We humans rely on a series of cues to recognize our friends, such as their smiles, their voices or the way they walk. Biologists have known for decades that dolphins form close friendships and that cetaceans identify pals by their unique whistles. Now, startling new research suggests that bottlenose dolphins use their sense of taste to discern the urine of their friends from unrelated dolphins.
Study leader Jason Bruck, a marine biologist at Stephen F. Austin State University in Texas, didn’t investigate whether bottlenose dolphins could identify themselves by their urine. Its original goal was to test whether dolphins use their signature whistles in the same way people rely on names. But to do that, he needed a second way for dolphins to identify themselves. (Radical effort to decode whale language launched).
To find out if dolphins could associate a whistle with a specific dolphin, Bruck turned to an unlikely substance: urine. A scientist had previously observed wild dolphins deliberately swimming through plumes of urine, leading Bruck to suspect they were picking up information from them.
“It was a shot in the dark,” says Bruck, whose study was published this week in the journal Scientists progress. “And I didn’t expect it to work, to be honest.”
In experiments using captive dolphins, the team found that dolphins paid more attention to their friends’ urine and whistles, suggesting they knew the animals that emitted them, he says.
The results are the first strong evidence of an animal identifying other members of its species using taste. They also show that by using at least two cues to identify individuals, dolphins have a complex understanding of their family and friends, just like humans.
“I was shocked, just shocked,” Bruck says. “I had a big smile on my face, like, Oh my God, it worked.”
In 2016 and 2017, Bruck and his colleagues observed several bottlenose dolphins at dolphin interaction facilities in Bermuda and Hawaii that also maintain a breeding consortium for the species. At these Dolphin Quest locations, the dolphins live in lagoons fed by natural seawater, which simulates their environment in the wild.
The researchers’ first step was to see if dolphins could detect urine in seawater. Over the course of evolution, bottlenose dolphins lost their sense of smell but retained a keen sense of taste.
In large pools containing temporarily separated dolphins, scientists poured water with ice into the water and then observed each animal’s reaction. Curious dolphins that explored the icy water were good candidates for the experiment. Next, the team needed to test whether the animals’ reactions to ice water and urine varied, and whether they reacted differently to familiar urine versus unfamiliar urine. (Read why dolphins have the longest memory in the animal kingdom.)
The team knew which dolphins knew each other based on those who had lived together for at least five years. So the researchers poured about 20 milliliters of familiar and unfamiliar dolphin urine into the pool, one after another, in the order determined by a coin toss.
The dolphins spent about three times longer studying familiar urine than unfamiliar urine, with a few individuals sampling the familiar substance for more than 20 seconds. The cetaceans paid little attention to the unfamiliar urine, only sampling it during the same period when they had freezing water.