Survival by the Numbers

In the choreography of life in a herd, animals sacrifice individuality for safety in a crowd.

  • Gary Turbak
  • Oct 01, 1993
From a low-flying plane, the scene on the tundra was a classic of nature at its tooth and-claw edge. As caribou cows watched helplessly, a lone wolf attacked their newborn calves, over and over. Ken Whitten, a research biologist with the Alaska Department of Fish and Game, could only watch as the wolf darted between a cow's legs, seized her day-old calf, killed it with a quick shake and ran on to the next pair. The wolf killed again and again; in five minutes, six calves were dead. Then a fog bank closed in, with no sign that the massacre was over.

In most cases, a wolf takes a single victim, the others run away and that's the end of it-but in 1988, the others were still too young, too "wobbly-legged and slow," says Whitten, to dash away. Snow conditions had forced the caribou to calve inland from their usual site on the coastal plain. And the helplessness of the newborns, practically on the doorsteps of wolf dens in the foothills, had this wolf in a killing frenzy. Whitten had never seen anything to match it. "For all I know, that wolf may have killed seven more calves," says Whitten.

That's not as callous as it may sound. As a biologist, Whitten knew that the slaughter in the snow represented a defensive adaptation of the caribou, no tragedy but a sacrifice for the survival of the species. In a strategy known as swamping, caribou cows travel hundreds of miles each spring to give birth on the same small patch where they themselves were calved. Thousands of births cluster into a brief three weeks. And usually the newborns are beyond the range of most predators.

For the wolves and grizzlies that prey on the young, the market is flooded. The calves can't possibly all be killed during the time they are at their most vulnerable, even when newborn, so most of them are guaranteed to come through. "If births were spread throughout the year," says David Barash, professor of zoology and psychology at the University of Washington, "each individual could be picked off in turn.

Swamping is an evolutionary adaptation used by many herding animals, including the wildebeest and Thompson's, gazelle of the African veldt. And herding itself is a rich field of study for zoologists intrigued with social behavior. Basically, it's a strategy for eating without being eaten, practiced by prey animals banding together to seek food and improve their chances against predators. In effect, they trade the Freedom and individuality of solitary life for the sheer safety of numbers. The herd that provides security and defense for its members also governs every aspect of their lives.

Herding animals usually move with the seasons, since their numbers are so big that they use up food supplies in one place after another, and their mating and birthing are timed by the migratory cycle. They must learn to communicate and settle conflicts without murderous battles. The herd instinct is so strong that in some circumstances members follow one another blindly to certain death. Humans have been studying herds ever since fur-covered hunters first took up the chase.

Caribou researchers have been particularly busy in the past few years trying to gauge the impact of proposed oil drilling in the Arctic National Wildlife Refuge, where the Porcupine herd, 180,000 strong, spends part of each summer calving and foraging. (The herd is named for a river it crosses during migration.)

Whitten and his colleagues monitor the calving every year, count the herd periodically using aerial photos and estimate the ratios of bulls and calves to cows. Individual animals are fitted with radio collars and tracked by satellite. The studies have added to the lore of herds. Whitten was surprised, for instance, to discover that a caribou may wander more miles trying to escape mosquitoes on its summer range than in its grueling fall and spring migrations.

Herding has obvious parallels in the schooling of fish, flocking of birds and swarming of insects. Among mammals, herding is just one extreme of a whole range of social behavior. At the other extreme, even such loners as male wolverines can be said to socialize, if only long enough to mate. Along the continuum, beavers have extended-family relationships, and white-tailed deer form seasonal herds to survive winter conditions. If food is abundant enough, mammals such as prairie dogs form permanent colonies-in effect, stationary herds-with complex social rules.

For the most part, herds have evolved among animals in open country: caribou, sheep, bison, elk, pronghorn and the ungulates of Africa. Their strategy is based on keen vision, running for protection and a basic selfishness: Each animal will sacrifice its neighbor to survive. A herd can consist of as few as a dozen musk-oxen or as many as the horizon-filling multitude of the Porcupine caribou. Some species herd by themselves, but the African plains are filled with mixed herds of antelope, zebra and wildebeest. Herds also have differing structures: A few species, such as mountain sheep, are led by the strongest males, but most seem to rely on older females as leaders.

