Dolphins Rub against Mucus-Oozing Corals to Soothe Skin

Like humans, dolphins sometimes suffer from irritated skin. But instead of lathering on soothing lotion, Indo-Pacific bottlenose dolphins (Tursiops aduncus) in the northern Red Sea head for the nearest coral reef. As if they were patrons at a popular spa, the dolphins line up to rub themselves against corals and sponges.

And some of these organisms may do more than just scratch a dolphin’s itch. In a new study published on Thursday in iScience, an international team of researchers discovered that the mucus oozing from some of the involved corals and sponges is loaded with antibacterial compounds and other potentially beneficial substances. The team posits that the local dolphins congregate near these useful invertebrates to actively treat skin infections.

Although rubbing behavior has also been observed in other cetaceans, such as orcas and beluga whales, examples of dolphins rubbing themselves on corals are rare. This is why the dolphins that frequent reefs off the Egyptian coast have garnered so much attention from researchers and tourists alike and have even starred in an episode of the BBC documentary Blue Planet II.

The unusual behavior is more complex than it initially appears, says study co-author Angela Ziltener, a marine biologist at the University of Zurich, who has monitored roughly 360 bottlenose dolphins in the Red Sea since 2009. “When I was diving in the Red Sea, I observed the dolphins doing this really unique behavior with [certain] corals, and I kept wondering, ‘What’s going on?’” she says.

The dolphins seem to be picky when it comes to choosing corals and sponges. They also appear to only rub specific areas of their body on particular specimens. More sensitive areas are scratched with bushy stalks of the soft gorgonian coral Rumphella aggregata—a practice researchers have appropriately termed “gorgoning”—whereas hardened areas, such as the head and tail fluke, are scraped on the wrinkled surface of a species of leather coral in the genus Sarcophyton and a species of stiff sea sponge in the genus Ircinia.

After observing the dolphins under the waves for more than a decade, Ziltener thought there must be something special about these corals and sponges. “There are so many other corals there that they ignore completely,” she says. “There seemed to be a relationship between these corals and sponges and the dolphins.”

To examine this relationship, Ziltener and her colleagues zeroed in on one aspect of the interaction that seemed to impact both creatures: mucus. As the dolphins grind against the corals and sponges, the friction causes the disturbed coral polyps to ooze fluid that sometimes stains the dolphins’ skin bright yellow or green. The researchers even observed one eager dolphin ripping a patch of leather coral off the reef and shaking the coral in its mouth—like a dog with a chew toy—apparently to cause mucus to seep out.

To examine the molecular makeup of the mucus, the researchers collected small samples of gorgonian coral, leather coral and Ircinia sponges from two reefs that serve as dolphin hubs. Back on the boat, the scientists put the samples on ice before sending them to the lab of Gertrud Morlock, an analytic chemist at Germany’s Justus Liebig University Giessen and lead author on the new study. After running the samples through multiple tests, Morlock and her colleagues identified 17 metabolic compounds that combatted multiple strains of bacteria, prevented some damaging cellular processes and balanced hormones in the dolphins’ skin.

The researchers suggest that the dolphins deliberately use these corals and sponges to cover infected areas of skin with the gooey, metabolite-packed mucus, much like a human applying ointment to soothe a rash. If this hypothesis is correct, it would add dolphins to a growing list of self-medicating animals. In the wild world of animal medication, known as zoopharmacognosy, various creatures ranging from great apes to insects utilize natural remedies to stay healthy. Examples include chimpanzees known to scarf down noxious leaves to aid digestion and fruit flies that ingest alcohol to rid themselves of deadly parasites.

Bruno Díaz López, a researcher at Spain’s Bottlenose Dolphin Research Institute, who was not involved with the study, says more behavioral work is needed before coral-scraping dolphins can join the proved ranks of self-medicating animals. In his own research, Lopez has observed dolphins enthusiastically rubbing against stones and submerged shipping lines—and even each other. “We really don’t know yet what is in the dolphin’s mind when they do this behavior,” Lopez says. “It could just be self-pleasure, like a bear scratching its back against a tree.” The metabolite-packed coral mucus, he adds, may simply be an unintended by-product of a dolphin’s natural urge to rub.

Ziltener agrees that more proof of deliberate self-treatment is required, but she says that the new findings are an intriguing step. “We needed to show that these corals and sponges have antibacterial substances to make this connection,” she says. “Now we need to continue to observe the dolphins under the water.”