Class: Mammalia
Order: Carnivora
Family: Canidae
Genus: Canus
Species: Lupus

Size and Weight: Gray wolves are the largest canids: on average, adults have a nose-to-tail length between 4.5 and 6ft (1.4 to 1.8m), a height at the shoulder from 26 to 32 inches (66 to 81cm), and a weight measuring between 50 and 110lbs (22.7 to 50kg). The largest wolf on record weighed 175lbs (79.3kg). Males are larger than females, and northern wolves are generally larger than those in southern areas.

Physical Features: The animal’s scientific name is Canis lupis, meaning “dog” and “wolf” in Latin. Domestic dogs are descended from wolves, which closely resemble some domestic breeds like the German shepherd or malamute. However, gray wolves are larger than most dogs, and appear even larger in the winter because of their thick undercoat. Their coloring is usually a mix of gray and brown (though white, red, and black coloration has been observed) with lighter fur on its legs and belly. They have relatively short ears, and a long bushy tail. For pushing through snow, wolves have narrow chests, and when walking on top of snow, big paws to help distribute their weight. Long legs and dense muscles make wolves excellent runners. They can reach 38 mph (61kph) sprinting, but will more frequently run long distances at around 5 mph (8kph). Marathon chases help wolves tire their prey which, once caught, are quickly killed by wolves’ powerful jaws and teeth. Their mouths contains 42 teeth including carnassial teeth, unique to carnivores for cutting through meat and bone, and can snap closed with pressure exceeding 1,000lbs per square inch (6895kPa).

Life span: The life spans of wild wolves vary dramatically. Although the average lifespan is between 6 and 8 years, many will die sooner, and some can reach 13. Wolves in captivity can live up to 17 years.

Diet: Wolves are carnivores, and tend to prey on large ungulates: hoofed animals like elk, deer, and boar. When livestock is readily available to them, wolves have been known to prey on animals like sheep and cows. When ideal prey is unavailable, wolves will eat smaller mammals, reptiles, insects, and fruits and berries. Since wolves may have to wait for days between big kills, they eat a lot when they can. They can consume as much as 22lbs (10kg) in one sitting.

Geography: Gray wolves and their relatives, red foxes, have the largest natural range of any land-based mammal besides humans and possibly some rodents. Even with their habitat and population severely constricted by human activity, various subspecies of gray wolves can be found in North America, Europe, and Asia, and as far north as the Canadian Arctic and as far south as India.

Habitat: As their range indicates, gray wolves are able to live in many biomes, from Arctic tundra to dense forests, to mountains, to dry shrublands.

Breeding and Social Structure: Wolves live in tight social units known as packs. The basic unit of a pack is a monogamous breeding pair. Beyond the founding male and female, wolf packs include their most recent litter, their offspring from previous years, and occasionally unrelated wolves. The average size is about 6, but packs exceeding 30 members have been observed. The breeding pair was once referred to as the alpha male and female, but some researchers believe that wolf hierarchy is not as rigid as those terms imply. In rare circumstances, if a pack faces a high mortality rate, or if there is an abundance of prey, other wolves in the pack may breed. Wolves mate once a year, generally in early spring. Pregnant females have a gestation period of about 63 days, and produce litters of about 4-7 pups. Pups are born unable to see or hear, and remain inside the den for about 4 weeks after they’re born. After about 10 weeks pups are weaned and become part of the pack. After a year or two a pup, now a young wolf, may leave in search of its own territory, or it may stay with the pack.

Wolves communicate in several ways, often to reinforce the breeding pair’s dominance and the rest of the pack’s submissive roles. Body language, such as eye contact, facial expressions, posture, and tail positions, can have specific meanings. Wolves also use sounds—whimpers, whines, barks, and howls—and scent marking to communicate with pack mates and strange wolves in their territory. Strengthening the hierarchical relationship in a pack may lead to physical conflict, but wolves try to avoid injuring members of their pack. Companionable behavior is much more common: wolves have been observed bringing food to incapacitated pack mates, and relating to dead pack mates in a way that suggests mourning.

Depending on prey density and other conditions, a pack’s territory can be small and close to other packs’ ranges, or larger and more spread out. Territories can be anywhere between 50 and 1,000 square miles (80–1,600km²).

