Like humans, dolphins sometimes suffer from skin irritation. But instead of lathering a soothing lotion, the Indo-Pacific bottlenose dolphins (Tursiops aduncus) in the northern part of the Red Sea head to the nearest coral reef. Like visitors to a popular spa, dolphins line up to rub against corals and sponges.
And some of these organisms can do more than just scratch dolphins itch. In a new study published Thursday iScienceAn international team of researchers has found that the mucus that comes from some corals and sponges is saturated with antibacterial compounds and other potentially beneficial substances. The team claims that local dolphins gather around these beneficial invertebrates for the active treatment of skin infections.
Although friction behaviors have been observed in other cetaceans such as killer whales and belugas, examples where dolphins rub against corals are rare. That’s why dolphins, which are common on reefs off the Egyptian coast, have attracted so much attention from both researchers and tourists and even starred in an episode of a BBC documentary Blue Planet II.
Unusual behavior is more complex than it seems at first glance, says study co-author Angela Ziltener, a marine biologist at the University of Zurich who has observed about 360 bottlenose dolphins in the Red Sea since 2009. “When I dived into the Red Sea Sea, I watched dolphins do this really unique behavior [certain] corals, and I kept wondering, “What’s going on?” She says.
Dolphins seem picky when it comes to choosing corals and sponges. They also seem to only rub certain parts of their body on certain patterns. More sensitive areas are scratched by bushy stems of soft coral gorgonians Rumphella aggregata-researchers-practitioners have appropriately called “gorgonization” – while hardened areas, such as the head and tail of the bivalve, are scraped off on the wrinkled surface of a species of leather coral of the genus Sarcophyton and a species of hard sea sponge in the genus Ircinia.
Watching 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 that they completely ignore,” she says. “There seemed to be a connection between these corals, sponges and dolphins.”
To explore this relationship, Ziltener and her colleagues focused on one aspect of the interaction that seemed to affect both beings: mucus. When dolphins rub against corals and sponges, friction causes disturbed coral polyps to leak fluid, which sometimes stains the dolphins ’skin bright yellow or green. Researchers even watched as one impatient dolphin plucked a piece of leather coral from the reef and shook the coral in its mouth – like a dog with a chewing toy – apparently to secrete mucus.
To study the molecular composition of mucus, researchers collected small samples of Gorgon coral, leather coral and Ircinia sponges from two reefs that serve as dolphin centers. Returning to the boat, the scientists put the samples on ice before sending them to the lab of Gertrude Morlach, a chemist-analyst at the German University of Just Liebig Giessen and lead author of the new study. After conducting samples through several tests, Morlac and her colleagues found 17 metabolic compounds that fought multiple strains of bacteria, prevented some damage to cellular processes, and balanced hormones in dolphin skin.
Researchers believe that dolphins deliberately use these corals and sponges to cover infected areas of skin with sticky mucus saturated with metabolites, similar to what a person applies ointment to soothe a rash. If this hypothesis is correct, it will add dolphins to the growing list of animals engaged in self-medication. In the wild animal medicine known as zoopharmacognosy, various creatures, ranging from large monkeys to insects, use natural remedies to stay healthy. Examples include chimpanzees known scarf down harmful leaves to aid digestion and fruit flies that consume alcohol to get rid of deadly parasites.
Bruno Diaz Lopez, a researcher at the Spanish bottlenose dolphin institute who was not involved in the study, says more behavioral work is needed before dolphins scraping out corals will be able to join the proven range of animals involved in self-medication. In his own research, Lopez watched dolphins enthusiastically rub against rocks and underwater shipping lines – and even each other. “We don’t yet know what a dolphin thinks when they behave like that,” Lopez says. “It can be just complacency, like a bear scratching its back against a tree.” Metabolic coral mucus, he adds, may simply be an unintended byproduct of dolphins ’natural desire to rub.
Ziltener agrees that more evidence of deliberate self-medication is required, but she says the new findings are an intriguing move. “We needed to show that these corals and sponges have antibacterial substances to make that connection,” she says. “Now we need to keep watching the dolphins underwater.”