Dolphins: Flipper or Killer?

Dolphins: Flipper or Killer? – evidence of infanticide among bottlenose dolphins

Melissa Stewart

New research reveals a dark side of the mammal.

In the summer of 1997, a dead baby bottlenose dolphin washed onto a Virginia beach. Its body was badly bruised; it had broken ribs and a punctured lung. One telltale clue gave scientists a grim surprise: bite marks that matched the exact pattern of the teeth of an adult bottlenose. Researchers concluded an adult dolphin had murdered a young baby or calf, a practice known in nature as infanticide.

“This is a dramatic change from the way people think of dolphins,” says Dale J. Dunn, a veterinarian pathologist, a specialist in animal diseases. No kidding. When most people think of dolphins, they think Flipper, not Killer. Since ancient Greece, dolphins have been celebrated in art and myth as frolicking creatures that protect shipwrecked sailors from ocean predators. Today, delighted fans still cheer dolphin antics in aquariums and marine parks, and swimming alongside captive dolphins in places like the Florida Keys has boomed into a tourist craze.

Now scientists are amassing startling evidence that suggests the beloved animals have a violent side as well. Dolphins seem to be killing porpoises, a related sea mammal, and baby dolphins in droves, wielding their long snouts as clubs and their jagged teeth to slash their victims to death. Can it be that dolphin behavior simply resembles that of most large animals, who are capable of being playful or violent by turns?

END OF A MYTH?

By the end of summer 1997, scientists in Virginia had discovered seven more dead dolphin calves that washed ashore with teeth marks, bruises, and damaged internal organs. “We immediately looked for other scientists researching dolphin deaths,” Dunn says. “That’s when we heard about researchers in Scotland.”

Scottish scientists started studying dolphins in 1990 after an increase of dead beached dolphins in northeast Scotland. At first they guessed that ongoing viral epidemics or fishing nets might be the culprit. Then in 1994 researchers discovered a porpoise washed ashore with bloody tooth marks that matched the teeth of an adult bottlenose.

When they took a closer look at the bodies of 105 porpoises recovered between 1991 to 1993, researchers realized 42 had been killed by bottlenose dolphins. Bottlenoses often swim close to shore rather than in mid-ocean, so this species (one of 26 identified dolphin species) was implicated in eyewitness accounts of dolphin attacks.

In one case, witnesses report a group of bottlenoses ramming a sole harbor porpoise with their heads and long snouts, sending it flying into the air. After more than 30 minutes of abuse, the beaten porpoise sank into the water.

“The animals I’ve been studying for 10 years are killing these porpoises!” says Ben Wilson, a dolphin expert at the University of Aberdeen in Scotland, who was shocked by his team’s discovery. Dolphins and porpoises are both marine mammals, or warm-blooded animals that nurse their young on milk. The two mammals belong to the same order called cetaceans (si-TAY-shins). But they eat different fishes and don’t usually compete for food (see food web, p. 10). So Wilson and his team are baffled by the apparently senseless porpoise murders.

VIOLENT DOLPHINS?

Researchers have known for decades that dolphins, like many animals, can behave aggressively. Male dolphins often compete for mates and food. They lunge at one another, slap their tails (or flukes), snap their jaws, and even bite. Scientists have also occasionally observed males threatening or fighting with females and calves, but aren’t sure what such behavior means.

But why would bottlenoses kill porpoises? Since harbor porpoises are roughly the same size as baby dolphins, Scottish researchers speculate that dolphins may practice their infant-killing techniques on porpoises. Infanticide is not uncommon in nature, especially among mammals. When food supplies dwindle, a mother gerbil, for example, may eat the weakest of her babies to ensure she has enough energy to produce food for her other infants. Scientists theorize that bottlenose dolphins may have more in common with these “cannibal animals” (see sidebar, right) than was previously thought.

Dolphins may be trying to destroy potential rivals, or may hope to free up females for mating. Female dolphins nurse their young for three or four years, Dunn explains. During that time females aren’t interested in males. But when a calf is prematurely killed, a female becomes ready to mate again, perhaps with the father’s rival.

Dolphin killers may also be driven by anger or aggression. If that’s true, they could share common characteristics with human murderers. Like humans, dolphins don’t eat their victims. Could dolphins possibly be killing for sport? No one knows for sure, but marine scientists hope to find out.

Killer whales, called orcas, also belong to the order of cetaceans and to the dolphin family of delphinidae (del-FIN-i-day). “Killer whales are just big dolphins,” says Daniel Odell, a research biologist at Sea World in Orlando, Florida. “If orcas are ferocious killer sea mammals, it shouldn’t be so surprising that dolphins are, too.”

