On Scotland’s west coast is the Firth of Clyde, a large saltwater inlet home to thousands of harbour porpoises—and one dolphin named Kylie.
Kylie hasn’t been observed with other common dolphins (Delphinus delphis) in at least 14 years—but she’s far from alone. On clear days in the Clyde, visitors to the marina can sometimes see Kylie swimming with harbour porpoises (Phocoena phocoena), cetacean cousins about two-thirds her size.
New research published in Bioacoustics this January suggests Kylie’s ties to porpoises are closer than scientists imagined. While a common dolphin’s vocal repertoire should include a diverse range of clicks, whistles, and pulse calls, Kylie doesn’t whistle. Instead, she “talks” more like harbour porpoises, which communicate using high-pitched bursts of clicks.
A view of the Firth of Clyde, a large body of water next to the Scottish Isle of Arran, on a bright winter day.
Photograph by Jim McDowall, Alamy
The study suggests that she may be communicating with the porpoises, or at least attempting to. It’s part of a growing body of work that illuminates a rich world of interactions between different cetacean species.
“Clearly, species in the wild interact much more than we thought,” says dolphin behaviour expert Denise Herzing.
Porpoise code
Years ago, the Clyde’s lone dolphin resident was partial to a buoy at the mouth of a loch called the Kyles of Bute, so locals took to calling her Kylie. But nobody knows where she came from, or exactly why dolphins end up alone, says David Nairn, founder and director of Clyde Porpoise, a local organisation devoted to researching and protecting marine mammals. (Kylie hasn’t been sighted in a year, but locals hope she will return soon.)
Some solitary dolphins end up alone after being separated from their natal groups by storms or human activity, or after being orphaned. Others still may simply be less sociable and prefer their privacy, according to a 2019 study on solitary dolphins worldwide.
To learn more about Kylie’s relationship with the porpoises, Nairn borrowed a hydrophone and towed it behind his sailing yacht, the Saorsa. Nairn captured audio of multiple encounters between Kylie and porpoises from 2016 to 2018.
“She definitely identifies as a porpoise,” says Nairn, who trained as an aquatic biologist in college.
Mel Cosentino, then a Ph.D. candidate at the University of Strathclyde in Glasgow, pored over thousands of ultrasonic cetacean clicks from the recordings.
While dolphins whistle almost constantly, porpoises never do. Instead, they communicate exclusively with what are called narrow-band, high-frequency (NBHF) clicks, with eight to fifteen amplitude peaks at around 130 kilohertz.
“To hear an NBHF click you have to play it about one hundred times slower,” Cosentino says. (When sounds are slowed down, pitch descends. Humans can hear between 20 hertz, roughly equivalent to the lowest pedal on a pipe organ, and 20 kilohertz.)
In the recordings, Cosentino identified lower-frequency clicks standard for common dolphins. But even when Kylie appeared to be alone, Cosentino found clicks with eight or more amplitude peaks at the key 130 kilohertz- mark—the frequency at which porpoises chat. In other words, Kylie talks like a porpoise even when solo. The researchers also found that Kyle never whistles, as other dolphins do.
Cosentino observed that the exchanges between Kylie and the porpoises had the rhythm of a “conversation” between members of the same species—turn-taking with little overlap—though naturally it’s unclear how much meaningful information is relayed in Kylie’s attempts at porpoise clicks.
“It might be me barking to my dog and him barking back,” Cosentino says.
Regardless, this behaviour represents “an attempt” at communication that the “porpoises probably recognise,” says Herzing, research director of the Wild Dolphin Project who has studied the behaviour of Atlantic spotted dolphins in the Bahamas for more than three decades. Herzing, who wasn’t involved in the study, commends the authors on the clever experimental design in a natural setting.
“The results are tantalising,” she says. “What’s really telling is that Kylie doesn’t make any whistles, because dolphins always make whistles and porpoises never do.”
Harbor porpoises near Shetland, Scotland. For the first time, scientists have found a dolphin that “identifies as a porpoise,” able to make the species’ unique clicking sounds.
