The male Indo-Pacific bottlenose dolphins of Australia’s Shark Bay are notorious for their ganglike behaviors. They form complex alliances to patrol large home ranges and corral fertile females for mating. Scientists have studied these mammals since the 1980s, intrigued by the tight, cooperative bonds between unrelated males—a type of social organization considered rare in the animal kingdom. Now, researchers report this male bonding has a big evolutionary payoff: Dolphins with the strongest buddy bonds father more offspring.
A second study reveals male dolphins use whistles to maintain their friendships—lending support to the idea that language evolved for long-distance social bonding. Together, the papers provide new insights into bottlenose dolphins’ complex social system, which is much like that of chimpanzees, says Liran Samuni, a primatologist at Harvard University who was not involved with either study.
Most male mammals compete for females and rarely cooperate with one another. Lions and chimpanzees were the previously known exceptions. Unrelated male lions sometimes work together to take over a pride of female lions, increasing their chances of fatherhood; male chimpanzees that form strong bonds with the alpha male are most likely to sire offspring.
Researchers had previously shown the male dolphins (Tursiops aduncus) of Shark Bay begin to form partnerships when they’re about 3 years old, after leaving their mothers. They soon join what scientists term a “first-order alliance” with a few nonkin pals. (Because male and female bottlenose dolphins are about the same size, a lone male cannot control a female.) The small alliances cooperate in larger second-order alliances comprised of as many as 14 dolphins; they fight other alliances over females. These alliances can endure for decades, and often band together in even larger third-order alliances to battle rivals.
In the new study, Livia Gerber, an evolutionary biologist at the University of New South Wales, Sydney, and colleagues analyzed which factors influenced male dolphins’ reproductive success. Using 30 years of behavioral data collected during surveys from a motorboat, the scientists examined 10 second-order alliances consisting of 85 males. They identified which males had the strongest bonds (based on how much time individuals spent together), and which were popular, sharing time with many members of their alliance.
Comparing genetic data collected from biopsy samples from these males with those from 256 calves born since 1994, the scientists determined that males who had the strongest social bonds and were friends with all members of their alliance had the most offspring. Other factors, including a male’s age or the size of his home range, did not predict paternity success, the researchers report today in Current Biology.
“It’s a great study,” says Frans de Waal, an emeritus primatologist at Emory University. “A lone male stands no chance in this system.”
“The study shows that male competition is not only about strength or body size—the male characteristics traditionally thought to underlie reproductive success,” Samuni adds. “By forming strong alliances with others, males can influence their own reproductive success in a way that wouldn’t be possible as single individuals.”
But how do male dolphins make and retain friends in the first place? “By spending time together—petting, rubbing, touching flippers, goosing each other, making synchronous dives, having sex,” says Emma Chereskin, a cetacean ethologist at the University of Bristol. Vocal exchanges also belong on the list, according to a second new study, which she led. (Watch males maintain friendships in the above video of a large, traveling alliance.)
Every dolphin has a signature contact whistle, a warbly, high-pitched “eeee,” they learn from their mother, and that they use to identify themselves. Mothers and calves and allied males use the whistles to stay in touch. To further investigate how adult males use them, Chereskin and her colleagues analyzed 92 whistle exchanges recorded by towing hydrophones from a boat. To count as an exchange, the recipient had to respond within 1 second.
The caller emits his whistle (basically saying, “Quasi, here. Quasi, here.”), and the receiver replies with his own whistle (“Imp, here. Imp, here.”).
Doing this “strengthens their bond,” says co-author Stephanie King, a behavioral biologist also at the University of Bristol. “It’s a low-cost way to maintain these relationships.” Male dolphins, the scientists found, whistle to “touch” partners that were 10 or more meters away and difficult to contact physically.
In these exchanges, the dolphins don’t call one another by name. Although they’re able to imitate another’s whistle, such vocal mimicry would be “unreliable,” King says. “They could never know for sure who was calling.” Instead, the dolphins are doing something like a roll call.
These exchanges never resulted in groups of dolphins merging together. Rather, males whistled simply to contact a second-order ally, who sometimes turned in the whistler’s direction while whistling his response, the team reports today in Current Biology. Intriguingly, the scientists report that the whistlers usually exchanged whistles with second-order allies with whom they were weakly bonded, rather than calling to a best buddy. In contrast, males with stronger bonds were more likely to be in close physical contact, petting and rubbing against each other.
The vocal exchanges are akin to primates’ social grooming—ruffling through a friend’s fur for nits and detritus, Chereskin says. She and her co-authors suggest the exchanges support a hypothesis evolutionary psychologist Robin Dunbar proposed nearly 3 decades ago: that contact calls help maintain relationships through “grooming at a distance.” (Indeed, such contact calls are how humans evolved language, Dunbar argued.) Yet numerous studies of nonhuman primates have never supported this idea—the animals exchange calls, but most often with those they’re most closely bonded to. “But no one had looked at this outside of primates,” Chereskin says.
“It’s an elegant test of Robin Dunbar’s hypothesis, using a sterling suite of data,” says Simon Townsend, a comparative psychologist at the University of Zürich. “They’ve supplied strong, surprising support from another species.”
That makes sense because “bottlenose dolphins are the only nonhuman mammals so far shown to have” certain vocal skills required for language, adds Karl Berg, an ornithologist at the University of Texas, Rio Grande Valley, and a parrot vocalization expert.
The findings provide further evidence of dolphins’ sophisticated social skills, de Waal says. Humans tend to think they’re unique in the animal kingdom, he says. But we’re clearly not.