Two adult bottlenose dolphins and one calf swim close to a sandy seafloor that’s dotted with coral.
Two adult bottlenose dolphins and one calf swim close to a sandy seafloor that’s dotted with coral.

Study Suggests Dolphins Use Coral Mucus as Medicine

Researchers observe that dolphins in a pod in the Red Sea regularly rub against certain corals and sponges, perhaps to sooth their skin by prompting the invertebrates to release mucus that contains antimicrobial compounds.

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Dan Robitzski

Dan is a Staff Writer and Editor at The Scientist. He typically works on the news desk and joined the team in 2021. He has a background in neuroscience and earned his master's in science journalism at New York University.

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May 19, 2022

ABOVE: Dolphins swim among coral that releases bioactive, potentially therapeutic mucus when agitated. ANGELA ZILTENER

Chimpanzees have learned to eat plants that kill or remove parasitic worms. Wood ants ward off pathogens by incorporating resin into their nests. Various bird species in Mexico City integrate cigarette butts into their nests to prevent tick infestations. Capuchin monkeys bite millipedes and rub the toxins they produce into their fur, keeping mosquitoes away. The animal kingdom is full of examples of animals that know, either innately or as a result of watching their parents or peers, to use plants and other animals for medicinal purposes. Dolphins may be another such example, according to research published today (May 19) in iScience.

The study makes two key findings: that Indo-Pacific bottlenose dolphins (Tursiops aduncus) repeatedly and deliberately rub against specific corals and sea sponges, and that when these invertebrates are agitated, they release mucus containing cocktails of bioactive compounds with antimicrobial, hormonal, and cytotoxic properties. The researchers extrapolate that the dolphins have learned to use select organisms to treat skin conditions or infections.

Study coauthor Angela Ziltener, a wildlife biologist at the University of Zurich, tells The Scientist that she started to observe this pod of dolphins in 2009 and has since been “adopted”: during her dives, the dolphins engage in seemingly normal behavior in her presence, appear to be comfortable with Ziltener being near their calves, and will return to her depth after surfacing for air and swim at her speed. During some dives, Ziltener says, she noticed that members of the pod would rub against particular types of coral and sponges, and she wanted to figure out what was going on.

Dolphins take turns swimming against coral, potentially for therapeutic purposes.
ANGELA ZILTENER

Ziltener teamed up with colleagues to conduct a detailed chemical analysis of the bioactive compounds secreted by gorgonian coral (Rumphella aggregate), leather coral (Sarcophyton sp.), and sponges (Ircinia sp.), three invertebrates against which the dolphins selectively rub their bellies, fins, and faces. For that part of the study, study coauthor Gertrud Morlock, an analytical chemist and food scientist at Justus-Liebig University Giessen in Germany, made the most of the one-centimeter cuttings of the coral and sponge that the team was allowed to take—and felt comfortable taking—from the reef. To do so, Morlock conducted planar separation chromatography, a technique that breaks a compound and individually tests its active ingredients, which allowed the team to conduct multiple analyses on the properties of each of their limited samples.

“We wanted to have a wide view on the effects” of the bioactive compounds, Morlock tells The Scientist. “We used ten different assays.”

The team found 17 different metabolites in the samples, which they say would be secreted in the invertebrates’ mucus. Some were previously known, providing proof-of-concept validation for their technique, Morlok says; others were new to science. Samples from all three species contained antibacterial compounds, while leather coral also included some hormones.

Ziltener tells The Scientist that they don’t have any evidence that these compounds actually help the dolphins as either a remedy or a prophylactic, however, which is why the study can’t definitively claim that the animals are self-medicating. And taking skin samples from the dolphins to find the answer isn’t feasible under the regulations protecting the dolphins—and might result in Ziltener’s rejection from the pod.

See “Chimps Appear to Treat Others’ Wounds Using Insects

Animal self-medication experts who spoke to The Scientist about the new study agree with Ziltener that the research falls short of conclusive proof of self-medication. Jaap de Roode, an Emory University biologist currently writing a book on animal self-medication, notes that the paper doesn’t show that the dolphins actually receive a medical benefit from the coral. Still, he says that he “wouldn’t be surprised at all” if that were the case. “It’s certainly plausible.” He adds that some of the antibacterial compounds identified could, theoretically, be explored for potential clinical use.

Wesleyan University biologist Michael Singer, who also didn’t work on the study, agrees that the self-medication hypothesis is plausible, but suggests other possibilities are that the dolphins are merely scratching an itch or using the mucus to camouflage their appearance or odor.

Two dolphins swim near the water’s surface. One has fungus growing on its dorsal fin.
A dolphin with a fungal infection on its dorsal fin
ANGELA ZILTENER

Even if the dolphins are indeed self-medicating, Ziltener says it’s not clear whether the animals are aware of doing so, or if they’re simply seeking out something that feels good on their skin, similar to how humans use hand cream. “It’s of course hard to tell that,” she says. “But because the substances have contact with the skin,” and because the dolphins deliberately seek out and press against specific species of invertebrates, the animals are likely seeking out the effects of the specific compounds. She adds that the dolphins will perform the behavior regardless of whether they’re alone or with the pod, sometimes rubbing against coral when they first wake up or before they sleep, as though it were a nighttime routine.

One conclusion Ziltener does draw is that applying the coral and sponge secretions is a learned behavior. Newborn dolphins hang back and watch the older members of the pod—both healthy and sick—as they line up and swim against the specific corals again and again, sometimes grabbing sponges in their mouths and shaking them, thereby coating their faces in the bioactive mucus. When the calves first try it out, they appear timid and uncertain, Ziltener explains, but eventually they get the hang of it and mirror the behavior of their elders, pressing their fins and torso against the corals and rubbing their faces into the sponges. Learned self-medication is common among intelligent animals, explains Jessica Abbott, an evolutionary biologist at Lund University in Sweden who also didn’t work on the study but has published a review on insect self-medication. Other animals, such as insects, appear to innately self-medicate, de Roode adds.

See “Natural-Born Doctors

Going forward, Ziltener plans to continue observing the dolphins in hopes of better understanding this behavior, including determining whether it varies between sexes or among individuals.

Abbott adds that she’d like to know whether dolphins around the world engage in similar behavior, or if this is an isolated cultural phenomenon within the pod that Ziltener observes. For his part, de Roode says he’s thrilled that the phenomenon of animal self-medication is getting more attention—especially given that most examples occur within terrestrial life.

“I think it’s exciting that as the field is growing, people are starting to interpret these behaviors in different ways,” de Roode says, “whereas 20 years ago people may have looked at these dolphins and said ‘I have no idea what they’re doing.’”