The contest between predators and prey is an even match. Muskoxen have the advantage of their circular defanse formation--rumps in the centre, heads towards the attacker, so that their flanks are protected and thwir weapons deployed. Caribou are protected by their erratic migrations, which make them hard to find, and by their sociability (it is difficult to make a kill in the midst of a stampeding herd). They and other members of the deer family have also acquired the ability to run a little faster than their enemies. Mountain goats, for their part, are able to escape up sheer cliff sides, where predators cannot trail them. Wolves have been one of the principal forces directing the evolution of these and other species, including moose, elk, bison and mountain sheep. These animals--the wolves' preferred prey--are specifically equipped to avoid wolf attacks. As the poet Robinson Jeffers once phrased it, "What but the wolf's tooth whittled so fine / The fleer limbs of the antelope?"

So it is not surprising that wolves are often unable to kill prey that is in prime condition. To be more accurate, they frequently do not bother to try. Biologists believe that wolves evaluate their for weaknesses that will permit an easy kill. In the case of caribou, they may test a herd by chasing if for a few minutes. If the prey stay bunched together and hurry nimbly away, the wolves immediately lose interest; but let one srumble or lag behind, and the wolves are quick to seize the advantage. Perhaps the brief attack on the muskoxen was just such a test. Since the animals all appeared strong, the wolves gave up. In general, that is the way most wolf hunts end. A kill is made only about one tme in ten.

What are the factors that put an ungulate in this unlucky percentage? Perhaps it has an infectious desease or a severe infestation of parasites, or maybe it is hampered by injuries or age. Perhaps it is genetically inferior. Or it could be that the ungulate population has outgrown its range and the individual has been unable to get enough food. Is all species by culling out the sick and the weak and helping to keep the population in check. If inferior and unproductive animals are removed, then there will be more food for healthy, well-adapted animals and their young.

A case in point comes from Isle Royale in Lake Superior, where moose populations plummeted in the 1940s because of overpopulation and overbrowsing. But after a small population of wolves established itself on the island late in that decade, moose gradually increased to about six hundred in 1960 and fifteen hundred in 1970. David Mech, who studied the animals throughout that period, concluded that the wolves were removing mostly aged and inferior animals. Although the predators were also killing young calves, they were obviously not overtaxing the moose, whose reproductive capabilities (like those of all ungulates) has a built-in allowance for such losses. These finding seemed to support the time-honoured belief that predator and prey maintain a finely tuned balance-a gentle rhythm of reciprocal ups and downs that never swings far from equilibrium. By and large, this generalization is still thought a hold true.