Why Hunt When You Can Rob? Stealing Food Is a Living
By the BrainSnail editorial team. How these articles are written and checked, and how to tell us when one is wrong.
Some animals obtain a large part of their food by taking it from other animals that caught it. The strategy is common, evolutionarily stable and considerably more complicated than simple theft.
What the strategy involves
The behaviour means acquiring food that another animal has already found, caught, killed or prepared, rather than obtaining it directly. It differs from predation, since the victim is usually not harmed and loses only the meal, and it differs from scavenging, since the food is taken from a living animal that still holds it. Methods range from open attack, with the robber chasing and harrying until the victim drops or disgorges its catch, to quiet appropriation, with the robber simply arriving at a carcass and displacing whoever is eating. Some species depend on it almost entirely and others use it opportunistically.
Who does it
The behaviour is scattered widely and some practitioners are well known:
- •Skuas and frigatebirds, which chase seabirds until they give up their catch
- •Bald eagles, which routinely take fish from ospreys
- •Hyenas and lions, each of which robs the other regularly
- •Gulls, which take food from other gulls and from people
- •Several spiders that live in another spider's web and eat from it
- •Various flies and wasps that take prey from hunting insects
Why it pays and when it stops paying
The calculation is straightforward and explains where the behaviour appears. Robbing is worthwhile when the effort of chasing and intimidating is less than the effort of catching the food directly, which holds when prey is hard to find, when victims are visible and predictable, and when the robber is larger or faster than the victim. It stops paying when victims become scarce, when they learn to avoid the robber, or when too many robbers compete for the same victims. That last effect makes the strategy self-limiting, since a population cannot consist entirely of thieves, and the proportion settles at a level where robbing and honest foraging yield about the same return.
The version inside a species
Robbing a member of one's own species is commoner than robbing another and raises different questions, since the thief and the victim are competing directly. Gulls at a colony take food from neighbours constantly. Ravens and crows watch where others cache food and raid the caches, which has produced experiments showing that a bird being watched while caching will move its food later, and will do so only if the observer was another bird rather than a person, which implies it is tracking who saw what. Chimpanzees take meat from each other after a kill, with the outcome depending on rank and on who has allies present. In each case the behaviour sits alongside cooperation in the same group.
How victims respond
Being robbed repeatedly exerts pressure of its own, and the defences are varied and identifiable. Swallowing prey immediately removes the opportunity, which is why many seabirds bolt a fish rather than carrying it visibly. Feeding in dense groups makes any individual harder to single out and lets victims mob the robber collectively. Carrying food low and fast, or taking an indirect route home, reduces interception. Some birds simply wait until the robber has left the area before returning to a cache. And a good deal of the aggression at seabird colonies and at carcasses is victims defending food rather than robbers attacking, which is easy to misread from a distance.
The takeaway
Taking food from an animal that already has it is worthwhile whenever harrying costs less than hunting, which holds where prey is scarce and victims are visible. The strategy is self-limiting, since a population of nothing but robbers has nobody to rob, and the proportion settles where both approaches pay equally. Victims respond by swallowing catches at once, feeding in groups and flying low and fast.