Ever been at a party where you recognize everyone’s faces but can’t think of their names? That wouldn’t happen if you were a bottlenose dolphin (Tursiops truncatus). The marine mammals can remember each other’s signature contact whistles—calls that function as names—for more than 20 years, the longest social memory ever recorded for a nonhuman animal, according to a new study.
“The ability to remember individuals is thought to be extremely important to the ‘social brain,’ ” says Janet Mann, a marine mammal biologist at Georgetown University in Washington, D.C., who was not involved in the research. Yet, she notes, no one has succeeded in designing a test for this talent in the great apes—our closest kin—let alone in dolphins.
Dolphins use their signature whistles to stay in touch. Each has its own unique whistle, and they learn and can repeat the whistles of other dolphins. A dolphin will answer when another dolphin mimics its whistle—just as we reply when someone calls our name. The calls enable the marine mammals to communicate over long distances—which is necessary because they live in “fission-fusion” societies, meaning that dolphins in one group split off to join other groups and later return. By whistling, they’re able to find each other again. Scientists don’t know how long dolphins are separated in the wild, but they do know the animals can live almost 50 years. So how long do the dolphins remember the calls of their friends?
To find out, Jason Bruck, a cognitive ethologist at the University of Chicago in Illinois, spent 5 years collecting 71 whistles from 43 dolphins at six captive facilities, including Brookfield Zoo near Chicago and Dolphin Quest in Bermuda. The six sites belong to a consortium that rotates the marine mammals for breeding and has decades-long records of which dolphins have lived together. The dolphins ranged in age from 4 months to 47 years and included males and females. Some of the animals had spent as little as 3 months together; others had been housed with each other for as long as 18.5 years before being separated and sent to another facility; and some had been apart for 20.5 years.
At each facility, Bruck placed a submerged speaker in the dolphins’ pool and waited for one of the animals to swim past. He then played a recording of a whistle that the dolphin had never heard before. “They don’t pay much attention to signature whistles of dolphins they don’t know,” he says. But when he played the whistle of a dolphin they had once lived with, the animals often swam immediately to the speaker. “They will hover around it, whistle at it, seemingly try to get a response,” he says.
Bruck also played recordings of an unfamiliar dolphin that was the same age and sex as the familiar animal—but these also did not elicit much of a response. “It was a striking pattern,” Bruck says. “They were potentially bored by unfamiliar calls but responded to whistles from the animals they’d known,” even if they had not heard the whistles in decades. “It seemed to be stimulating to them. In Bermuda, a mother dolphin even brought her calf over to listen to the whistles of dolphins she’d known,” Bruck says. Sometimes the dolphins got upset, slapping their tails in protest, when Bruck removed the speaker from the pool; but they quickly settled down again after he put it back in the water.
In one case, Bruck played the whistle of Allie, a female dolphin at the Brookfield Zoo, for Bailey, a female in Bermuda. They had lived together at the Dolphin Connection in the Florida Keys when Allie was 4 and Bailey was 2. Twenty years and 6 months had passed—yet Bailey instantly recognized Allie’s whistle, Bruck says, as evidenced by her close attentiveness to the speaker.
The dolphins often responded as if they were picturing their long-ago social pals, Bruck reports online today in the Proceedings of the Royal Society B. For instance, two younger dolphins, Kai and D.J., became watchful and alert when they heard the whistles of Lucky and Hastings, two dominant males they had spent time with at the Brookfield Zoo. “Their whistles elicit a certain vigor and spirit in males that hear them,” says Bruck about the responses of Kai and D.J. “It looked as if those whistles put the image of those two dominant males in the heads of Kai and D.J.,” although he adds this has yet to be shown experimentally.
The study demonstrates the “long-term stability of the dolphins’ whistles,” Mann says. “Even though dolphins may change in size and physical characteristics—getting scars and speckles—their whistles provide a reliable means of identification.” And that in turn enables them to “track relationships and connections between individuals,” she says.
“We know they have relationships in the wild that last decades,” adds Richard Connor, an animal behaviorist at the University of Massachusetts, Dartmouth. “Remembering a particular individual—even in the absence of that individual—could help them navigate their current social milieu.”
Bruck’s study, however, did not test whether the dolphins mentally picture the correct dolphin when they hear his or her signature whistle. So far, scientists have only been able to demonstrate this ability in horses. Researchers from the University of Sussex in the United Kingdom videoed individual horses while a herd member was led past them and out of view. The scientists then played the whinny of that horse or of a different horse. If the whinny was from the horse that had just walked by, the watching horse continued doing whatever it had been doing before; but if the whinny came from a different stable-mate, the watcher instantly turned to look in the direction of the call, as if saying, “that didn’t sound like you.”
A similar experiment now needs to be done with dolphins, says Stephanie King, a biologist at the University of St. Andrews in the United Kingdom. She wonders if the animals are paying attention to the whistles of their former pool-pals because the sounds are familiar—or because they “evoke a mental representation of the absent animal in the dolphin’s mind.” In other words, does Kai mentally picture Lucky when he hears the dominant male’s brassy whistle erupting from the speaker? Or does he merely register, “That call takes me right back to Chicago.” Stay tuned—Bruck has a test in the works to find out.