In 2013, a group of 52 Atlantic spotted dolphins, driven to migrate by unknown forces, left their home on the Little Bahamas Bank in the northern Bahamas. They traveled 100 miles south to the Bimini island chain, a destination already inhabited by a community of 120 Atlantic spotted dolphins.
When groups of social mammals get together, things can get tense. Run-ins between chimpanzee communities, for example, are notorious for their violence. Adult male mammals, in particular, want to protect 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 too far, according to scientists.
Two teams of researchers recently published papers about the growing dolphin community. Unlike the dolphins, their analyzes did not merge and offered independent confirmation that dolphins from different groups formed strong bonds in a short period of time. The rare occurrence provides new clues about how these brainy mammals organize their complex societies, and could help predict what might happen if climate change pushes populations together.
Denise Herzing, a marine mammal behavioral biologist at the nonprofit Wild Dolphin Project, and her colleagues had been watching the dolphins on the Bahamas’ Little Bank for nearly 30 years and began tracking the 52 dolphins when they left.
“We were curious how they were integrating,” she said. “It’s kind of a natural experiment.”
Another team, the Dolphin Communication Project, saw dolphins at Bimini for 20 years. “Suddenly we were seeing so many adults we didn’t know,” said Nicole Danaher-Garcia, a behavioral ecologist with the group. She was referring to the dolphins, not the other dolphin researchers, of course.
The aquatic mammals often spend their entire lives forming close bonds within their home group, Dr. Danaher-Garcia said. But at Bimini, they were just forming new friendships with strangers in a year.
The team of Dr. Danaher-Garcia which dolphins spent time together from 2013 to 2018 and analyzed how individual animals interacted with each other. “Often you will see them rubbing their pectoral fins against each other. It looks like they’re playing patty-cake,” she said. A dolphin may rub its forehead on a friend’s belly, indicating an even stronger bond. “They have to like you,” she said, “and if they allow you to do it, they have to 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 did not observe aggression between the newcomers and the original Bimini team, the type of conflict often seen in nature when groups of mammals meet.
“That’s very unusual,” Dr. Danaher-Garcia said. Instead, her team saw the animals socializing, playing and crossing original group lines, behavior more akin to that of bonobos.
She said it was possible that “like bonobos, they use sexual behaviors to relieve tension.” Sometimes, this bacchanal can look like a ball of dolphins. “You can’t really tell who’s doing it and what’s going on,” she said.
Like both bonobos and chimpanzees, dolphins live in fission fusion societies where they form strong bonds between individuals but can break those bonds and create new ones. This connection 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. “It’s quite exciting to see such social flexibility within groups that didn’t live together before,” said Dr. Reiss.
The team of Dr. Danaher-Garcia suspects Bimini’s geography, with its many shallows as well as abundant access to deep water for foraging, creates more friendly interactions because the dolphins probably don’t have to fight over space.
But that doesn’t mean it was all frictionless. Dr. Herzing’s group observed some aggressive behaviors, such as dolphins slapping or ramming heads, which are typical when males are fighting over mating opportunities. Her group mapped cetacean associations from 2015 to 2020 and reported the results last week in the journal Marine Mammal Science. But the fight that Dr. Herzing’s team observed is not unusual and can occur within a single group of dolphins.
Dr.’s group has not yet published its analysis. Herzing on the types of contact that occurred in the newly mixed group. That team stayed on a boat further offshore watching the dolphins over longer days during the summer. In contrast, Dr.’s project had a limited sample size. Danaher-Garcia focused more on men, compared to men and women, said Dr. Herzing, and may have missed some aggressive contacts.
“They probably didn’t see an attack, probably because there was nothing to fight,” she said.
There may also be a difference in how the two studies classify what constitutes aggression, Dr. Herzing and Dr. Danaher-Garcia noted.
More research is needed to determine whether mixed groups of dolphins are becoming more inclusive through mating. The team of Dr. Herzing enlists the help of the Wild Dolphin Project as they collect dolphin feces and analyze the genetic material they contain to reveal the parentage of the dolphins.
Guido J. Parra, a behavioral ecologist at Flinders University who was not involved in either study, said there was value in researching these interactions. Understanding social connections may help to show how groups of animals may respond to environmental change and aid conservation. Researchers still have a lot to learn about the ecological factors that drive grouping, the role of individuals in shaping social structure and the costs and benefits of banding together, Dr Parra said.
That will be important because different populations of dolphins could be pushed together. For example, in Bangladesh, rising seas encroached on a land border and brought river dolphins into contact with another dolphin species in the ocean, Dr. Herzing said.
“We don’t know exactly how species will fare” she said.