- Number in Yellowstone area: 400–450 wolves in the greater Yellowstone area.
- ±124 individuals live in the park.
- Where to see: They inhabit most of the park now, look at dawn and dusk.
Behavior & Size
- 26–36 inches high at the shoulder, 4–6 feet long from nose to tail tip; males weigh 100–130 pounds, females weigh 80–110 pounds.
- Home range: 18–540 square miles; varies with pack size, food, season.
- Typically live 3–4 years in wild; can live up to 11 years in wild.
- Three color phases: gray, black, and white; gray is the most common; white is usually in the high Arctic; and black is common only in the Rockies.
- Prey primarily on hoofed animals. In Yellowstone, 90% of their winter diet is elk; more deer in summer; also eat a variety of smaller mammals like beavers.
- Mate in February; give birth to average of five pups in April after a gestation period of 63 days; young emerge from den at 10–14 days; pack remains at the den for 3–10 weeks unless disturbed.
- Human-caused death is the highest mortality factor for wolves outside the park; the leading natural cause is wolves killing other wolves.
Wolves ranged widely throughout North America in pre-Columbian times. Worldwide, all wolves, except the red wolf (Canis rufus) of the southeastern United States, are the same species (Canis lupus). Wolves are highly social animals and live in packs. In Yellowstone, the average pack numbers eleven animals; some are more than twice that size. Pack size varies based on the size of its main prey. The pack is a complex social family, with leaders (the alpha male and alpha female) and subordinates, each having individual personality traits and roles in the pack. Packs generally command territory that they mark by urine scenting and defend against intrusion by other wolves (individuals or packs).
Wolves consume a wide variety of prey, large and small. They efficiently hunt large prey that other predators cannot usually kill. They also compete with coyotes (and, to a lesser extent, foxes) for smaller prey. In Yellowstone, 90 percent of their winter prey is elk; 25 percent of their summer prey is deer. They also can kill adult bison. Many other animals benefit from wolf kills. For example, when wolves kill an elk, ravens 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 magpies to foxes—consume the remains.
Changes in Their Prey
From 1995 to 2000, in early winter, elk calves comprised 50 percent of wolf prey and bull elk comprised 25 percent. That ratio reversed from 2001 to 2007. Scientists are examining why this happened and what it means. They know that bull elk are entering winter in worse condition than before. Therefore, bulls are easier to kill than before, and one bull provides much more meat than one calf or cow.
Wolves may be consuming as many pounds of meat each year, but working harder for that food. When such “food stress” occurs, it can lead to increased wolf mortality—which was seen in 2008. Food stress may increase because a large number of older, unhealthy elk—those easiest to kill—died last winter.
From their confined beginnings in a few pens, the wolves have expanded their population and range, and now are found throughout the Greater Yellowstone Ecosystem. While their exact numbers are not known, scientists know that the wolf population fluctuates. Disease periodically kills a number of pups. The first serious outbreak was 1999, then six years later in 2005. That year, distemper killed two-thirds of the pups. The next outbreak was just three years later, in 2008,when all but 22 of the pups died. This shortened interval concerns scientists. Infectious canine hepatitis and sarcopitic mange also have been documented among adult wolves, but their effect on mortality is unknown.
Adult wolves kill each other in territory disputes. Such disputes happen each year, but increase when food is less abundant. This may have been why so many adult wolves died in fights during 2008. In addition, scientists found two wolves that had died of starvation—only the second time such deaths have been documented since wolves returned to Yellowstone.
In 2009, mange, distemper, and parvovirus continue to present challenges to the wolf populations in Yellowstone. Wolf populations are currently at their lowest since their re-introduction in 1995. The following map shows the current distribution of organized wolf packs in the park.
In the 1800s, westward expansion brought settlers and their livestock into direct contact with native predator and prey species. Much of the wolves’ prey base was destroyed as agriculture flourished. With the prey base removed, wolves began to prey on domestic stock, which resulted in humans eliminating wolves from most of their historic range. (Other predators such as bears, cougars, and coyotes were also killed to protect livestock and “more desirable” wildlife species, such as deer and elk.) By the mid 20th century, wolves had been almost entirely eliminated from the 48 states.
Today, it is difficult for many people to understand why early park managers would have participated in the extermination of wolves. After all, the Yellowstone National Park Act of 1872 stated that the Secretary of the Interior “shall provide against the wanton destruction of the fish and game found within said Park.” But this was an era before people, including many biologists, understood the concepts of ecosystem and the interconnectedness of species. At the time, the wolves’ habit of killing prey species was considered “wanton destruction” of the animals.
Between 1914 and 1926, at least 136 wolves were killed in the park; by the 1940s, wolves were rarely reported. In the 1960s, National Park Service wildlife management policy changed to allow populations to manage themselves. Many suggested at the time that for such regulation to succeed, the wolf had to be a part of the picture.
Also in the 1960s and 1970s, national awareness of environmental issues and consequences led to the passage of many laws designed to correct the mistakes of the past and help prevent similar mistakes in the future. One such law was the Endangered Species Act, passed in 1973. The U.S. Fish and Wildlife Service is required by this law to restore endangered species that have been eliminated, if possible. (National Park Service policy also calls for restoration of native species where possible.)
Wolves & Humans
Wolves are not normally a danger to humans, unless humans habituate them by providing them with food. No wolf has attacked a human in Yellowstone, but a few attacks have occurred in other places. Most were from wolves that had become conditioned to human foods. Like coyotes, wolves can quickly learn to associate campgrounds, picnic areas, and roads with easy food. This often leads to aggressive behavior toward humans.
Source: Yellowstone Resources and Issues, 2009