother teaches her calf to rub against medicinal coral. Angela Ziltener

In the waters of the Red Sea off the coast of Egypt, bottlenose dolphins have a highly unusual relationship with their reef habitat. Divers frequently witness the marine mammals rubbing their faces and bodies against certain corals and sponges, even waiting in orderly lines to approach the invertebrates. 

“You get very excited to see that for the first time,” says Angela Ziltener, a wildlife biologist at the University of Zurich who has studied dolphins in this region for more than a decade. When she initially witnessed the behavior, she wondered if the dolphins were brushing up against the corals because it simply felt good or if there was something more to it.

To find out, Ziltener and her collaborators analyzed tissue samples from the invertebrates. They identified 17 chemicals that were bioactive, or likely to have an effect on living cells or tissues. This suggests that rubbing against the corals and sponges might actually help the dolphins keep their skin healthy and fight infections, the team reported on May 19 in the journal iScience.

“These are potent active compounds that we found,” says Gertrud Morlock, an analytical chemist and food scientist at Justus Liebig University Giessen in Germany and another coauthor of the paper. When they come into contact with the body, she says, the coral compounds may work similarly to the antibacterial creams people put on their skin.

Scientists have occasionally observed dolphins performing similar rubbing behaviors in the Bahamas and Florida, she and her colleagues wrote. The population they focused on included 360 Indo-Pacific bottlenose dolphins in the Northern Red Sea. As the researchers watched the dolphins visiting the corals and sponges, they noticed a few patterns. 

Calves under one year of age hung back and watched the adults brush themselves against the invertebrates. The adults only did the rubbing under calm and quiet conditions. “If they’re disturbed by boats, you don’t see this behavior,” Ziltener says. “They need to be relaxed.”

Groups of dolphins regularly queued up behind each other and took turns gliding towards and rubbing against the coral or sponge. “It’s very organized,” Ziltener comments.

And not just any invertebrate would do. The dolphins zeroed in on soft, branching gorgonian corals as well as harder leather corals and certain sponges. They often slid into the branches of gorgonian corals multiple times, allowing several body parts to brush against them. By contrast, the dolphins pushed a particular body part such as the head or fluke forcefully against the surfaces of the more compact organisms. On occasion, the dolphins would pull a leather coral from the seabed, carry it in their mouths for a few minutes, and then wave the coral around until yellowish or greenish substances leaked out and stained their head and snout.

The researchers identified a variety of intriguing properties in compounds extracted from tiny pieces of the dolphins’ favored corals. Many of the molecules combatted bacteria, Morlock says, and several resembled the hormone estrogen (which in humans helps keep the skin firm and moisturized).

[Related: Supercharged blood helps some dolphins dive 1,000 meters]

While the findings are suggestive, they aren’t enough to prove that the dolphins are seeking out the invertebrates to self-medicate, Ziltener acknowledges. “So far in this publication, we just can show the link between the invertebrates and the dolphins,” she says. 

While skin samples from the dolphins might provide more conclusive evidence that secretions from the corals and sponges help prevent or control infections, the researchers intend to disturb the marine mammals as little as possible. However, Ziltener says, investigating the dolphins’ behavior in more detail may offer insights about when and why they rub against the marine invertebrates. She plans to track how much coral-rubbing differs based on the age or sex of the dolphins, and which body parts the dolphins press against the organisms most often. 

Moreover, Ziltener adds, the findings highlight the importance of conserving vulnerable reef ecosystems.

“There’s so much more to discover in coral reefs,” she says.