We humans rely on a suite of cues to recognize our friends, such as their smiles, their voices, or the way they walk. Biologists have known for several decades that dolphins form close friendships, and that the cetaceans identify pals by their unique whistles. Now new surprising research suggests bottlenose dolphins use their sense of taste to discern their friends’ urine from unrelated dolphins.
Study leader Jason Bruck, a marine biologist at Stephen F. Austin State University in Texas, didn’t set out to test whether bottlenose dolphins could identify each other through their urine. His 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 each other. (Learn more about how dolphins respond to their “names.”)
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 purposefully swimming through plumes of urine, leading Bruck to suspect they were gathering information from it.
“It was a shot in the dark,” says Bruck, whose study was published this week in the journal Science Advances. “And I was not expecting it to work, to be honest.”
In experiments using captive dolphins, the team discovered that dolphins paid more attention to their friends’ urine and whistles, suggesting they knew the animals that issued them, he says.
The findings are the first robust 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—much as humans do.
“I was shocked, just shocked,” says Bruck. “I had a big grin on my face, like, Oh my God, this worked.”
In 2016 and 2017, Bruck and 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, dolphins live in lagoons fed by natural seawater, which simulates their environment in the wild.
The researchers’ first step was to see whether dolphins could detect urine in the seawater. Over evolutionary time, bottlenose dolphins have lost their sense of smell but retained a strong sense of taste.
In expansive pools containing temporarily separated dolphins, the scientists poured water with ice into the water and then watched to see how each animal responded. Curious dolphins that explored the ice 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 responded differently to familiar versus unfamiliar urine. (Read why dolphins have the longest memories in the animal kingdom.)
The team knew which dolphins were familiar with each other based on who had lived together for at least five years. So the researchers poured about 20 milliliters of both familiar and unfamiliar dolphin urine into the pool, one after another, with the order determined by coin flip.
The dolphins spent about three times longer investigating the familiar urine than the unfamiliar urine, with a few individuals sampling the familiar substance for more than 20 seconds. The cetaceans paid little attention to the unfamiliar urine, sampling it only for the same time they had ice water.
“The dolphins were very, very keen to participate,” says Bruck, who added they weren’t rewarded with food. “Usually, dolphins get bored with my experiments. We were tapping into something that is part of the dolphins’ world.”
The final test investigated whether dolphins had a paired understanding of other dolphins’ cues—in other words, if an individual’s whistle and urine were connected in their minds.
To do this, Bruck did what behavioral ecologists call an “expectation violation” experiment: showing animals something that doesn’t make sense and seeing how they react. In people, this would be like seeing your best friend’s face but hearing a different voice. (Read how dolphins have bold and shy personalities, like us.)
For this final experiment, Bruck tested different urine-whistle combinations on 10 dolphins, five of which were the same animals in the previous trials.
When exposed to an incorrect urine-whistle combination, the dolphins didn’t pay too much attention—perhaps a useful innovation for the wild, where the mammals would be inundated with mismatched whistles and urine, he says.
But when a dolphin encountered the correct urine-whistle pair, the animal explored the area for an average of 10 seconds longer than the mismatched pair. Two individuals hung around for more than 40 seconds—the compelling evidence the team needed that they could recognize their friends.
A taste of success
“It’s very difficult to demonstrate that a concept exists in an animal’s mind, so these types of experiments trying to answer that question are very interesting and useful,” says Bruno Díaz López, chief biologist of the Spain-based Bottlenose Dolphin Research Institute, who was not involved in the study.
López, who would like to see similar studies attempted in the wild, adds “it’s a good approach and a good first step” to understanding the role taste might play in dolphin recognition. (How dolphins use tools, teamwork, and trickery to get their dinner.)
“This really deepens our understanding of how dolphins keep track of each other, which we know is really important to them,” says Laela Sayigh, a marine biologist at Woods Hole Oceanographic Institute in Massachusetts, who also was not involved in the study.
“It opens doors to other types of questions around what all they might be learning” from urine trails, she says.
As for Bruck, he’s eager to delve next into what biological mechanism the dolphins use to taste urine.
A promising possibility is lipids in urine, which can be detected with a physical “antenna” on their taste buds. It’s especially pressing, he adds, as it’s unknown what impact human-made pollution has on the dolphins’ taste-based abilities.