When wildlife biologist Angela Ziltener went on deep-sea dives, she often observed something extraordinary. A group of Indo-Pacific bottlenose dolphins (Tursiops aduncus) lined up and took turns rubbing their bodies against corals or sea sponges along the seafloor. After observing the behavior for over a decade as an "adopted member of the pod," the University of Zurich scientist and her team found that the sea mammals may be using the invertebrates as pharmacies to self-medicate, reports Erin Garcia de Jesús for Science News. The study detailing the find was published this month in iScience.
The dolphins brush up against soft gorgonian corals (Rumphella aggregata), sturdier leather corals (Sarcophyton sp.) and other specific sponges (Ircinia sp.). The sea mammals slide into the branches multiple times, and push body parts like their heads or flukes against more compact corals and sponges, Popular Science's Kate Baggaley reports.
Some dolphins even rip out leather coral from the seabed, hold it in their mouths, and wave it around until a yellow and green substance oozes out and stains their snouts and bodies. The invertebrates may make antibacterial compounds that they release into the water that give the dolphins healthy skin, per Science News. Overall, the gathering is peaceful, and dolphins won't fight each other to rub against the coral. "It's not like they're fighting each other for the turn," Ziltener says to Science News. "No, they wait, and then they go through." Previous studies have found that dolphins are vulnerable to skin ailments like poxvirus infections or fungal diseases like lobomycosis, Science reports.
Researchers also observed adult dolphins teaching young calves the behavior. When a group of 360 dolphins visited corals located in the Northern Red Sea, they noticed that calves under one year old would watch adults brush themselves against the coral, reports Popular Science. Adult dolphins only rubbed against the invertebrates under quiet and calm conditions. If boats disturbed the area, dolphins didn't engage in the behavior.
“This is very valuable work,” Michael Huffman, an expert on animal self-medication at Kyoto University not involved with the research, tells Tess Joose for Science. “I’ve long awaited a really solid study of self-medication in a marine animal species.” After analyzing the pieces of the dolphins' preferred coral and sponges for rubbing, researchers found 17 total compounds in the invertebrates, per Science News. Ten of the compounds had antibacterial properties or antimicrobial activity. Other compounds resembled the estrogen hormone. Estrogen helps keep the skin firm and moisturized in humans, Popular Science reports.
More research is needed to confirm if the dolphins are buffing against coral and sponges to treat skin infections since experts have not observed or seen proof of a coral cure, Science News reports. The team plans to look into how the coral rubbing behavior differs in dolphins of different sexes and ages—and which areas of the body dolphins brush the most. Ziltener tells Popular Science that while more studies are needed, the find reveals the importance of conserving coral reef systems. "So far in this publication, we just can show the link between the invertebrates and the dolphins," Zeiltener tells Popular Science.