Whitten's satellite tracking shows caribou wandering almost at random in the vast herd, joining one grazing bunch after another. The herd instinct seems to involve a generalized gregariousness, he says: "When they wander off from the group they've been with, they're real good at finding other caribou. And they have scent glands in their hooves that leave tracks for the others to follow."

Grouping together provides a key defense against predators. Antelope feeding with their heads down are vulnerable to stalking leopards, but a herd contains many eyes for lookout duty, and only a few need look up at any given time. Animals can bunch in defensive formation. Bison and musk-oxen form circles facing outward, with the weakest inside and a formidable ring of horns. As for breeding, animals in herds have an evolutionary edge: The herd efficiently assembles the whole population and gives the strongest males maximum opportunity to plant their seed. Above all, there's safety in sheer numbers. In normal circumstances, predators kill just one or two animals at a time, picking off the slowest and weakest individuals. "The chap in the middle of [he herd is always safe," says Valerius Geist, professor of environmental design and biology at the University of Calgary in Alberta, Canada. "And those on the edge only need to run faster than their slowest herdmate."

A running herd also presents a flowing image of confusion to a predator looking for weakness; the lioness chasing the zebra sees only glimpses ofhooves, tails, wild eyes and patches of stripes in the cloud of dust. Some scientists even theorize that the zebra's stripes and the pronghorn's white-onbrown coloration are adaptations to create a dizzily stroboscopic effect when the herd is in flight. As Barash notes, however, "There are no free rides in nature," and herd animals pay for their safety in numbers. For one thing, "It's fine to have help finding food, but once it is found, it must be shared." In the end, a herd is a vast feeding machine, guaranteed to eat up the supply and force all the perils of migration.

Animals in herds are also vulnerable to epidemics. They give powerful signals of their presence and may actually attract predators and parasites. Caribou on their summer feeding grounds are so tormented by thick clouds of bloodthirsty flies and mosquitoes that they turn their heads into the slightest breeze, seek out ice patches to make the bugs more sluggish, wade into the frigid sea and wander incessantly in search of relief. "An animal will go right across the Brooks range, and four days later be back on the coast," says Whitten.

The herd instinct itself can be a fatal tyranny. Native Americans killed bison in the thousands by heading the herd over a cliff, each animal inexorably following the others. Caribou crossing rivers won't turn back even when the current is too rough and all ahead are drowning. In September 1984, the rain-swelled Caniapiscau River killed 10,000 of Quebec's George River herd; bloated bodies littered the banks for 50 miles.

The most enduring mystery of herds is the riddle of their wanderings-not why they move, but how they decide where and when to go. Dating back to Pliny the Elder in ancient Rome, observers have speculated about a group mind or collective consciousness driving bands of animals. "It's an appealing idea, but my scientific training doesn't let me accept it," says Frank Heppner, a zoologist at the University of Rhode Island. Animals in groups probably coordinate their behavior by picking up cues from such variable factors as air currents, the weather, hormonal changes, approaching predators and each other's movements.

But those cues are subtle. In the herd's seasonal movements, says Geist, there's an overall rhythm: "The cycle of migration is tied to the mating season, and that in turn is tied to the light. In the autumn, the gradual decline in daylight triggers the hormonal cycle, and when an animal is reminded that it's time to reproduce, he's also reminded that he's in the wrong place." In the spring, pregnant caribou know by hormonal changes and the swelling of their bellies that it's time to get back to the calving grounds.

Large questions are still open. Why, for instance, are some herd animals less regular in migrating than others? Barring sudden storms that can trigger an early start, a mountain ram will begin his migration every year within two days of the previous year's date. With caribou, the main spring migration may be in full swing in mid-March one year, but not until late April the next. One group may begin moving two weeks ahead of a group on another range. Lengthening days, variations in weather and the changing snow cover are clearly factors in starting the movement. But within the herd individual animals seem to play a key part.

"There are variations in itchiness," explains Geist. "Some animals want to go; some hang back because the food is still too good to leave. It's not a group consciousness, but before the major movement, agitated individuals begin to affect the others. They look restless, they gaze off into the distance, they start wandering off and then return to the herd." It takes two or three days before the whole group gets infected, he says, "but the restlessness spreads, and eventually they all go."

On the move, there is no overall leader; from time to time, one band or another pushes ahead of the broad front of migration. But the bands have leaders, most often older females, who feel the most urgency and communicate it best. They're the ones with the tightest budgets, says Geist: On the way north, they are pregnant and need to get to the calving grounds; heading south, they are depleted by lactation and crave the richer grazing.

