Bottle-Nosed Dolphins' Aggressive BehaviorBottle-nosed dolphins have long had the image of being intelligent guardians of the sea who speak their own language, enjoy the company of humans, and have even saved shipwrecked sailors from certain doom by leading them to land. Although these frolicking mammals may look friendly with their toothy smiles and playful gestures, marine biologists in mid-1999 were learning that dolphins also have a dark side that may include the capacity to kill.
In Shark Bay, on the coast of Western Australia, researchers from the University of Massachusetts in Dartmouth have observed gangs of male dolphins harassing and intimidating sexually receptive mature female dolphins. The researchers, led by biologist Richard Connor, documented several cases in which male dolphins, working in groups of twos and threes, “kidnapped” females from other dolphin groups. The females were then held captive for as long as one month.
This behavior appeared to be rooted in something other than simply aggressive tendencies. Based on observations of male dolphins in Shark Bay he had made since the 1980's, Connor believes that these gangs of male dolphins may be one of the few examples in nature in which the males of a species form stable long-term bonds with one another. In some cases, those bonds last more than 10 years.
Shark Bay, part of the Indian Ocean, has the largest sea grass beds in the world. The beds contain an abundance of fish and, because of the food source, a large number of dolphins. Because of this high concentration of dolphins, Connor and his team of researchers were able to study the formation and interactions of male social groups from boats in the clear waters.
Studying A Large Gang of Dolphins
As part of one two-year study that began in 1995, Connor studied a “supergang” of 14 adult male bottle-nosed dolphins in Shark Bay. Although he had studied other groups of dolphins that teamed up in twos or threes, this appeared to be an alliance of several smaller gangs united to form the supergang.
Connor reported that this alliance was the first documented evidence that male dolphins would form not only primary but also secondary social bonds—‘bonds extending beyond an original group to encompass new individuals‘—to fight rival males and stalk potential mates. Connor reported that during the course of his two-year study, as he and his associates observed the supergang, the dolphins never lost a fight.
Many biologists considered Connor's findings interesting because in most mammals, such as elephants, whales, and many species of apes, the females usually establish closer social alliances than males do. Typically, the females of a species are more cooperative, less violent, and more likely to use complex vocalizations than males. With some animals, such as killer whales, the social bonds among females are so strong that female calves never leave their mothers.
Aggressive Behavior Toward Captive Females
Although Connor's research revealed that male dolphins form cooperative units, the purpose of these units is very different from those of the females of the species. For the male dolphins, the motivation is purely sexual. The roving groups of males observed by Connor would almost always mate with their female captives, either individually or several at a time, during the course of the females' captivity. In some cases, the teams of male dolphins would ram and bite the females to prevent them from escaping.
However, Connor said that his research found no evidence that male dolphins ever forced female dolphins to mate. In fact, some females did not seem to be bothered by being herded around by their male cohorts. But by outnumbering the females, Connor said, the male gangs do bully, and may injure, females with powerful blows from their beaks or sharp teeth.
Such aggression is not uncommon in mammals. But what amazed Connor and his colleagues was how organized the dolphins' aggressive behavior was. Connor said the herding of females by male gangs may be, in part, a reaction to female promiscuity. Biologists know that female dolphins are very sexually active and may want to mate with as many males as they can. Some researchers thus speculated that herding by male gangs is a strategy to prevent females from reproducing with rival dolphins. In this way, a male alliance may increase the chances that the group's members will breed the next generation of dolphins.
Contrary Views On Herding Behavior
However, not all biologists agreed that herding females is a successful reproductive tactic—‘or even that it is a common type of behavior. Other research has shown that dolphins seem to have highly complex social orders that vary widely from place to place. For example, captive bottle-nosed dolphins do not form roving gangs. In fact, herding and supergangs have not been documented in any dolphin populations except those in Shark Bay.
Moreover, some studies have revealed that female dolphins have many strategies to control with which male dolphin they will conceive with. Because females dolphins spend three to six years raising a single calf, they tend to be very selective about who sires their offspring. Marine biologists said it would take genetic testing to establish patterns of paternity among the Shark Bay dolphins and evaluate the success of male herding behavior and violence in dolphin reproduction.
Dolphin Attacks On Harbor Porpoises
Although herding may be confined to the coast of Australia, biologists have reported that other types of aggressive dolphin behavior are more widespread than many people believe. Only bottle-nosed dolphins, however, have been observed to show these mysteriously violent tendencies. In the late 1990's, researchers in Virginia and Scotland were surprised by the discovery of several instances of extreme violence by bottle-nosed dolphins.
Some of the first evidence of dolphin brutality began washing up on the shores of Moray Firth, a large bay on the northeast coast of Scotland, in 1990. Researchers found hundreds of harbor porpoise carcasses on the beach. The bodies did not look badly injured, but when scientists performed autopsies, they discovered that all the bodies had sustained serious injuries. Most of the dead porpoises had fractured bones, ripped tissue, and bruised organs.
Researchers originally theorized that boat propellers or fishing nets might have been responsible for the death of the porpoises. But in 1994, researchers discovered a dead porpoise with fresh bite marks. Scientists measured the marks and discovered that they exactly matched the spacing of the teeth on an adult bottle-nosed dolphin. Why the dolphins were killing porpoises was a mystery, since the two animals do not compete for the same fish for food.
Killing Their Young
The mystery deepened when the bodies of young dolphins bearing the same kinds of wounds also began washing up on Scottish beaches. By 1998, the researchers concluded that bottle-nosed dolphins were killing not only young porpoises but also their own babies.
Such incidents of infanticide (the killing of an infant) among dolphins were not confined to Scotland. In Virginia Beach, Virginia, in the late 1990's, biologists found other battered carcasses of baby bottle-nosed dolphins washed up with the tides. All of the bodies had wounds similar to those seen on the bodies of the porpoises and dolphins in Scotland.
The researchers reported that all of the dead baby dolphins discovered in the United States and Scotland were less than 1 year old, an age at which the infants are normally under the full-time care of their mothers. Instead, somehow, these baby dolphins had been bitten and bludgeoned to death by adults.
Because male and female dolphins look nearly identical, teams of researchers have been unable to determine whether it was males or females who were killing the young. But some biologists have speculated that adult males were doing the killing. They based their conclusion on the fact that infanticide among other animals is more commonly committed by males.
In addition, scientists know that a female dolphin, while caring for her calf, can remain sexually inactive for several years. But a female becomes fertile and ready to mate within one to two weeks of losing an infant. Therefore, researchers speculated that killing a mother's calf might be a male strategy designed not only to destroy the offspring of a rival but also to bring the female into a sexually receptive state.
By mid-2000, the deaths of the porpoises still remained a mystery, since these animals have no obvious connection to any reproductive advantages. Scientists in Scotland, however, theorized that if the male dolphins were responsible for the large number of reported deaths, they may have used the porpoises as “target practice” for later attacks on infant dolphins. Other experts, however, theorized that the attacks might instead have stemmed from simple aggression or from confusing the baby porpoises for baby dolphins in the murky waters.
Researchers agreed that reports of such aggressive behavior should serve as a warning to people who enjoy swimming with dolphins. Furthermore, many scientists said that such studies were just the first step in gaining a better understanding of the nature of bottle-nosed dolphins.
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