Researchers at Germany’s University of Giessen have found that bottlenose dolphins inhabiting the Red Sea routinely rub themselves against specific types of coral, possibly to benefit from the coral’s antibacterial properties. Using Dolphin Watch Alliance’s long-term observation of hundreds of dolphins as a jumping off point, the team’s research focuses mainly on why the dolphins participate in coral-rubbing: specifically how they learn to do it, and what they have to glean from it.
Dolphin Watch Alliance has witnessed numerous coral-rubbing (or “gorgoning”) behaviors since 2009, when it began observing the dolphins’ activity via boat surveys and SCUBA sessions. Since the beginning, the dolphins have appeared to be picky about which types of coral they use to self-medicate. In a paper published last week in iScience’s Cell Press, the researchers from the University of Giessen describe how bottlenose dolphins will rub any part of their body on gorgonian coral. Meanwhile, the dolphins selectively rub their heads, bellies, and tail fins on softer leather coral, as well as sea sponges. The dolphins will ignore all of these options if they’re growing too close to venomous species that might irritate the skin.
An adult dolphin rubbing against gorgonian coral (Image: Angela Zillener/Dolphin Watch Alliance)
But the dolphins don’t appear to be using corals to scratch an itch. Gorgonian coral and leather coral release mucus when agitated—mucus that has now been found to contain antibacterial properties. The researchers write that a coating of this mucus might help dolphins maintain skin homeostasis and avoid microbial infections. There’s also the chance that the mucus might feel soothing.
The bottlenose dolphins even ensure others in their pods get a turn with the coral, making group gorgoning events a polite affair. “When in groups, dolphins are often observed queueing up behind each other to wait their turn to approach the invertebrate,” the paper reads. Juvenile dolphins (or calves) were not witnessed rubbing against coral until they were at least a year old, at which point they carefully joined in with their pods. This suggests the dolphins are not biologically predisposed to self-medicating via coral, but instead learn it in a social environment.
The scientists point out that this is yet another motive to protect coral reefs, which have experienced massive losses in recent decades thanks to overfishing, pollution, careless tourism, and other dangerous phenomena. Coral has previously been understood to participate in symbiotic relationships with other aquatic species like algae and sea stars. By adding dolphins to the list, conservationists may be able to argue for new and enhanced marine protections.
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