Tales of Two Orcas
The more scientists learn about killer whales, the more differences they find between resident pods and transient travelers
- Doug Stewart
- Dec 01, 2000
One of the many things killer whales do well is turn on a dime in tight quarters. Eva Saulitis, a marine biologist with the nonprofit North Gulf Oceanic Society in Alaska, has often followed the animals as they hunt for harbor seals in the rocky inlets of Prince William Sound. The predators are typically longer than the 20-foot skiff she uses and many tons heavier. "But we find them in the most unexpected places, like tiny coves where it´s too narrow to anchor," she says. "We hit rocks trying to follow them."
Like other dolphins, killer whales can make clicking sounds to locate underwater objects using their own sonar, but those that hunt seals are mostly silent (to avoid tipping off their prey). That means the crafty hunters of Prince William Sound, with nearly 1,000 miles of shoreline to prowl, apparently memorize the contours of the whole, convoluted coast.
Agility and brains are just two of the outstanding traits of killer whales, or orcas, the largest and fastest of the dolphins. Long viewed by fishermen as maritime pests, killer whales also have become the crowd-pleasing stars of whale-watch tours, aquarium shows and the occasional movie. And marine biologists, using microphones, DNA sampling and patient observation, have been accumulating surprising new information about the species´ aquatic life-style and its hazards. "There´s much more scrutiny of these animals than there used to be," says John K.B. Ford, head of marine mammal research at the Vancouver Aquarium in British Columbia. "Around here, they´ve really become sacred cows."
These killer whales--those that ply the coastal waters of the Pacific Northwest from the Gulf of Alaska to Washington´s Puget Sound and sometimes down into Northern California--are also among the best studied in the world. One of the more striking findings over the past two decades is that these dolphins, and perhaps orcas worldwide, fall into at least two utterly distinct groups, called residents and transients. They often share a territory, yet they do not mix, do not mate and do not even seem to notice one another.
Transient killer whales are meat-eaters. Traveling widely in small, shifting groups of three to seven members, they prey on other marine mammals: seals, sea lions, porpoises and occasionally much larger whales. Residents, by contrast, swim in noisy, gregarious pods of 10 or 20 members or more. They eat only fish. "The transients tend to be stealthy and a little secretive and generally spread out and work as a loose pack when they´re hunting," says Ken Balcomb, a research biologist at the Center for Whale Research on Washington´s San Juan Island. "Whereas residents are very vivacious; they splash and jump and breach and tail-slap a lot." Residents can afford to be noisy, as herding fish is not as sneaky an enterprise as nabbing harbor seals. The fish hunters sometimes stun their prey with powerful tail-slaps.
Both kinds of killer whales belong to the same species, Orcinus orca, but they have remained apart long enough to have begun diverging physically. A transient´s dorsal fin is pointier and the patch of gray on its back, called a saddle patch, is slightly longer.
"The DNA samples of the two groups are well-enough differentiated to pretty much rule out intermatings for hundreds and possibly thousands of generations," says geneticist Lance Barrett-Lennard of the University of British Columbia, who has studied DNA from killer whales of both types, mostly from the Canadian coast and Alaska´s Prince William Sound. "They probably haven´t diverged enough genetically to be incompatible," he adds. "The only thing keeping them apart so far are cultural differences."
Among residents, "culture" means what mothers teach their calves. (Transients are harder for biologists to track and observe, so their culture is more of a mystery.) Residential pods are amazingly stable groups, organized for life around mother-calf bonds. "The babies stay with the mother even when they become adults and have babies themselves," says Balcomb. "So you end up with a big matrilineal family tree."
Decades ago, scientists assumed the biggest bull in a pod was some kind of harem master. Not so: He is probably a grown offspring still hanging around mom. Mating, which people rarely see in the wild, may take place most commonly during summer gatherings of the pods when a male slips away from his mother for a brief encounter with a female from another pod.
Other than humans, killer whales are the most widely distributed mammals in the world. They are found in every ocean, from the Arctic to the Antarctic, but are most numerous in the waters covering high-latitude continental shelves. They also are unchallenged as the top predator of the waters they inhabit. Lacking natural enemies, they are capable of growing very large and very old. Adult males average 27 feet in length and weigh more than four tons. Females are slightly smaller. In the wild, males can live more than 50 years and females can survive into their eighties.
Calves, typically 8 feet long and weighing between 300 and 400 pounds at birth, nurse underwater for a year or more. When not nursing, a calf swims just behind the mother´s dorsal fin, slipstreaming in her wake. From this vantage point, it evidently picks up pointers, from how to make sounds to how to catch fish.
Mothers are not the only members of the pod capable of giving lessons, Balcomb recently noticed. Last fall off the Washington coast, he watched two recently orphaned males. The younger of the two had probably not been fully weaned. "The older brother would chase salmon in circles around and around the younger whale," he says. "Each time he caught one, he would push it with his tongue toward the younger whale´s mouth." The older brother was clearly trying to teach his orphaned brother to catch and eat fish, Balcomb says.
