That December, Billie followed a regatta of sailing ships out of Port River and ended up trapped in a particularly polluted harbor. A nearby dolphinarium called Marineland rescued her and kept her at its facility for three weeks. There, she lived alongside five captive dolphins that had been trained to tail-walk in public shows. Billie never received any training, but she didn’t need it. She learned to tail-walk just by watching them.

That became clear after she was released back to Port River. She tail-walked around Bossley’s boat. She tail-walked in the bow of ships—the only dolphin ever known to do so. Then, in 2007, Bossley and his team of volunteer observers saw another female, called Wave, perform the trick. Her proficiency grew as Billie’s health started to falter. And after Billie died of kidney failure in 2009, “Wave’s tail-walking exploded, and she started doing it all the time,” says Luke Rendell from the University of St. Andrews. “The sheer number of times she did it was probably the influence that got other dolphins to do it, too.”

Indeed, Wave’s daughter Ripple also picked it up, as did four other adult females in the group, and four other juveniles. Some still do it, but the fad is fading; it peaked in 2011, and has declined since then.

No one really knows why Billie learned to tail-walk from the captive dolphins, or why her wild peers learned it from her. Many animals have been seen imitating one another’s actions, but most of these examples of wild culture involve techniques for getting food or attracting mates. Tail-walking seems to carry no benefit. There are only a few examples of such apparently arbitrary traditions, including orcas that started carrying dead salmon for a few weeks, macaque monkeys that began playing with stones, and capuchin monkeys that poke one another in the eye as a greeting.

Bossley says that tail-walking is unlikely to be a straightforwardly playful behavior: When one of her calves died, Wave could be seen tail-walking beside its body. It’s probably not just a call for attention either, since the dolphins do it when alone.

“We know that dolphins are social learners,” says Diana Reiss, a dolphin expert at Hunter College. “There have been past reports of captive cetaceans imitating the behavior and vocalizations of other dolphins, and even other species, with which they are housed. In doing so, perhaps they’re trying to fit in or bond with the others.”

Rendell agrees with that idea, and notes that humans do something similar. “Human children copy irrelevant actions as part of belonging,” he says. “It’s the Salesman 101 technique: Just copy the person you want to sell to and they feel warmed toward you.”

But why is it that only adult females learned to tail-walk, and adult males never did? “I have absolutely no idea,” Rendell says. “The only males seen to do it were calves swimming with their mothers. I can only speculate that copying someone is such a strong social signal for a male bottlenose to give that they’ll only do it in very strong circumstances.”