Dolphins were spotted in 2009 doing something unusual. A new study sheds light on the reasons behind it

A dolphin with a fungal infection on its dorsal fin. Rubbing against certain corals may protect dolphins against skin complaints.

(CNN)In the Red Sea off the coast of Egypt, bottlenose dolphins were spotted in 2009 doing something unusual. They lined up to rub their bodies against coral.

They were choosy about which type of corals they rubbed against, noticed wildlife biologist Angela Ziltener, a guest researcher at the University of Zurich who spent the next 13 years trying to unravel the perplexing behavior.
The results of her extensive research on the community of 360 dolphins were published on Thursday.
    By observing the dolphins and studying the properties of the coral, Ziltener and her colleagues discovered that the dolphins appear to use the reef as a medicine chest: Bioactive compounds in mucus released by two different types of coral and a sea sponge likely help the dolphins protect their skin.
      It's the first time this type of behavior has been witnessed in cetaceans -- the scientific order of marine mammals that includes dolphins, whales and porpoises -- the study said. However, some birds, mammals, insects and reptiles have previously been observed using plant parts or other substances to combat pathogens or parasites.

      Building trust

      It took Ziltener years of scuba diving with the local dolphin population to earn their trust. "You have to be kind of adopted by the dolphins. It took time to actually see all their secrets," she said.
        The dolphins only rubbed against a gorgonian coral known as Rumphella aggregata, the leather coral Sarcophyton sp., and the sea sponge Ircinia sp., Ziltener observed. What's more, they used the organisms in different ways.
        With the leather corals and sponges -- which are more compact and harder in texture than the soft gorgonian coral branches -- the dolphins tended to push an isolated body part in and twist it around, the study found. In contrast, they slid their entire bodies into the gorgonian coral several times, rubbing multiple body parts at once.
        The dolphin behavior of rubbing against the gorgonian coral, dubbed gorgoning, and Ziltener's research were first revealed in 2017 in the BBC documentary "Blue Planet II" and a number of other nature documentaries. However, this is the first time a detailed study of the behavior has been published in a scientific journal.
        When in groups, the dolphins often lined up and took turns rubbing against the gorgonian coral. Interacting with the leather coral didn't appear to be a group activity.
        With leather coral, a dolphin would sometimes uproot it from the ground and carry it in its mouth for a few minutes, swinging it around -- an action that caused the compounds to leak out of the coral and spread around the dolphin's head, staining it yellow and green.

        Coral samples

        Because the reef is protected, the team obtained permission to take small samples -- just a centimeter -- of the corals and sea sponge used by the dolphins. The study analysis found that these organisms contained 17 bioactive compounds, with different properties, such as antibacterial, antioxidative or hormonal attributes, said coauthor Gertrud Morlock, an analytical chemist and professor of food science at Justus Liebig University Giessen in Germany.
        The three different organisms showed similar, and some different, effects, Morlock said.
        It's not only humans who are right-handed. Dolphins also have a dominant side