Wolf, any of two species of wild doglike carnivores. The gray, or timber, wolf (Canis lupus) is the better known. It is the largest nondomestic member of the dog family (Canidae) and inhabits vast areas of the Northern Hemisphere.

Pervasive in mythology, folklore, and language, the gray wolf has had an impact on the human imagination and has been the victim of levels of misunderstanding that few animals have shared. With the exception of humans and the lion, the gray wolf once had a larger distribution than any other land mammal, once ranging over all of North America from Alaska and Arctic Canada southward to central Mexico and throughout Europe and Asia above 20° N latitude. It lived in every type of habitat except tropical forests and the most arid deserts, and it was the premier hunter of the large hoofed mammals. Several subspecies occur throughout North America.

The wolf is built for travel. Its long legs, large feet, and deep but narrow chest suit it well for life on the move. Keen senses, large canine teeth, powerful jaws, and the ability to pursue prey at 60 km (37 miles) per hour equip the wolf well for a predatory way of life.

Wolves play a key role in keeping ecosystems healthy. They help keep deer and elk populations in check, which can benefit many other plant and animal species.

The carcasses of their prey also help to redistribute nutrients and provide food for other wildlife species, like grizzly bears and scavengers.

Many other animals benefit from wolf kills. For example, when wolves kill an elk, ravens and magpies arrive almost immediately. Coyotes arrive soon after, waiting nearby until the wolves are sated. Bears will attempt to chase the wolves away, and are usually successful. Many other animals—from eagles to invertebrates—consume the remains.

Biologists are often faced with the grim task of documenting the cascade effects of what happens when a species is removed from an ecosystem, by local extirpation or even extinction. In Yellowstone, biologists have had the rare, almost unique, opportunity to document what happens when an ecosystem becomes whole again, what happens when a key species is added back into the ecosystem equation.

In Yellowstone National Park in the late 1800's, wolf packs roamed the park. But, by the end of the 1920's, grey wolves had been hunted to eradication. Wolves had been pursued with more determination than any other animal in United States history.

In the 1930s, the wolf was killed off in Yellowstone. Even though Yellowstone elk were still preyed upon by black and grizzly bears, cougars and, to a lesser extent, coyotes, the absence of wolves took a huge amount of predatory pressure off the elk. As a result, elk populations did very well, perhaps too well. Two things happened: the elk pushed the limits of Yellowstone’s carrying capacity, and they didn’t move around much in the winter-browsing heavily on young willow, aspen and cottonwood plants. That was tough for beaver, who need willows to survive in winter.

As attitudes towards wild ecosystems changed, people began questioning whether a wolf-less Yellowstone environment was a healthy one. Once the wolves were gone, the elk population exploded and they grazed their way across the landscape killing young brush and trees. As early as the 1930s, scientists were alarmed by the degradation and were worried about erosion and plants dying off.

To protect declining species from the shortsightedness of man, the Endangered Species Act was created. In 1974 the gray wolf was added to the list.

Biologists in Yellowstone began exploring the idea of bringing Canadian wolves to the park and on January 12, 1995 the first eight wolves arrived from Jasper National Park in Alberta, Canada.

Wolves have a large roaming area and a homing instinct. It was feared that the expensive, transplanted wolves would simply head north to home. To make the wolves establish a home in the park, Yellowstone built three acclimation pens to house 14 wolves for several weeks. Carcasses of elk were covertly “planted” to give wolves a taste of their new environment.

In the years that followed, wolves brought the elk population down and protected the open valleys from overgrazing. However, the number of elk killed was double than estimated and many local hunters stirred controversy by protesting that the wolves will end up killing ALL of the elk. Today the debate is still strong. Inside the park, scientists joyously exclaim that the wolves have saved Yellowstone. Cross the park border into a gateway town and you will surely hear how wolves kill for the pleasure of killing and are terrorizing ranches and wildlife.

Research in Yellowstone since reintroduction has highlighted the adaptive value of social living in wolves – from cooperative care of offspring, group hunting of large prey, defense of territory and prey carcasses, and even survival benefits to infirmed individuals.