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 cache or abandon the remainder. The term was invented by Dutch biologist Hans Kruuk after studying spotted hyenas in Africa and red foxes in England. Other than humans, surplus killing has been observed among zooplankton, damselfly naiads, predaceous mites, martens, weasels, honey badgers, wolves, orcas, red foxes, leopards, lions, spotted hyenas, spiders, brown and black and polar bears, coyotes, lynx, mink, raccoons, dogs, and house cats.
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 from 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.
- Mills, L. Scott. Conservation of wildlife populations: demography, genetics, and management (2nd ed.). Hoboken, NJ: Wiley-Blackwell. p. 148. ISBN 9780470671504.
- Moskowitz, David. Wolves in the Land of Salmon. Timber Press. p. 144. ISBN 1604692278.
- Kruuk, Hans (1972). The Spotted Hyena: A study of predation and social behaviour. p. 335. ISBN 0-563-20844-9.
- Macdonald, David (1987). Running with the Fox. p. 224. ISBN 0-04-440199-X.
- "Far North Grizzlies Develop Taste for Muskoxen, Alaska Science Forum".
- "Wildlife Management Institute".
- Mech, edited by L. David; Boitani, Luigi (2003). Wolves: behavior, ecology, and conservation. Chicago [u.a.]: Univ. of Chicago Press. p. 145. ISBN 9780226516967.
- Stuart, C. T. (1986). "The incidence of surplus killing by Panthera pardus and Felis caracal in Cape Province, South Africa". Mammalia. 50 (4): 556–558. doi:10.1515/mamm.1922.214.171.1243. ISSN 0025-1461.
- Skinner, J. D. (1979). "Feeding behaviour in Caracal Felis caracal". Journal of Zoology. 189 (4): 523–525. doi:10.1111/j.1469-7998.1979.tb03979.x.
- Moskowitz, David. Wolves in the Land of Salmon. Timber Press. p. 145. ISBN 1604692278.
- "Wyoming wolf pack kills 19 elk in rare 'surplus killing'".
- Hansen, Kevin (2006). Bobcat: master of survival ([Online-Ausg.] ed.). New York: Oxford University Press. p. 114. ISBN 0195183037.
- Jennifer L. Maupin and Susan Reichert, Superfluous killing in spiders.
- Joseph K. Gaydos, Stephen Raverty,Robin W. Baird, and Richard W. Osborne, SUSPECTED SURPLUS KILLING OF HARBOR SEAL PUPS (PHOCA VITULINA) BY KILLER WHALES (ORCINUS ORCA).
- William G. George and Timothy Kimmel, A Slaughter of Mice by Common Crows.
- Wildlife Online: Foxes-Surplus Killing, Why do foxes kill to excess....
- For Wolves: Ralph Maughan Wolf Report, Jackson Trio makes some surplus kills.
- High Country News, Zachary Smith, Wolf pack wiped out for ‘surplus killing’.
- Victor Van Ballenberghe, Technical Information on Wolf Ecology and Wolf/Prey Relationships.
- Pierre-Yves Daoust, Andrew Boyne, Ted D’Eon, Surplus killing of Roseate Terns and Common Terns by a mink.