Surplus killing, also known as excessive killing and henhouse syndrome, is a common behavior exhibited by predators, in which they kill more prey than they can immediately eat and then they either cache or they abandon the remainder. The term was invented by Dutch biologist Hans Kruuk after studying spotted hyenas in Africa and red foxes in England. Some of the animals which have been observed engaging in surplus killing include zooplankton, damselfly naiads, predaceous mites, martens, weasels, honey badgers, wolves, orcas, red foxes, leopards, lions, spotted hyenas, spiders, brown bears, American black bears, polar bears, coyotes, lynx, mink, raccoons, dogs, and humans .
There are many documented examples of predators exhibiting surplus killing. For example, researchers in Canada's Northwest Territories once found the bodies of 34 neonatal caribou calves that had been killed by wolves and scattered—some half-eaten and some completely untouched—over 3 square kilometres (1.2 sq mi).
In Australia, over several days a single fox once killed eleven wallabies and 74 penguins, eating almost none. One leopard in Cape Province, South Africa killed 51 sheep and lambs in a single incident. Similarly, two caracal in Cape Province killed 22 sheep in one night, eating only part of the buttock of one carcass. Up to 19 spotted hyenas once killed 82 Thomson's gazelle and badly injured 27, eating just 16%.
In late autumn, least weasels often surplus kill vole and then dig them up and eat them on winter days when it is too cold to hunt. Surplus killing by wolves has mainly been observed when snow is unusually deep in late winter or early spring, and the wolves have frequently cached their prey for eating days or weeks later. On February 7, 1991, in Denali National Park, six wolves killed at least 17 caribou and left many untouched. By February 12, 30–95% of each carcass had been eaten or cached, and by April 16, several had been dug up and fed upon again. In March 2016, a Wyoming wolf pack of 9 wolves were found to have slaughtered 19 elk. John Lund, of the Wyoming Game and Fish Department, claimed to have never documented surplus killings to that extreme.
In surplus killing, predators eat only the most-preferred animals and animal parts. Bears engaging in surplus killing of salmon are likelier to eat unspawned fish because of their higher muscle quality, and high-energy parts such as brains and eggs. Surplus killing can deplete the overall food supply, waste predator energy and risk them being injured. Nonetheless, researchers say animals surplus kill whenever they can, in order to procure food for offspring and others, to gain valuable killing experience, and to create the opportunity to eat the carcass later when they are hungry again.
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