Machiavellins of the Deep?
New research suggests that among some bottlenose dolphins, violence and aggression play a key role in their social lives
- Roger Di Silvestro
- Aug 01, 1998
Scientists began discovering dead harbor porpoises washing up on Scottish shores eight years ago. In many instances, the little porpoises´ bodies looked relatively unscathed but postmortem work found that the animals´ internal organs were shattered. In subsequent years, dozens of the dead, battered creatures surfaced in Scottish waters.
"The cause of their internal injuries was a mystery for several years," says Ben Wilson, a dolphin expert at the University of Aberdeen who was part of a team that eventually solved the puzzle with the aid of a videotape shot by an amateur dolphin watcher. The deaths, Wilson discovered, are caused by beatings delivered by the harbor porpoise´s larger relative, the bottlenose dolphin, a creature familiar to television watchers from the series Flipper. The bottlenose, it seems, can deliver devastating, quick blows with its beak and tail. The discovery is just one of several recent findings that are changing the way we look at one of the ocean´s most fascinating creatures.
Behind the dolphin´s fixed, smilelike gaze and remarkable intelligence lurks a creature that sometimes indulges in acts of violence against both other species and its own kin. That fact is being substantiated by dolphin-behavior expert Richard Conner, whose research reveals that male Indian Ocean bottlenose dolphins form groups that function much like roving gangs of human hoodlums. They clash with other groups and rob one another of the great prize of the dolphin realm: mates. Sometimes, Connor notes, two groups of males will form an alliance to fight another group or alliance of groups. His work is preliminary, a scant hint at the secrets still to be wrested from dolphin society.
Connor teaches at the University of Massachusetts, Dartmouth, but focuses his research on dolphins half a world away in Australia´s Shark Bay, an area that attracted him because it provides easy access to dolphins. This inlet of crystal blue waters is home to some Indian Ocean bottlenose dolphins that frequently come close to shore and even allow human visitors to wade next to them and touch them. Connor started studying the dolphins there in 1982. Along with other researchers at the site, he has identified about 400 individual bottlenose dolphins, usually distinguishing them by their distinctive dorsal fins.
The scientist´s work is patterned after a study of Atlantic bottlenose dolphins that has been conducted in waters in and around Florida´s Sarasota Bay since 1970. Headed by Chicago Zoological Society biologist Randall Wells, that study has cataloged more than 2,500 individual dolphins by their dorsal fins. Wells says that half the animals that he first identified in 1970 are still alive. About 100 of the cataloged dolphins are year-round residents of the Sarasota Bay area; the others live in surrounding waters.
Taxonomists are not sure how many dolphin species exist worldwide, though the number seems to be around 30, with about 20 ranging in U.S. waters. Of those species, the bottlenose is probably the most studied since it occurs in coastal waters and survives well in captivity. But the dolphins in Sarasota Bay and those off Shark Bay, although both called bottlenose, may be different species. Scientists are uncertain. They do know that the Indian Ocean bottlenose dolphin has distinct physical and behavioral differences from the Atlantic bottlenose.
Off Florida, Wells found that the marine mammals stay with their mothers for 3 to 6 years, then join groups made up of juvenile animals. Females reach sexual maturity when they are between 5 and 10 years old. "Right after they give birth, some females go off alone," Wells says. "Then they often group with other mothers that have young of the same age." These nursery groups may consist of 10 to 14 individuals, including the young. "You see tremendous flexibility," he adds. "Some females raise their young alone."
Genetic analyses by Portland State University researcher Debbie Duffield show that males at Sarasota Bay do not breed successfully until they are in their twenties, about a decade after they reach sexual maturity. Like females, males may continue breeding into their forties. Few dolphins reach 50 years of age, and those that do are usually female. Males usually are more scarred than females, suggesting a higher level of violence in their lives and a possible explanation for why they do not live as long as females.
Although some mature males become loners, most pair up with other males when they reach sexual maturity. Wells has observed strongly bonded pairs of males together for as long as two decades. But there is no evidence that these males have better breeding success than lone males.
