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The Value of Running in Packs

 

Today's Paper Science / Stephen Strauss / August 28, 2004

The existence of most carnivores is akin to an unanswered ad in the personals column, as 85 to 90 per cent of meat eaters live solitary lives. But for science the question has remained: Why do the rest of them run in pairs or packs.

At first blush, it would seem that hunting is the clue, because packing together should be a more efficient way of capturing prey, particularly when what is being pursued is something fast and large. The problem is that while it may be easier for groups to bring down a big animal, packs also mean that there are many more hungry mouths to share kills with.

When scientists looked at the relationship between optimal pack size and maximal food consumption, they often found that pack animals apparently got less meat in their mouths than they would have if hunting alone or in pairs.

Now, an analysis of data garnered from 27 years of observation of moose-hunting wolves in Isle Royale, the U.S. national park in Lake Superior, has shown a largely unappreciated virtue to packing: It's an effective raven swatter.

American scientists calculated that a pair of wolves loses about 37 per cent of a moose carcass to ravens, which can at times form 100-bird flocks.

If a single wolf takes a run at one or more scavenging ravens, it leaves the carcass open to be picked at by other birds.

When a moose-hunting wolf pack numbers six animals, some eat and some guard, and the result is that only 17 per cent of the moose meat goes down the ravens' voracious throats.

If there is less to eat -- some wolf packs feast mostly on deer -- the animals adjust to the change in food needed to be guarded by hunting is smaller packs.

Given their new appreciation of the scavengers' ability to cut down on the wolves' dinner, the scientists are hoping to see whether they have found an explanation for packing that fits animals worldwide.