Why does Amaroq pursue the third caribou, leaving the first two alone in "Julie of the Wolves"?
In a stark example of the Darwinian concept of "survival of the fittest", Amaroq recognizes that the third caribou is the weakest of the three that he tests. He knows he has a good chance of being able to overtake it, whereas that would probably not be the case with the other two.
The first caribou that passes through "open(s) his stride and gracefully gain(s) ground" when the hunters in the pack speed towards it. It is obvious that this one could "easily outrun the wolves", and so Amaroq wisely refrains from chasing it himself. Instead, he chooses another one to test, his first. As Amaroq approaches, this caribou quickly outdistances him, so he "swerve(s) and test(s) another". This one likewise is capable of running too fast, so Amaroq "put(s) on full speed and (bears) down on a third".
It is evident from the beginning that this third animal cannot outrun Amaroq. It is the weakest, and wolves, with an innate understanding of their limitations when going against larger prey, "take the old and sick". Amaroq attacks, and the caribou tries to strike back with its "powerful hoofs", but the Amaroz is too quick and dodges the blow. The other wolves join the chase, and the caribou bellows in despair, "zigzag(s) over a frost heave", and disappears with the pack closing in. It will not be long until he meets his death (Part 1).