Surplus killing is where an animal kills prey that is not immediately used as food. Surplus killing is something that upsets some people and it is used by farmers as a reason to kill wild cats that attack livestock. It does look like crazed, wanton and unjustified killing for the fun of it.
Surplus killing and playing with prey by domestic cats is also an activity that is used by people who don’t like cats to argue against cats.
I am writing about surplus killing by cats in this instance. Other animals do it including, humans, wolves, brown bear, foxes, hyena, spiders and mink.
There are two types of surplus killing (a) when additional prey is killed ancillary to the prime target. This is better described as multiple killing. Lions acting together do this sometimes. Lynx also make multiple kills sometimes when hunting hares. The extra kills can be hidden (cached) for use in the future so this is strictly speaking not surplus killing and (b) true surplus killing — killing prey that is not utilised.
The big topic in relation to surplus killing is why does it happen?
The victims of surplus killing are normally livestock. They are penned in and cannot escape from the cat. Some wildcats have been recorded as being involved in surplus killing (a) caracal preying on sheep (b) snow leopards preying on sheep and goats and (c) lions preying on cattle. I have also read about cougars preying on livestock and indulging in surplus killing.
The first reason why it happens is because the drive to hunt and kill is separate and independent of the present urge to eat – hunger. Cats don’t hunt prey as a direct response to immediately satisfy hunger.
The second reason is that the prey (livestock) cannot escape. If surplus killing takes place on wild animals it may occur when environmental conditions are particularly helpful for the predator e.g. dark, wet and windy.
The third reason is that when cats are making multiple kills they are using their energy very efficiently. Andrew Kitchener in The Natural History of The Wild Cats (ISBN 0-8014-8498-7) says that they “maximise energy over time”. It seems to be an energy efficient way to kill and killing prey is an instinctive survival activity even if disconnected from the immediate need to eat.
There may be a fourth factor. When a cat or predator starts killing it may be difficult to switch off and return to the normal urges to hunt.
However, the downside of any attack on prey is the potential for the predator to get injured. An injured predator might not be able to hunt efficiently.
Unfortunately the wildcat is unable to rationalise the long term downside of surplus killing as it leads to retaliatory killing by farmers. The snow leopard is particularly susceptible as it is quite a trusting wildcat and is not as suspicious of the human as other large wild cats. They are relatively easy to kill it appears.