One of the most important skills killer whales teach their offspring is vocalization. Like other dolphins and humpback whales, orcas use birdlike whistles, low grunts and various calls and squeals to communicate with one another. For the most part, scientists have failed to match specific calls with specific behaviors, although Alaska researcher Saulitis offers one telltale call used by transients in Prince William Sound. "A male sometimes goes off by itself for a while, maybe to scout for new hunting areas," she says. "When it wants to get back in touch with the group, it makes a stereotyped sequence of three calls, over and over again." The calls are unusually long and sound something like an underwater siren: Here I am--where are you? "They´re really loud and dramatic and sort of eerie-sounding," she adds.
Marine mammalogist Ford has discovered that killer whale calls vary from clan to clan, pod to pod and even from individual to individual. (A clan is a set of related residential pods that may have split off from a single ancestral pod.) "The different dialects give us a nice opportunity to track groups of whales by their distinctive sounds, because they´re pretty darn hard to track any other way," he says. For the convenience of eavesdroppers, the aquarium operates a 24-hour-a-day radio station, dubbed ORCA-FM, that broadcasts orca and other underwater sounds from a hydrophone placed off the coast of Vancouver. (You can tune in via the Internet at www.whalelink.org.)
Dialects are slow to change because of killer whales´ long life spans and rigidly matrilineal pods, so they offer clues to social groupings going back centuries or more. Ford suspects these vocal distinctions play a role in the way killer whales pick mates. "Here you have a social system where there´s no dispersal from the natal group and therefore a high risk of inbreeding," he says. "At the same time, you´ve got a great mechanism for identifying relatedness." To avoid mating with near cousins, orca bulls probably choose mates with vocal styles as different as possible from their own. They are turned on by females with heavy accents, so to speak.
Although killer whales worldwide are not endangered, individual populations are showing signs of trouble. Inbreeding is just one of the threats. The Exxon Valdez oil spill in 1989 was as catastrophic for some orcas as for other aquatic life in the Gulf of Alaska. One pod of 36 residents in Prince William Sound lost seven members almost immediately; another six died later that year, and six more in subsequent years. The oil´s toxicity is the number one suspect. With new births, the pod has struggled back to 23 members.
Hardest hit of all was a group of 22 transients that frequented the sound at the time of the spill. Today, 11 survive, with the last live birth in 1984. Scientists fear the group may be doomed.
A more insidious threat to killer whales comes from persistent toxic contaminants. Among the pods around Puget Sound, Vancouver Island and southern British Columbia in the past five years, birth rates are down and calf mortality is up. "We´re seeing a fair number of young adults dying too," says Balcomb. The fat tissues of all the animals examined so far contain extremely high levels of PCBs, which are known to affect immune and reproductive systems. Manufacture of the industrial chemicals is now banned in the United States and Canada, but they are long-lived, and weather systems can spread them from other countries. Not surprisingly, transient killer whales, which feed on other carnivores, carry particularly high levels of these poisons, which accumulate in ever-higher doses as they move up the food chain.
Another possible culprit in the decline of some orca populations is a shortage of food locally, whether fish or marine mammals. The effects can upset an entire ecosystem. Take the case of the disappearing sea otters along a 600-mile stretch of the Aleutian Islands. Starting in the mid-1990s, the population dropped by nine-tenths. At the same time, the numbers of seals and endangered Steller sea lions in the area also dwindled, possibly due to declining fish stocks.
Research group leader James Estes of the U.S. Geological Survey and the University of California, Santa Cruz, realized that killer whales in the area had shifted their diet--no doubt reluctantly--from blubbery pinnipeds to the much scrawnier sea otters. "We calculated that it would only take three or four killer whales eating nothing but sea otters to explain the drop in the sea otter population," he says.
Sea otters eat sea urchins, which eat kelp, which provide habitat for fish, which provide food for eagles and other fish-eaters. With sea otters almost wiped out in this part of the Aleutians, the sea urchin population has exploded, ravaging the kelp beds. Researchers have not yet had a chance to measure the effect on species dependent on the kelp ecosystem, but Estes expects the impact to be significant.
For all the attention that killer whales earn from human observers, much about them remains mysterious. "We´re only seeing one-eighth of their activity when we see them on the water´s surface," says population biologist Marilyn Dahlheim of the National Marine Mammal Laboratory in Seattle. "And our window for observing them is May to September." The rest of the year is too cold and too dark for chasing them around. She adds, "Even the best-known pod of killer whales in the world, here in Puget Sound--which has been studied for close to 30 years--we don´t know where they go in the wintertime!"
For Dahlheim, their mystery is part of their allure. "Sometimes I feel really insignificant around killer whales," she says. "I´m sitting in a boat stuck on the surface, and they´re diving down underwater, doing things I can´t see, and I think about how beautiful their world must be."
Doug Stewart, a Massachusetts writer, wrote about another aquatic carnivore, the American alligator, in the June/July issue.