Risks: Pup mortality rates can be as high as 60%, and starvation is one of the main causes of natural death. Territory fights with other wolves and scuffles with large prey can lead to injuries and death, and wolves are subject to diseases like Lyme disease, and those endemic to canids, like canine mange, parvovirus, and distemper. When in close proximity to wolves, humans pose a major threat to the animals. Humans have a long history of hunting and trapping wolves. These practices are still legal in some places, and still occur even in areas where wolves are protected. Ranchers may kill or poison wolves to protect their livestock. Human destruction of wolves’ preferred habitat has forced the animals to move closer to developed areas in search of food. And when roads divide the wilderness, wolf populations can become isolated, inbred, and more susceptible to disease.

Additional Facts:

• Wolves’ sense of smell is estimated to be 100 times stronger than humans. They are reportedly able to smell prey from miles away.
• DNA sequencing now shows that domestic dogs, once thought to be bred from a mix of canids, in fact descended solely from gray wolves.
• The Egyptian golden jackal, once thought to be a sub-species of the golden jackal, is actually a type of gray wolf.
• Not much is known about wolves’ ability to perceive color, but one experiment found that they can detect red and yellow more easily than blue or green. This may be related to how important blood and urine are as sources of information.
• Wolves tend to initiate play with other wolves by bowing.
• At about 5 inches long and 4 inches wide, all wolf tracks are larger than coyote tracks, but not all dog tracks. To distinguish wolf prints from dog prints, compare patterns. Unlike dogs, wolves tend to travel at a regular pace in straight lines.
• Wolves appear in Roman, Turkish, Norse, Japanese, and Native American myths and legends.

When the gray wolf was eradicated from Yellowstone National Park in the 1920s, more was lost than just the noble and fascinating predator. The park’s entire ecosystem changed. Now, nearly a dozen years since the wolves returned, the recovery of that system to its natural balance is well underway, say ecologists William Ripple and Robert Beschta of Oregon State University.

The researchers began studying the interaction of wolves with other parts of the ecosystem somewhat indirectly. “Back in 1997, I became aware that the aspen trees in Yellowstone were declining,” Ripple explains. “There was disagreement and confusion as to why these trees were disappearing, so I set out with graduate students to unravel this mystery.”

“We went out to the park and we cored the trees and studied the tree rings which show the annual growth, and we were able to age the trees that are still there,” Ripple says. The tree ring analysis indicated that the aspen, which usually regenerate themselves by sending off new shoots rather than by producing seeds, had stopped producing new trees during the first half of the 20th century.

Ripple and his colleagues looked at several possible variables that could be affecting the trees, from climate fluctuations to a changing natural forest fire regime. But the only factor that fit, Ripple says, was the browsing patterns of elk, which like to feed on the seedlings of aspen trees, and which are also a favored food of gray wolves: “The wolves were killed off from Yellowstone in the 1920s, which correlated with the start of the aspen decline. That led us to develop the hypothesis that the wolves were connected in some way to the aspen trees.” That connection, Ripple concluded, was mediated through elk: “We connected the dots: wolves affect elk; elk affect aspen; and therefore wolves affect aspen.”

Aspen grove (photo: NPS/J Schmidt; 1977)

Ripple and his colleagues subsequently discovered other changes. In some areas, willows — small, scrubby trees that grow in wet areas along stream beds — were starting to grow taller, because they were escaping predation by elk. In other areas, however, the willows continued to be heavily grazed upon. The same patchy changes were also seen with cottonwood trees, which also grow along streams.

“The more I looked at it the more I could see that what is going on may be an ecology of fear,” Ripple says. “The theory goes like this: the browser — in this case the elk — need to make behavioral decisions and tradeoffs as to how much time and energy to put into eating food versus how much time to be staying in safe places.” Those decisions affect where the animals concentrate their feeding efforts, and therefore the distribution of the vegetation they eat. “What we started noticing is that the plants were doing better where the terrain might favor the wolf a little bit more than the elk,” he says. For example, the elk might browse less in areas with poorer visibility (more dangerous to the elk because they can’t see if wolves are on the scene), or regions littered with heavy debris (a risk because it becomes an impediment to escape in the event of an attack).

