Wolf Depredation
7. Surplus Killing
What Is Surplus Killing?
Surplus killing is so rare that it first only really came to the attention of biologists in the early 1970's (Kruuk 1974). It is the killing on a hunt of several prey animals too numerous to eat at one sitting. Surplus killing has been documented in many species of predator from tiny weasels up to bears. The proverbial example of surplus killing is the fox in the chicken coop. A fox gets in among the chickens and kills and seriously wounds several birds before escaping, perhaps with just one chicken.
Rare incidents of surplus killing by wolves in Minnesota are reported to leave up to 35 sheep killed and injured in flocks and losses of 50 to 200 birds in turkey flocks (Paul et al 1994). Wildlife are also on the receiving end. An instance of surplus killing by wolves was belatedly witnessed by researchers in Canada's Northwest Territories. They came across 34 new-born caribou scattered over three square kilometres. All had been evidently killed the previous day within minutes of each other by a wolf pack. The wolves had eaten only a few parts from half the calves and not touched the rest (Miller et al 1985).
Wolves In The News
Wolves are sometimes charged in the news media with killing hundreds of livestock animals in a single night. The Independent, a London based newspaper, carried a story about the recent deaths of 403 sheep falling over a ravine in the Mercantour massif in the French Alps (Litchfield 2002).
The newspaper first attributed the deaths of the sheep to a violent storm or an attack by feral dogs. But when the corpses were examined
"...the carcasses showed several animals were bitten by the powerful jaws of animals, almost certainly wolves."
Apparently the sheep had hurled themselves off the ravine in their panic to escape the savage onslaught. No other evidence was offered by the reporter, save the circumstantial fact that wolves are recolonising the region.
Opposition To Wolves
Surplus killing by wolves and other predators is one of the reasons why they arouse opposition. The sight of dozens of slaughtered livestock will not put a farmer in a philosophical mood. Nor will hunters be positively impressed when they see their quarry cut down and scattered all about, especially if wild herds are declining locally.
Livestock farmers and hunters tend to blame predators as wanton killers, killing for the fun of it. To add insult to injury, wolves sometimes only eat a part of their surplus-killed prey, like the tail. Some farmers and hunters see this as outrageous, unmitigated mutilation.
Moreover, a massive surplus kill can be a calamity for some farmers and a disaster for livestock raisers in Third World countries. An impoverished small farmer on marginal land might have his entire stock wiped out in one night. It is therefore not surprising that many farmers and hunters protest at simply having wolves around and wish to kill them as a preventive measure.
Why 'Trash' Prey?
What makes wolves and other predators kill like this, catching and killing more animals than they can eat? It certainly looks like a waste of a predator's time and energy to 'trash' prey and leave them to rot.
Observations in the wild indicate that surplus killing may occur when prey animals are incapacitated by adverse environmental conditions, enabling a predator to sneak up and kill several of them (Kruuk 1974). Imagine a very dark stormy night in an especially severe winter with deep snow. In conditions like these, deer are vulnerable because they cannot flee properly and wolves may find it easy to pull down several.
It is said, as an explanation of surplus killing, that in ordinary conditions, prey animals in the vicinity of a kill have fled by the time a victim has been captured. Then the predator's killing behaviour comes to a halt as there are no more prey to catch. But during a surplus-kill, a predator's killing mechanism is continually sparked off by the stimulus of so many prey animals unable to escape so that the predator cannot stop killing (Kruuk 1974).
How Frequent Is Surplus Killing?
Conditions in nature which favour surplus killing are unusual. Consequently surplus killing in the wild is rare. However, the conditions for surplus killing are fairly common in domestic situations. Domestic animals are often confined at high density in a field, an enclosure, or indoors, so cannot escape. If a predator is suitably motivated by hunger and can get in among them, many animals may be killed before the predator finds his way out or the farmer arrives.
Is Surplus Killing A Waste?
A prey animal brought down by wolves is ordinarily well consumed, with the exception of some bits and pieces. But in a surplus-kill wolves tend to eat only the internal organs and leave much of the rest. However, it has been suggested that these internal organs are the most nutritious parts of the prey, so that the wolves are eating the best portions.
The remaining parts of a carcass may appear to be wasted, but wolves tend repeatedly to return to their kills and therefore these parts can be a vital buffer against starvation and death, especially in periods of severe and unpredictable weather, the very circumstances which set off the surplus killing.
Scavengers also benefit from wolf surplus kills as the remains are fed on by the likes of foxes, martens, eagles and ravens. Waste is a human term for human economics.
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