Dolphins in the Red Sea could use coral for ‘self-medication’

Thirteen years ago, Angela Ziltener was diving in the Red Sea off the Egyptian coast and observed a group of Indo-Pacific bottlenose dolphins as they played in and around a coral reef about 45 feet below the water’s surface. A marine biologist and president of the nonprofit Dolphin Watch Alliance, Ziltener routinely dives with the animals, but what she saw that day was unexpected.

“All of a sudden they were very organized,” she recalls. The dolphins lined up, politely waited their turn, and one by one rubbed their bodies against a particular type of coral. “It was clear that the dolphins knew exactly which coral they wanted to use,” Ziltener says. “I knew there had to be a reason.”

After more than a decade of underwater observation of some 360 ​​dolphins, as well as chemical analysis of the particular coral they preferred, Ziltener and her colleagues may have figured out what that reason is. In a recently published article in iScience, they described how the animals could “self-medicate” by using mucus that the coral releases when the animals rub against it. Co-author and chemist Gertrud Morlock of the University of Giessen, who led the analysis of the coral samples, says they contain “antibacterial, antioxidant, hormonal and toxic compounds” that the dolphins could use to prevent or treat skin infections.

While both scientists note that further research is needed to confirm their findings, the work demonstrates the value of observing animal behavior up close in the wild. Most research on dolphins is conducted in aquatic facilities or from the surface of the water, but Ziltener enjoys a dolphin perspective on the animals, thanks to her skills as a diver and her ability to put them at ease, “until they make us part of the pod.”

Ziltener says that gaining their trust means being patient and approaching the dolphins carefully and respectfully: “Some are curious, some are more shy. Eventually the shy people come into play.” The dolphins often greet divers by swimming in circles around them. “As a scientist, you try to be neutral and just observe,” she says. “But you must also greet them.”

Once hellos are out of the way, Ziltener says the dolphins quickly settle down and do their own thing — and that includes interacting with coral, which plays an integral role in their lives. The marine biologist often observed the dolphins waking up from a nap and immediately performing their coral-rubbing rituals. “It’s almost like they’re taking a shower, cleaning themselves before they… get up for the day.”

Ziltener noted that the dolphins preferred two specific types of coral, mushroom leather and the gorgonian bristly sea rod, as well as ircinia, a sea sponge. The animals seemed to use them the way humans use an exfoliant, rubbing a body part against the hard coral to abrade the skin, then absorbing the mucus it secreted with extra passes. Sometimes the dolphins would pluck the mushroom’s leather coral from the seafloor and swung it around, causing the coral to release green and yellow compounds that were visible in the water. Like a person walking through a spritz of perfume, the dolphins would then swim through the connections, engulfing themselves in the process. These behaviors, Morlock says, “are almost like a ceremony.”

Dolphin calves generally watched from the sidelines as the adults took turns playing with the coral. But as the calves grew older, they also started to participate, implying that “knowledge about the effects of bioactive compounds in the invertebrates is not innate, but … is passed on socially to the next generations,” the paper said.

Michael Walsh, a University of Florida professor and aquatic veterinarian who was not involved in the study, says he’s not surprised to learn that adult dolphins seem to be instructing the younger ones; it’s a behavior he’s seen in aquarium environments as well. However, he believes there are “alternative explanations” behind coral rubbing practices.

“They like to rub. They rub everything,” he says. “It’s common for them to constantly peel the skin and rubbing it helps them get it off.” Walsh adds that this is why aquariums often put large, soft brushes in dolphin and whale tanks. While he has no doubts that the coral contains medically beneficial compounds, he’s not sure those compounds could have much of an impact on the animals. For starters, a dolphin’s skin is very impenetrable. Second, in an underwater environment, it would be difficult to keep those compounds on the skin for long. Walsh thinks the dolphins rub coral for a very simple reason: “It feels good.”

Scientists collected samples of coral and sea sponge species that the dolphins seemed to prefer. © Angela Ziltener

Whether the wild dolphins are motivated by feeling good or good health, Ziltener has no intention of stopping interacting with them. Her deep affection for them is obvious: While she can identify most members of the pod by the individual’s unique dorsal fin, Ziltener and her team also give many of them names. There’s Sefa, a young man they met as a calf and who is now most often seen with “his friends,” and Laura, a 17-year-old friendly woman who is “very popular” with tourists swimming with dolphins. So kind, in fact, that Ziltener and her colleagues were concerned that Laura was spending too much time with people—and not enough time fapping. (When Covid-19 virtually ended all recreational and research diving, Laura finally got her first calf, which she “proudly” presented to Ziltener when the scientist returned to the water.)

As tourism returns to pre-pandemic levels, concerns about the stress humans are causing to animals are growing. It’s one of the reasons Ziltener’s team at Dolphin Watch Alliance want to create an international code of conduct for guides and tourists diving with dolphins – the other reason is to protect the coral the animals rely on to provide refuge, rest and maybe even skin care. The researchers also plan to continue investigating the link between dolphins and the chemicals in that coral. Morlock says the beneficial compounds found in marine invertebrates may also be beneficial to humans, quickly adding, “But let it be for the dolphins.”

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