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Of wolf packs and ravens

Posted: February 17, 2012 - 7:38am
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A trail camera captured the images of wolves scavenging a well-worn moose carcass near Gustavus.  Photo courtesy of Kevin White
Photo courtesy of Kevin White
A trail camera captured the images of wolves scavenging a well-worn moose carcass near Gustavus.

A major source of mortality for wolves, in areas without much human interference, is starvation. Because food-deprivation is a real risk, much of what wolves do is driven by the search for food. An adult wolf can gobble down prodigious amounts of meat in one day. For example, on Isle Royale in Lake Superior, where wolves have been studied for decades, adults averaged about six kilograms (over thirteen pounds) of meat (usually moose) per day.

The traditional explanation for why wolves often hunt in packs is that there is a greater chance of a successful hunt if more wolves participate. That supposition often may be true, for wolves hunting moose or deer or other ungulates. Even so, on Isle Royale, only about six percent of moose chases were successful, even for packs of ten to sixteen wolves. 

However, more successful hunts by large packs don’t mean more meat for each wolf, because the meat is divided among all the members of the pack. It turns out, from studies on Isle Royale and in the Yukon, that when wolves are hunting moose or deer, a pack of only two wolves obtains the highest yield of meat per wolf. 

But many wolf packs are larger than two, and some are much larger. In some cases, a family with offspring will hunt together while the young are learning their techniques. Many packs, however, are composed of unrelated adults, sometime ten or twenty or even more.

So the question is why are wolf packs often so large, given that large packs do not result in the best fed wolves?This is where the ravens come into the story. Ravens are well-known to attend wolf kills (of ungulates). They often follow a hunting pack of wolves and may even notify nearby wolves of an available winter-killed carcass. But ravens cannot get at the meat, except for the eyeballs and perhaps the tongue, until the carcass is opened by the slicing teeth of the wolves. When the wolves are there, they open the carcass and ravens can feed. 

A single raven may “steal” as much as two kilograms (about four and a half pounds) of meat from a large carcass, eating some on the spot and caching the rest. And sometimes there are many ravens attending a single carcass; five or ten ravens at a carcass is not uncommon, and occasionally, over a hundred ravens have come to one carcass. 

Although wolves and ravens sometimes feed placidly side by side, in many instances the wolves defend their carcass from ravens. Wolves don’t conceal their kills the way the grizzlies or pumas often do. Instead, they commonly rest near the carcass in between bouts of feeding and try to chase the ravens away. They eat very fast, too, which helps to reduce the loss of meat to scavengers (and to other wolves in the pack). Nevertheless, big packs are better at excluding the ravens than small packs are. So, where ravens are likely to “steal” a sizable proportion of the meat, a big wolf pack will end up with more meat per wolf, despite having to share with each other.

 

• Mary F. Willson is a retired professor of ecology.


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