Like a marine mammal version of Facebook, male and female bottlenose dolphins spend their days courting friends and building alliances. Two new studies show just how important such friendships are to dolphins—and the role friends and alliances play in life's biggest game: the race to reproduce.
Male bottlenose dolphins (Tursiops sp.) form tight bonds with friends and allies that are as intricate and devious as those of humans. Researchers already know, for example, that males team up as duos or trios—known as first-level alliances—so that they can mate with a female without her swimming away. (Females come into estrus only every 4 to 5 years and are thus a rare prize.) But rival males will often try to steal the female, causing the duo or trio to join forces with other duos and trios in what's known as a second-level alliance.
"There can be huge battles over a single female," says Richard Connor, an animal behaviorist at the University of Massachusetts, Dartmouth, who has been studying wild dolphins in Shark Bay, Australia, for 24 years. "A trio under attack will get help from their buddies."
Now Connor and colleagues have found an even higher level of alliance. In the biggest fights, the team found, the second-level alliance may receive help from another group of male dolphins, forming what the researchers call a "third-level" alliance. Even among chimpanzees, scientists have not witnessed such sophisticated partnerships, where one group of animals receives help from another group in a fight. The need to keep track of these complex and shifting alliances may help explain why dolphins have such large brains, the researchers report online today in Biology Letters.
The extensive male partnerships are puzzling, says Connor, because receptive females are so rare. "These are rival males, not relatives, and, theoretically, they should avoid each other." They might help allies keep a captured female because at some point in the future, they will be the ones calling for help, he speculates.
Female bottlenose dolphins also have a strong network of female family and friends—and in the second study, Connor and another team of researchers found that this helps them have more calves. The research—reported online this week in the Proceedings of the National Academy of Sciences—combined genetic analysis with observations collected by the scientists on 52 female dolphins in Shark Bay. The team found that female dolphins have more calves that survive to age 3 if they have friends that have also raised calves to that age, when a dolphin calf usually becomes independent. (A similar connection has been seen in horses.)
"It's not just a matter of genetics," of having relatives who are successful mothers, says Celine Frere, the study's lead author, who analyzed the data while at the University of New South Wales, Australia; she is now an evolutionary biologist at the University of Queensland. "It's who they hang out with" that's important to a female dolphin's ability to give birth and raise a calf, perhaps because young calves can be easy prey for sharks.
The "well-done" papers "add new insight to the dolphin's complex and fascinating social system," says Hal Whitehead, a cetacean biologist at the Dalhousie University in Canada. In future studies, the scientists hope to learn "how the dolphins may use their communication calls" to build and maintain these complex relationships," says Connor.
Correction: This article was changed to include the name of the University of New South Wales, Australia, where Celine Frere carried out her research for the study.