The male dolphins of Shark Bay, Australia, are known to marine biologists for their messy social entanglements. Their relationships with each other are so unusual—they're more like the intricate webs of the Mafia than the vertical hierarchies of chimpanzees—that, in a new paper, one team of scientists argues that the dolphins live in a social system that is "unique among mammals." Intriguingly, the researchers also suggest that these complex, and often cooperative, relationships may stem in part from one simple, unexpected factor: the dolphins' low cruising speed.
Mammals have evolved a variety of social structures. For example, chimpanzees live in what ethologists call "semiclosed groups"—that is, a community comprised of individuals who are well-known to each other. The members generally aren't friendly to chimps in other communities; the males practice what's known as community defense, patrolling and guarding their territory and fighting their neighbors. Inside that tight group, the chimpanzees also have male-male alliances.
At first glance, dolphins seem to have a somewhat similar social system. Two or three adult males form a tight alliance and cooperate to herd a female for mating. (Female dolphins rarely form strong alliances.) Other male teams may try to spirit away the female—particularly if she is in estrus. To fight back, the first-level alliances form partnerships with other first-level alliances, thus creating a larger second-level alliance. Some of these second-level alliances have as many as 14 dolphins and can last 15 years or more. On some occasions, the second-level alliance can call in the troops from yet another group, "a third-order alliance," as the researchers call them—leading to huge battles with more than 20 dolphins biting and bashing each other with their heads and tails over the right to keep or steal a single female.
But are these dolphin battles analogous to what male chimpanzees do? That is, are the dolphin alliances also fighting over territory? To find out, a research team headed by Richard Connor, a cetacean biologist at the University of Massachusetts, Dartmouth, tracked 12 of the second-order alliances in Shark Bay—a 13,000-square-kilometer bay in western Australia—during the peak mating season from July to November over 6 years. The scientists monitored a 600-square-kilometer region of the bay, keeping tabs on every member in each alliance, the ranges, or areas, they regularly traveled, their behaviors, whether the males had a female with them, and—when there was a battle—which groups came to each other's aid. Connor's group then calculated the total home range for each alliance and mapped the degree of overlap between ranges.
The team discovered that, unlike chimps, none of the male groups were patrolling and defending a large community territory. Instead, the dolphins live in a society with a mosaic of many overlapping male and female ranges, without any apparent boundaries. "There isn't a community border that males or females are patrolling," says Connor, whose team reports its findings online today in the Proceedings of the Royal Society B. Instead, he says, they live in an open society, with groups teaming up for a bit and splitting apart—all the while doing what Connor summarizes as "soap operatics," trying to stay on top of who did what to whom, while deciding whether they should stay friends or become foes.
"It's just unprecedented; there's nothing like this in other mammal societies," says Srđan Randić, the study's lead author and Connor's former graduate student, who is now a doctoral candidate at Paris University-South XI.
Although bonobos, orangutans, and Western gorillas have less hostile relationships with neighboring groups than do chimpanzees, none of these species has the tolerance of the dolphins, or their ability to form alliances outside of their immediate community. Among mammals, only elephants come close; though they live in matrilineal groups, elephants maintain relationships outside of these, forming large, stratified societies. But even these large societies are still primarily with close kin and are not changeable as are the dolphins' alliances.
Because female dolphins give birth to only single calves that are separated by several years, the males cannot count on forming alliances with close kin. Instead, male dolphins must learn how to make and maintain friendships—demanding social skills that are likely to have contributed to the dolphins' large brains, says Connor. But it's not just the number of social relationships the dolphins must maintain, he adds. "It's the uncertainty of those third-level alliances. It's those guys you rarely see. What have they been up to since the last time you met them? Are they still on your side?"
Among mammals, humans, elephants, and dolphins are ranked highly by scientists for their level of social cognition—a convergence that Connor's team suggests may be due in part to the minimal amount of energy these species expend when just cruising along. The dolphins, they add, offer a model for how a low cruising speed may lead to social smarts. Because the Shark Bay dolphin population is large and has overlapping territories, it doesn't take long for one group of dolphins moving at their normal speed to meet up with another, possibly competitive, group. In those situations, the dolphins are forced to do the two things that scientists say enhance social cognition: make many friends and form group alliances. Or as Connor puts it, "If you're going to run into your enemies, you better be with your friends, or have some that are close by, willing to be recruited."
"It's an extraordinary study and shows what you can learn from a long-term project on a fairly undisturbed [dolphin] population," says Peter Corkeron, a cetacean biologist with the Northeast Fisheries Science Center in Woods Hole, Massachusetts.
As for the suggestion that there may be a connection between social complexity and low-cost locomotion, just think about what happened when humans got the wheel, says Richard Wrangham, a primatologist at Harvard University. "Invent the chariot, and win an empire!"