Female dolphins in Connor´s study may form loose assocations with other females in small groups. Mature males, however, engage in two more complex levels of social behavior. "On one level, you have stable pairs or trios," Connor says. "On another level, you get alliances between the stable groups." The latter represents what Connor calls "alliances of alliances."
Wells has not seen alliances of alliances in the Atlantic bottlenose. In fact, only one other species is known to engage in this type of social behavior, says Connor: humans. People form alliances of kin groups, villages and nations that fight other kin groups, villages or nations. Similarly, male dolphin groups form alliances to fight other male dolphin groups. This makes dolphin social life "extremely intense," says Connor, "like an urban street gang or a tribal society where the cost of social mistakes may be pronounced physical costs."
Relationships with females may be one of the underpinnings of the male-to-male bonds. When a female is two to four years old, she becomes attractive to the males. A male group may then begin directing a special popping sound at the female. "This induces the females to stay close to the males and is backed up with aggression," says Connor. "It´s a threat. It´s another, not-so-appealing aspect of their behavior. The females try to get away. There´s a conflict of interest." He does not know yet if all the males in a group mate with the female or if only one does.
The scientist has found, however, that males forming an alliance "shadow one another. The physical association is quite clear," he says. "They surface together with almost military synchronicity."
An alliance of two male groups may prowl the sea together, or a new, complicated twist might take place: A third group might appear, bond with one of the other groups already in an alliance and then team with that group to drive off its former ally. The social undercurrents driving these shifting alliances remain unknown.
When male groups clash, the disputes can lead to intense fighting. In combat the dolphins may use teeth and fins, which can deliver powerful blows, but the most potentially lethal weapon is the tail. "It´s very, very powerful," Connor says. "They could just blast you with their tails. If one did it to you, you´d be dead. Your ribs would be broken, your liver shredded, your lungs collapsed."
Although the males may use their tails against one another, whether they do so against other enemies is not clear. Biologists have evidence that the bottlenose uses only its head and beak on the harbor porpoises found dead in Scottish waters. But those porpoises are smaller than the dolphins and seemingly pose no threat.
What about a potentially dangerous adversary, such as a shark? Popular legend says that dolphins attack sharks as natural enemies, ramming the sharks with their heads. In fact, this behavior is unlikely to occur, both Connor and Wells contend. About 31 percent of Sarasota Bay´s bottlenose dolphins bear healed shark bites. However, says Wells, "Sharks and dolphins show a natural toleration of each other. They are not the ultimate mortal enemies."
Most sharks are small and harmless, Connor adds. Only some of the larger species are threats to dolphins, but even if one of these comes into the vicinity of dolphins, a direct confrontation is unlikely. "Our little dolphins are not going to beat up a large tiger shark," notes the scientist. "It would be like hitting a tank." Generally, the dolphins flee from dangerous sharks. On one occasion, Connor saw an 8-foot-long juvenile great white shark quietly cruise in among a group of resting dolphins. "The dolphins started leaping out of the water and racing away and didn´t stop for 10 minutes," he says. "No one has seen wild dolphins beat up sharks."
Shark Bay dolphins do occasionally put the bite on human bathers, however. Dolphins in the bay often let people touch them, but sometimes the touching gets to be too much for the dolphins and they warn away people by thrashing their tails and otherwise showing agitation. "Humans don´t respect what they don´t fear," says Connor, so people petting dolphins often ignore the sign language. An agitated dolphin may bite the offender. The wound is usually superficial, however, indicating another warning signal.
In the open sea, dolphins doubtless are less tolerant about letting other large creatures approach them. In the fish-eat-fish world of the ocean, where there are few places to hide, even a fishlike mammal can never let down its guard. "They always have to be alert," says Connor.
That perhaps explains the need for alliances and even alliances of alliances among these creatures. Extra sets of eyes, ears and tails may be essential to survival in the ocean, even for dolphins. "The sea," adds Connor, "is sometimes a scary-as-hell place to live."
Virginia writer Roger DiSilvestro formerly was a senior editor of this magazine.