Indeed, Ripple says, “we found that aspen were growing the tallest along streamside areas that had some downed woody debris or some downed logs nearby.”

Elk behavior and vegetation distribution aren’t the only factors impacted by the return of the Yellowstone’s wolves. Ripple suspects that the ripples of their recovery are reverberating throughout the entire ecosystem, in birds, fish, insects, as well as in other plants and animal species. Beavers, for example, are probably affected, he says. “The park service has been monitoring beaver since the wolves returned, and found that they have increased in numbers every year in the northern part of Yellowstone. Before the wolves returned, there really wasn’t much food for the beaver. But now with this growth of these plants — especially the willow — the beavers have more food, and they are also using the willows to build their lodges and their dams, which may be contributing to beaver population increases.”

“We are at the beginning of a grand ecological experiment,” Ripple says. “We were without wolves for seventy years, and we’ve just had them back in for 11 years, so we’re only just starting to see changes. It could take many decades for the ecosystem to recover.”

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The epic history of the Druids, one of more than a dozen packs now occupying the 2.2 million acres of Yellowstone, is documented in NATURE’s In the Valley of the Wolves, was produced and shot in High Definition by Emmy-award winning filmmaker Bob Landis.

On the Web site for In the Valley of the Wolves, you’ll learn how the successful reintroduction of Yellowstone’s apex predator has changed the entire ecosystem of the park, and about the threats that these majestic animals continue to face on their road to recovery.

Watch an online-exclusive video. In this video, Emmy Award-winning wildlife cinematographer Bob Landis discusses the making of the film, including the ideal circumstances for filming a predation scene; the importance of spending a vast amount of time in the field; the uniqueness of Yellowstone’s Druid wolf pack, and more.

To order a copy of In the Valley of the Wolves, visit the NATURE Shop.

Online content for In the Valley of the Wolves was originally posted November 2007.

A grizzly in his prime is the undisputed master, able to take down prey as easily as he is able to dominate the fresh kills of other predators. The wolves of Yellowstone must use their numbers to their advantage, organizing to take down their prey and defend the spoils of their hunts from the bears. Together, a strong pack of wolves can challenge a solitary grizzly, and defeat it. As these two fierce competitors test their strategies for survival against each other, an entire food chain of scavengers survive in their wake. Crafty coyotes and cunning crows take advantage of the predators’ feuds to make the most of any felled feast.

It’s a test of skills and wills in the often harsh environment of Yellowstone as two of the continent’s greatest hunters take each other’s measure. Now, join NATURE and discover who will triumph in Clash: Encounters of Bears and Wolves.

Gray Wolf (Canis lupus)

• Type: Mammal
• Family: Canidae
• Habitat: Open tundra, forests, prairies
• Location: Some areas of the continental U.S., Alaska, Canada, Mexico, Europe, and Asia
• Diet: Carnivore
• Average lifespan in the wild: 10 – 18 years
• Size: Shoulder height: 26 – 36 inches (.6 – .9 m); overall length, 4′3 – 6′9 (130 – 205 cm)
• Weight: 70 – 130 lb (26 – 59 kg)

Once the world’s most widely distributed land mammal, the gray wolf, or timber wolf, was systematically eradicated from the continental United States in the 20th century and was added to the endangered species list in 1973. Gray wolves are considered a “keystone” species — one with a dramatic effect on its environment. For example, the recent reintroduction of gray wolves to Yellowstone National Park has led to the recovery of the entire ecosystem, from vegetation to beavers to hawks and eagles. The animal is the direct descendent of the ancestor to dogs. Wolves are distinguished from dogs by their long, powerful muzzle, yellow eyes, longer legs, larger paws (which are slightly webbed for walking more easily on snow) and bigger teeth. The wolf has a two-layered coat, which is generally gray, but can vary in color from white to black.

The highly social animal lives in a pack of around five to nine members, led by an alpha pair — generally the only pack members allowed to breed, and mated for life — plus their offspring and a few unrelated wolves. All pack members care for young, and the pack hunts cooperatively, usually at night, to chase down prey such as moose, elk, bison, reindeer, beaver, and rabbit. The wolves slash at the hind leg tendons of their quarry to disable it, or herd it back toward waiting pack members. Wolf howls and other vocalizations allow the pack to communicate, and advertise their territories (which can cover from 100 to 260 square miles) to neighboring packs.