Nobody knows for sure what causes the urge to return to specific breeding and calving grounds, but getting there is at least partly learned behavior. There are indications that older animals teach young ones about the trail. And there's evidence that where and when to go can also be unlearned. After some Laplanders tried carrying their reindeer from winter to summer grounds in trucks for several years, the animals were helpless to migrate on their own.

From birth to death, the herd rules life for its individual members. The young must be ready to move with the herd and flee predators, so herd animals give birth to precocious offspring, usually just one a year. The newborns have long legs and robust lungs, and are ready to use them: Barely five minutes after birth, a young African wildebeest can run full tilt, 40 miles per hour.

Males compete for mates by display and occasional battle. Male kob stake out individual territories within the herd for stiff-legged prancings that attract the females, while bull elk gather harems of as many as 40 cows. A bull may lose 100 pounds while fighting off rivals and servicing his cows in the month-long course of the rut.

Like all social behavior, herding demands communication. Biologists have found vocabularies of as many as 30 individual signals used by a single species, expressed in an endless variety of grunts, puffs, postures, cries, head shakes, lip curls and the like.

The most important signals warn of danger. A pronghorn flaring its white rump hairs can be seen for a mile. Prairie dogs bark warnings that send the whole colony underground. If a herd of perpetually grunting wildebeest suddenly hushes, a predator is near. Similarly, African antelope react to danger by freezing into rigid stillness which not only alerts the herd, but makes the signaler less conspicuous to the stalking cheetah.

Other signals support the social contract. Elk have bones in their legs that click like knuckles cracking when they move, helping them identify each other in wooded places. Similarly, caribou have tendons in their hooves that click as they walk, an adaptation that keeps the herd in touch in the dense fogs of the Alaskan coastal plain. There are even signals that seem designed to taunt predators. Some African antelope indulge in spronking, reacting to a predator by repeatedly jumping straight up and down, stiff-legged, like so many pogo sticks. Spronking may fluster the prowling lioness or simply warn her off. David Scheel, a University of Houston ecologist who has worked in Africa's Serengeti Plain, translates: "You've been seen, and we are in such good shape that we will surely escape, so don't waste your time hunting us."

Living cheek by jowl with so many other animals increases competition, and there are inevitable disputes over food as well as mates. Caribou must paw through the snow to graze, and there are frequent squabbles over choice feeding craters. To even the odds, most females of herd species are nearly as large as the males. Female caribou, pronghorn, bighorn sheep and bison even grow horns or antlers for self-protection, though not quite as large as those of males. Wildebeest females actually have a tuft of hair mimicking the male penis-apparently, biologists say, as a token of vigor and a warning to steer clear.

But herd animals have also adapted to minimize injury in conflicts within the group. Posturing and aggressive display often substitute for actual battles. And when animals do fight, seemingly formidable weapons turn out to be mutual defense mechanisms. Elk antlers, for instance, interlock rather than pierce; most bouts are sumo wrestling matches instead of duels to the death.

So the herds endure, in their grunting, chewing, prancing, clicking, spronking multitudes-feeding, breeding, birthing and dying, always on the move. Every two years, when Ken Whitten sets out with his crew to make a spot check on the Porcupine herd on its spring migration, he gets a renewed sense of this timeless mystery.

The helicopter finds the herd and sets the observers down in its path, half a mile apart, human beads dotted across the tundra. At first there is only silence. Then a few antlered heads bob on the horizon. The heads become a swell of caribou and finally a surging tidal wave. The air fills with cacophonous grunting, a doglike smell of wet fur, the clicking of hooves. Fifty yards from Whitten, the leaders pause, dubious about this alien speck on the tundra, but the mass of the herd pushes them forward.

The great wave of caribou divides to pass him, a few feet on either side, melding again behind him. He struggles to focus on individuals and groups: How fat are the calves? How many cows per bull? Sometimes a calf pauses to sniff at his trouser leg. Fully two hours later, the wave thins, then the last animal passes and the horizon empties once again, leaving trampled earth and a lingering musky smell. Whitten feels light-headed. "This is how I imagine the Pleistocene must have been," he says later. "What a riveting, awe-inspiring experience."

Writer Gary Turbak lives in Montana, far from the bigger human herds.

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