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Yellowstone wolves eating more bison

Yellowstone wolves eating more bison

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Paying tribute

Offspring of Mollie's pack in Yellowstone Park show respect to their mother and father.

Bison are playing a larger role in the sustenance of park wolves compared to past years as Yellowstone National Park’s bison herd has swelled and elk numbers have declined, according to the Yellowstone Wolf Project’s 2012 report.

Not all of those bison are being killed by wolves, said Wolf Project leader Doug Smith, some of them are dying of natural causes and the wolves are scavenging.

“If wolves have a choice, they’ll always choose elk over bison,” Smith added, simply because elk are easier to kill.

Out of the 255 animals that wolves killed and that were detected by the Wolf Project staff, 159 were elk (62 percent) while 32 were bison (13 percent). That’s the highest proportion of bison kills recorded since wolf monitoring began. The majority of the bison killed (17) were calves.

Those numbers compared to 343 staff-detected wolf kills in 2011, 267 of which were elk and 15 of which were bison. In 2010, out of 268 wolf kills 211 were elk and 25 were bison.

One reason bison are playing a larger role in wolf diets is that their numbers swelled in Yellowstone to 4,600 in 2013, up 9 percent from 2011, while elk in the park’s northern range fell to about 3,900 from 4,100 in 2011.

Stable growth

Smith said wolf numbers inside Yellowstone seemed to have stabilized since 2008, following a peak in 2003 at 174 animals. In 2012, there were 83 wolves in 10 packs. Seven wolves were killed by other wolves and 12 were shot by hunters in the surrounding states of Wyoming, Montana and Idaho.

So far this year, the number of wolves in the park is estimated at about 86 with more pups born than in 2012 and no park wolves taken in hunts in the surrounding states.

“This year, very few wolves have spilled out of the park,” Smith said.

A less dense wolf population has meant no disease outbreaks since 2008 and fewer incidents of mange, which can cause wolves to lose their hair and even die in severe cases.

As wolf numbers have stabilized, there’s also been a shift in where they live. In the past, more wolves were found on the park’s northern range. Now, “Northern range wolves have declined 60 percent since 2007, compared to only 23 percent for interior wolves,” the report stated.

“Interior wolves depend on elk, deer, bison and moose and those food sources are not as boom bust like elk on the northern range,” Smith said.

The exception to the thriving interior packs is the Mollie’s pack, which fell from 19 wolves in 2011 to 10 in 2012. The pack was one of the first to consistently feed on bison in the park’s interior. But by 2012 the pack’s age structure was not conducive to hunting bison — 13 of the 19 wolves were pups or yearlings and four of the remaining six were older females. Big, mature males are the most successful bison hunters.

More data

The most unusual park wolf to die in 2012 was a 3- to 4-year-old male, 827M, that wandered more than 400 miles to the east where it was hit and killed by a vehicle in southwestern South Dakota. It took less than two months for the wolf to make the long journey from where the Delta pack lives in Yellowstone’s remote southeastern corner.

The park’s wolves are some of the most popular and hated animals in the West. Because of where they live, they are also some of the most studied wolves in U.S. history. The Wolf Project releases annual reports that are always two years behind the calendar year.

Other interesting facts from the report include:

Wolves are opportunistic feeders. The park staff detected one bighorn sheep, two Canada geese, four pronghorns, one grizzly bear cub and a coyote among the prey animals on the wolves’ diet.

Seven of the 11 Yellowstone packs had at least one wolf taken by hunters when they wandered into surrounding states.

Of the 15 confirmed mortalities of collared park wolves, the majority (nine) were females and five of those nine were females killed by rival packs.

Two packs disbanded — the Agate Creek pack, which had totaled only three wolves the prior year, and the Mary Mountain pack, which contained 10 wolves the previous year.

The Agate Creek pack included female 471F, a nearly 9-year-old wolf that was killed by a rival pack and was about two weeks away from whelping seven pups. Smith said biologists don’t know how long wolves will reproduce, but their average age is nine and the oldest documented lived to 11.

The Agate Creek pack was unusual in that it had stable leadership, four alpha males and females in the pack’s 10-and-a-half years. 472F was the alpha female for eight years, the longest reigning alpha female on record.

Out of 18 Agate-born wolves collared over nine years, 11 went on to become the leaders of their own packs.

The Yellowstone Delta pack contained some of the largest wolves in the park, including the alpha and beta males. When captured in February 2012, two of the pack’s yearlings weighed 145 and 106 pounds. This was the same pack that the male left to walk to South Dakota.

Among the elk killed by wolves, 54 percent were newborn calves, 34 percent were cows.

The Wolf Project is collaborating with UCLA, which is sequencing the entire genome of wolf 302M.

Volunteers donated more than 6,800 hours to the Wolf Project.

It’s estimated that 27,500 visitors to the park saw wolves in 2012, up from 25,000 in 2011.



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