Scavengers do not actively hunt and kill prey. Instead, they must seek out carrion across broad distances on landscapes. But carrion is highly ephemeral in space and time and so it can be quite difficult to find it unless one can search wide distances quickly and efficiently. Most scavengers do not have this searching ability. So they beat the odds against finding carcasses by associating themselves with species that actively hunt and kill prey.
A classic example of such association is between wolves and ravens. Ravens are typically present at wolf-killed carcasses and in some locations such as on Isle Royale in Lake Superior they are omnipresent. There are even cases in which ravens are rarely found on the landscape except in close association with wolves. Ravens can derive a very good livelihood from scavenging carcasses. An individual can ingest and hoard between 0.5 and 2 kg of wolf-killed prey per day. Thus, wolves may routinely lose between 2 and 20 kg of food per day to flocks of ravens. There are notable cases in which flocks of ravens devour up to half of the moose carcasses.
Such a high level of scavenging imposes strong competitive pressure on wolves to the extent that it may alter wolf grouping dynamics. A classically held belief is that groups of wolves comprise related kin in which altruistic behavior of the kin contribute toward overall family welfare (survival and reproduction). But recent research shows that wolfpacks contain unrelated individuals. Moreover, pack sizes are often larger than one would expect if individual wolves were attempting to maximize their foraging returns. Such behavior is not expected to be favored by natural selection. This counterintuitive behavior can, however, be reconciled if we add in the costs of food loss to scavengers. In the absence of scavenging, wolves maximize their foraging returns by associating in groups of two or three individuals because one individual alone is inefficient at killing prey and beyond two or three individuals competition for access to a carcass increases with group size leading to diminishing per individual foraging return rate. Loss of food to scavengers may change this structure because it forces wolves to hunt more frequently. Larger packs tend to be more efficient at killing prey frequently. Also, individual foraging return varies little with group size under conditions of scavenging and frequent hunting. Thus, the foraging cost of living in large groups may be offset by the benefit of frequent prey capture in wolves and perhaps in other social carnivores like lions that also face competition with scavengers.
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