NEW YORK: In 2013, a group of 52 Atlantic spotted dolphins, driven to migrate by unknown forces, left their home on the Little Bahama Bank in the northern Bahamas. They traveled 100 miles south to the island chain of Bimini, a destination already inhabited by a community of 120 Atlantic spotted dolphins. When groups of social mammals meet, things can get tense. Run-ins between chimpanzee communities, for instance, are known for their violence. Adult male mammals, especially, are keen to defend territory and access to females. But for the Atlantic spotted dolphins of Little Bahama Bank and Bimini, the mixing and mingling seems to have gone rather swimmingly, scientists found.
Two teams of researchers published papers recently about the growing dolphin community. Their analyses, unlike the dolphins, were not blended, and offered independent confirmation that dolphins from different groups formed strong bonds in a short time frame. The rare event provides new clues about how these brainy mammals organise their complex societies, and may help predict what may occur if climate change pushes populations together.
Denise Herzing, a marine mammal behavioural biologist at the nonprofit Wild Dolphin Project, and her colleagues watched dolphins on the Little Bahama Bank for almost 30 years and started tracking the 52 dolphins when they left. “We were curious how they were integrating,” she said. “It’s a kind of a natural experiment.”
Another team, the Dolphin Communication Project, observed dolphins at Bimini for 20 years. “All of a sudden we were seeing so many adults that we didn’t know,” said Nicole Danaher-Garcia, a behavioural ecologist with the group. She was referring to the dolphins, not the other dolphin researchers, of course. The aquatic mammals often spend their whole lives forming close bonds within their home group, Dr. Danaher-Garcia said. But at Bimini, they were forming new friendships with strangers in only a year.
Dr. Danaher-Garcia’s team tracked which dolphins spent time together from 2013 to 2018 and analysed how individual animals touched each other. “A lot of times you’ll see them rubbing their pectoral fins against one another. It kind of looks like they’re playing patty-cake,” she said. A dolphin may rub its forehead on a pal’s belly, indicating an even stronger bond. “You must like them,” she said, “and if they’re allowing you to do it, they must trust you.” Such friendly gestures were common between males from the different groups, the team reported this week in the journal Royal Society Open Science.
The team didn’t observe aggression between the newcomers and the original Bimini crew, the kind of conflict often seen in nature when mammal groups merge. “That’s very unusual,” Dr. Danaher-Garcia said. Instead, her team saw the animals socialising, playing and getting frisky across original group lines, behavior more akin to that of bonobos.
She said it was possible that “like bonobos, they use sexual behaviours to ease the tension.” At times, this bacchanal can look like a ball of dolphins. “You can’t really tell who’s touching whom and what’s going on,” she said. Like both bonobos and chimpanzees, dolphins live in fission-fusion societies where they make strong bonds between individuals but can break those bonds and forge new ones. This kind of bonding between individuals in different groups is not seen among many mammals, said Diana Reiss, a marine mammal scientist and cognitive psychologist at Hunter College who was not part of either study. To see such social flexibility within groups that didn’t previously live together “is pretty exciting,” Dr. Reiss said.