Photograph by Scotland: The Big Picture, Nature Picture Library
One of the greatest challenges of marine bioacoustics is identifying which creatures made what sound, says Laela Sayigh, an associate professor of animal behaviour at Hampshire College. “They don’t make any external movement associated with sound, and most of the time you can’t see them anyways,” Sayigh says.
However, Kylie can be distinguished in this case—by her accent. “It still looks like she’s struggling” to get as high-pitched as the porpoises, Cosentino says—the peaks on her clicks aren’t as crisp as they should be, and there are some lower-frequency sounds mixed in with the high notes.
“If they were singers, Kylie would be Pavarotti and the porpoises would be Mariah Carey.”
Cetaceans in captivity are capable vocal mimics, Herzing notes, pointing to killer whales and belugas that mimicked bottlenose dolphin tankmates. And a 2016 bioacoustics study found that a cross-fostered Risso’s dolphin in an Italian marine park whistled more like the bottlenose dolphins it was raised with than wild members of its species.
However, that Kylie makes NBHF-like clicks while alone “draws into question” if she’s clicking to communicate with harbour porpoises or just mimicking the sound, Sayigh says.
Melon-headed conversation
Dolphins, porpoises, and whales are all cetaceans, descendants of land-dwelling mammals that made their way back to the water over millions of years. As they re-adapted to life in the ocean, “evolutionarily, the nostrils became the blowhole,” Cosentino says.
While toothed whales like dolphins and porpoises have only one open nostril, both nasal cavities are still present below the surface, each capped by a muscular structure called “monkey lips.” (Cetacean anatomy is often described in colourful terms, originating from descriptions by whalers). The monkey lips are somewhat analogous to our own vocal cords, controlling airflow—and when air is forced from the lungs through the “lips” on the left nasal cavity “it’s like letting the air out of a balloon,” creating warbling whistles, Cosentino says.
The right nasal cavity is responsible for the clicks used in both communication and navigation. It dead-ends next to a fatty deposit on the toothed whale’s forehead called a melon, which amplifies and focuses the cetacean’s vocalisations. Since both sets of monkey lips operate independently, some cetaceans, including bottlenose dolphins, can click and whistle at the same time—kind of like Mongolian throat-singing. (Learn more: The pioneering science that unlocked the secrets of whale culture.)
Kylie’s story is part of a broad field of research into how cetaceans interact with members of other species. “They’re very social, they’re very sexual, and they’re very communicative,” says Herzing. “These animals survive and adapt socially, and sound is a natural way they do it.”
Well-documented interspecies adoptions also demonstrate that species divisions may not be as clear-cut as once thought. Examples include a Canadian beluga pod that took in a narwhal calf and a spinner dolphin that lived among Tahiti bottlenose for 20 years.
Recent DNA analyses also demonstrate we’ve only scratched the surface of the extent of hybridisation, Herzing emphasises. Bottlenose dolphins have hybridised with at least 10 species in captivity and in the wild, including cetaceans as disparate as the pilot whale and the Guiana dolphin. Researchers hypothesise that cetaceans are able to hybridise so successfully because of their shared DNA—their species diverged only within the past 10 million years.
Besides attempts at communication, Kylie seems close to porpoises in other ways. On multiple occasions, Nairn has seen female porpoises bring their young calves to interact with Kylie. Since porpoise calves usually stick very close to mom until they’re weaned, Nairn was surprised to watch them swim with the dolphin in echelon, a position just behind her pectoral fin that researchers say is the cetacean equivalent to “carrying” a baby, Nairn explains.
Nairn has also observed male porpoises attempting to mount Kylie. But does she entertain their advances? “I would even say she courts, aye,” Nairn admits with a chuckle. Mating is theoretically anatomically possible, although there haven’t been any scientifically documented dolphin-porpoise hybrids, Herzing says.
Ever since a week of intense storms in February 2021 caused a massive drilling ship to become unmoored near her favourite buoy, Kylie has been missing. Nairn says it’s not out of character for her to relocate after a big disturbance to one of her “holiday buoys” elsewhere in the Clyde for months at a stretch, even up to a year, but he can’t help but worry.
Nairn and his colleagues say they’re eager to look—and listen—for Kylie as soon as the spring field season begins—and see what else she might teach us.