We’re not the only creatures who pop a pill or slather on an ointment when we’re ill or itchy. Chimpanzees swallow rough leaves to rid their intestines of parasites, for example, and some ants eat foods rich in hydrogen peroxide to expel fungal infections. Now, scientists might be adding dolphins to the list. Researchers have discovered that bottlenose dolphins in the Red Sea rub against corals and sponges that have medicinal properties, possibly to ward off pathogens that cause skin diseases.
“This is very valuable work,” says Michael Huffman, a primatologist and expert on animal self-medication at Kyoto University who wasn’t involved with the research. “I’ve long awaited a really solid study of self-medication in a marine animal species.”
Huffman has studied primate self-medication since the 1980s. He also collects information on instances of the phenomenon across the animal kingdom. But he says such behavior has been tough to study in the ocean because of the logistical challenges of observing animals at sea.
Enter Angela Ziltener, a wildlife biologist with the University of Zürich and the conservation nonprofit Dolphin Watch Alliance. Since 2009, Ziltener has scuba dived in Egypt’s Red Sea among a population of 360 Indo-Pacific bottlenose dolphins (Tursiops aduncus), gaining their trust and observing their behavior up close.
Over the years, she noticed the animals rubbing up against certain corals and sponges. The behavior wasn’t random: The dolphins rubbed their heads on some corals, scraped their bellies on others, and avoided some species altogether (see video, above). Adult dolphins would queue up to rub, whereas the younger members of the group watched closely and slowly began to mimic their actions.
It seemed like the dolphins were getting more than just a good back scratch, Ziltener says. Not only were they selecting specific corals, but as they rubbed, mucus coatings puffed off the corals and sponges, clouding the water and coloring the dolphins’ skin. “They’d get yellowish or greenish,” Ziltener says.
She thought the mucus might feel good or even help heal skin ailments. Past studies have shown dolphins are vulnerable to infections of poxvirus, which causes ringlike lesions on the skin, and fungal diseases such as lobomycosis, which causes cystlike nodules.
To find out whether coatings from the corals’ mucus had medicinal properties, Ziltener teamed up with Gertrud Morlock, an analytical chemist at Justus Liebig University Giessen. The duo took tiny samples of mucus from the dolphins’ three preferred rubbing surfaces: gorgonian coral (Rumphella aggregata), leather coral (Sarcophyton sp.), and a sponge (Ircinia sp.).
The researchers put the samples on ice and brought them to Morlock’s lab, where they used a technique called high-performance thin-layer chromatography to analyze the mucus’ chemical makeup. Similar to how black ink drawn on thin paper bleeds into many colored pigments when moistened with water, the coral mixtures could be separated out into their component parts when applied with chemicals to a special silica plate.
After separating the mucus into its component chemicals, the scientists tested those parts for 10 types of molecules that might have healing or medicinal activities. The corals and sponge coatings contained 17 different biologically active compounds that had antibacterial, antioxidative, or hormonal properties. Those chemicals could potentially treat skin conditions, the team reports today in iScience.
Showing the mucus has healing properties is a good first step, says Eric Angel Ramos, a marine biologist at Rockefeller University who wasn’t involved with the work. To prove that the animals are truly self-medicating, however, “I’d want to see what kind of skin ailments these dolphins are experiencing, and if these corals actually improve their health,” he says.
In his own studies, Ramos has seen bottlenose dolphins in Belize rub against certain corals, too. In light of the new findings, “I want to look back at my own observations,” he says. The animals he’s observed could be self-medicating, or they might just like the way the corals feel. “Dolphins love to touch and play with things,” Ramos says. “They’re like babies that way.”
Science, 19 May 2022