In the summer of 2013, dolphin researcher Nicole Danaher-Garcia spotted something rare and remarkable in the animal world. As she stood on top of the bridge of a sport fishing yacht near Bimini in the Bahamas, she spied 10 adult Atlantic spotted dolphins she had never seen before—speeding into the waters of another group of dolphins.
Most mammals attack intruders, but war wasn’t on the menu that day. Instead, the newcomers—eventually 46 in all—joined up with the resident dolphins, some 120 in number. Today, the two groups of Atlantic spotted dolphins (Stenella frontalis) have partially integrated, diving and swimming together, forming fast friendships, and likely even mating.
It’s a “striking” display of tranquility between animals scientists usually consider rivals, says Richard Wrangham, a primatologist at Harvard University who was not involved with the study. Most mammals fight to protect mates and other resources if they encounter strangers entering their territory, he notes. This research, he says, may ultimately lead to a better understanding of the evolution of peacefulness.
Danaher-Garcia, a behavioral ecologist, and her colleagues at the Dolphin Communication Project observed the two groups of dolphins in Bimini for 5 years, carrying out nearly 300 surveys. At first, the scientists only saw one small group of mixed Bimini and newcomer dolphins. But the next year, the scientists spotted a larger group of males and females of all ages from both communities mixing without “any signs of aggression,” she says. The dolphins continued their friendly behaviors through 2018, leading the team to suspect the two groups were merging. (Because of COVID-19 concerns, the scientists put their studies on hold in 2020.)
The scientists discovered the newcomers had migrated from Little Bahama Bank, an area some 160 kilometers to the north known for its shallow seas, coral reefs, and sand banks. They were part of the White Sand Ridge (WSR) spotted dolphin community that another scientific team has been studying since the mid-1980s.
Whenever a member of the team saw the larger, more heavily spotted WSR dolphins with the Bimini residents, she entered the water to film them. The scientists recorded which dolphins spent the most time together, touching, rubbing against one another (as in the above video), and courting—all signs of burgeoning friendships. In no time at all, WSR males were courting Bimini females, swimming belly-up beneath them in the dolphins’ mating position. The scientists suspect some of these interactions led to actual matings, but they’re waiting for genetic proof. The smaller Bimini males weren’t seen courting WSR females; the scientists don’t know why.
In all, the two communities were becoming well-integrated, the researchers report today in Royal Society Open Science.
“It’s a fascinating study” that shows just how socially adaptable dolphins are, says Peter Corkeron, a marine biologist and cetacean expert at the New England Aquarium. He hopes the findings are an indication that dolphins may “find ways to deal with social changes” that are likely to come as the oceans warm.
Other dolphin species, such as the Indo-Pacific bottlenose dolphins of Australia’s Shark Bay, are notorious for their battles over females. However, the male Bimini dolphins behaved more like bonobos, primates that are known for their usually peaceful relationships with neighbors. Not only did male and female dolphins from the two groups get along, the local males became chummy with the outsiders. Male-male friendships are central to dolphin society, and the scientists observed the range of tactile behaviors that signal male dolphins’ friendships: touching pectoral fins, swimming and diving closely together, swimming in the mother-calf position, goosing each other with their rostrums, and engaging in male-male sex, bonding behaviors common to dolphins and porpoises.
Normally, juvenile and subadult dolphins form such friendships, but the Bimini and newcomer males were adults. “You’d think the Bimini males wouldn’t want these other adults around simply because of the competition for mates,” Danaher-Garcia says. But they were “almost going out of their way to make friends” with the newbies, she says. The scientists speculate in their study that these tightly bonded males may join forces to defend against predators and to hunt deep-water prey.
This kind of gregariousness between unrelated groups is rare in birds and mammals. And “peaceful immigration of entire groups is even rarer,” says Elizabeth Lonsdorf, a primatologist at Emory University. Some dolphin species may be more likely to get along with strangers because they don’t have defined territories and don’t guard resources, such as prey, she says.
In any case, scientists now have another species (in addition to bonobos) to investigate as they begin to grapple with the notion of peaceful relationships among groups, Wrangham says. This is such a new area of research, he notes, scientists don’t yet have a “general theory” to explain peacefulness. But by understanding such tendencies in other species, he says, we may in time begin to understand how peacefulness evolved in humans, too.