Did you know? The gray wolf can gallop over short distances at speeds of more than 30 miles per hour, but will usually give up the hunt if it doesn’t overtake its victim within the first 1,000 yards.

Related Episodes:

• In the Valley of the Wolves

The original five members of the Druid Peak pack — #38 and #39, the alpha male and female, and female pups #40, #41, and #42 — were captured near Fort St. John in British Columbia and relocated to Yellowstone’s acclimation pens before being released in April 1996 in the Park’s scenic Lamar Valley. The nearly treeless Lamar Valley is often considered Yellowstone’s most prized hunting grounds, and the most visible wolf territory in the Park.

On this public stage, the Druids displayed early signs of the upheaval and drama that would eventually come to characterize the group. During that first year in Yellowstone, a yearling male, #31, dispersed from the nearby Chief Joseph pack and joined the group, while alpha female #39 left the pack completely to become a lone wolf — perhaps driven off by #40, her own ruthless daughter, who began a terrible reign as the pack’s alpha female.

In 1997, pups were born to #41 and #42, the subordinate females, but none to the aggressive alpha female, #40. Lone wolf #39 reunited with the pack briefly, then left once again in November — this time with her daughter, #41 (who also may have been driven off by #40). The pack’s two males, #31 and #38 were shot and killed in December, setting the stage for the dominance of a new male, #21, dispersed from the Rose Creek Pack. By the end of 1998, the Lamar Valley Druids had seven members, and a growing reputation for conflict. The constant harassment of beta female #42 by her sister, #40, earned #42 the nickname “Cinderella” by the Yellowstone researchers. The put-upon Cinderella created a den and gave birth to pups in 1998, but none survived; the following year #40 attacked #42 in her den, and she again produced no offspring.

Casanova

Cinderella finally reached the ball in 2000, after a violent turn of events that put her at the head of the pack. She and the other female members of the pack, perhaps tired of #40’s iron-pawed leadership, turned on the alpha female, and killed her. At least three litters were born to the liberated females; 20 of the 21 survived. The Druids, 27 strong, became the largest pack in Yellowstone. In 2001, another 10 pups were added to the group, and the 37-member Druid pack became perhaps the largest wolf pack ever documented.

Like all dynasties, however, the Druids were destined for a fall. In 2002, the massive pack reached critical mass, and splintered. Three new packs, the Agate Creek, Geode Creek, and Slough Creek packs, were created, each anchored by a former Druid female born at the same den in Lamar Valley in 1997. The Druids were left with 11 members by 2002’s end, including the matriarch, Cinderella, and the long-time alpha male, #21. The pack expanded to 17 members by the end of 2003, aided by the arrival of a lone black male, #302, formerly of the Leopold pack. #302 may have fathered all of the pups not born to the alpha female. To wolf researchers, he was “Casanova” — a lover, not a fighter, who wooed the females in the group while staying appropriately submissive to alpha male, #21.

In 2004, the Druids once again suffered terrible losses; longtime alpha female #42 was killed by members of a rival pack, and the aging patriarch was found dead in the summer. At the same time, however, the neighboring Slough Creek pack began to spend more time on the northwestern boundary of Druid territory. Their incursions into Druid turf culminated in a decisive battle in 2005 that ousted the formerly dominant Druid wolves from the Lamar Valley. Two adult female Druids died that year — one killed by the Sloughs — and no pups survived. The pack was reduced to just four members, and looked to be nearing its end.

In true soap opera fashion, however, the Druids’ epic tale does not conclude with their exile. In 2006, from their new location in an area called Cache Creek, aided by Casanova and #480, the new alpha male, the pack began to rebuild. Both of the pack’s adult females successfully bred, producing eight surviving pups. The Druids pushed back against the Slough Creek pack — which suffered its own losses earlier in the year after a run-in with an unknown pack from the north — and reclaimed their traditional territory in the Soda Butte and Lamar Valleys; six pups were born there in 2007. The Druids, for now, are home.