REALITY CHECK

In the end, scientists have much to learn about dolphin behavior. But they’ve concluded that, like other large mammals, wild dolphins engage in violent acts and can be dangerous to other animals and even humans. In recent years, travel agents have begun wooing tourists with boat trips that highlight swimming with wild dolphins. Experts are concerned about reports that wild dolphins have bitten, hit, and bodyslammed swimmers. “Wild dolphins need to be viewed with respect and kept at a safe distance,” says Dunn.

On the other hand, captive dolphins pose little threat to people in swim programs, where animals are usually well-trained and closely supervised by experts working in pools and enclosed bays. Like trained dogs and chimpanzees, trained dolphins interact safely with humans. In the end, they may not be the do-gooders they were once made out to be, but dolphins are probably no more violent or dangerous than other large mammals–including humans.

RELATED ARTICLE: Dolphins & Company

Name Bottlenose Dolphin Harbor Porpoise

Family Delphinidae Phocoinedae

Class Mammal Mammal

Species The bottlenose is the only “Porpoise,” often used to

Info one of 26 dolphin species mean any small dolphin, is

identified as a killer of actually a distinct mammal

porpoises and baby with 6 species. The harbor

dolphins. Bottlenose swim porpoise is a shy loner,

close to shore, while and is sometimes the prey

other dolphin species swim of bottlenose gangs, who

in mid-ocean. kill but don’t feed on it.

Size and Some adults grow to 3.9 m Adults rarely grow beyond

Weight (12.5 ft) and weigh 650 kg 1.5 m (5 ft) and weigh 65

(1,450 lb). kg (145 lb).

Where All oceans and in remote Coastal waters of the

Found places like the Black Sea. northern hemisphere. They

swim up rivers.

Great Their pointy snout and From underneath, their

Features curved mouth resembles a white belly looks like the

wide smile. sky to predators, thus

protecting them.

Cool Dolphins have a Healthy porpoises often

facts melon-shaped cavity in support injured ones and

their head for bring them up to the

echolocation–finding food surface to breath.

by bouncing sound waves

off of prey. The cavity

focuses the sounds into

a beam of short clicks.

Name Killer Whale Dolphin Fish

Family Delphinidae Coryphaenidae

Class Mammal Fish

Species Killer whales, or orcas, Some people confuse

Info are one of only 6 “whale” dolphin fish with

species that belong in the dolphins, but these salt

dolphin family. The killer water (and fresh-water)

whale doesn’t show fish, called dorado, bear

aggression to its own no relation to dolphins.

close group, and it There are 2 species of

doesn’t attack swimmers dolphin fish.

in the wild.

Size and Adults grow up to 10 m (33 Dolphin fish can grow to 2

Weight ft) long and can weigh 9 cm (6.5 ft) long and weigh

tons. 40 kg (100 lb).

Where All oceans–they even lurk Away from coasts in warm

Found near beaches to hunt for tropical waters.

seals.

Great Males have high fins–1.8 Their body-long fin looks

Features m (6 ft)–which they slap prehistoric.

on the water to signal

each other.

Cool Orcas leap out of the Dolphin fish often jump

facts water, spin, and land on out of the water to catch

their backs. Juveniles flying fish, their

spin more than adults. favorite meal!

RELATED ARTICLE: The Seafood Web

Dolphins belong to a food web that interconnects all marine organisms. Above, each yellow arrow leads from food to predator. In general, smaller creatures are food for larger creatures. But tiny microorganisms feed on and decompose even the largest critters after they die and sink to the sea floor (red squiggles). Humans, also part of the web, net many kinds of sea animals for food. They often trap dolphins by accident in fishing nets.

RELATED ARTICLE: Cannibal Animals

Dolphins have joined a growing list of animals that researchers now realize are vicious killers.

Experts have identified more than 1,300 animal species that kill their own kind. Many of these animals eat their victims and are called cannibals. These include praying mantises, black widow spiders, tiger salamanders, horned frogs, sharks, damselfish, great egrets, lions and bears.

The idea of cannibalism may make your skin crawl, but from an evolutionary point of view it makes sense. Male brown bears sometimes attack and devour bear cubs that aren’t theirs in order to get cub moms to mate with them instead.

Dolphins, however, are more like hyenas since they kill but don’t eat their own kind. When a female hyena has twins, one usually kills the other to eliminate a potential rival for mates and social rank.3

COPYRIGHT 1999 Scholastic, Inc.

COPYRIGHT 2000 Gale Group