Why do soldiers seldom "fight to the death" even if they are going to be killed anyway? At the siege of Masada, a group of heavily outnumbered Jewish soldiers elected to commit suicide en masse, rather than to be captured by the besieging Romans, who would probably have committed them to a tortuous death by crucifixion. Such an action is highly unusual, almost unique in the annals of history.

During the Spartacus slave revolt, some 6,000 rebels were captured and crucified. Why did they choose this over death in battle? (As rebelling slaves, they were outlaws, not foreign prisoners who could expect to be allowed to live.) And why might this be true of others in similar straits (e.g. partisans captured by Nazis in World War II)?
Why does prey, when caught in the jaws of an animal whose bite and build is powerful enough to carry them, cease to struggle? I remember an old quote from psychology, recounting first-hand, a lion attack. I cannot find it now. The narrator said that once the lion had him in its mouth, it shook him (physically) and thus disoriented him. He said that he felt no pain, and he said that it must have been because of adrenaline. I forget how, I think a local intervened, but he survived to describe the experience.
Why does a fish, when laid on ground, cease to struggle? Yet waggle when touched and escape when released? Morale?
Why do bucks, when vying for supremacy by engaging in contests of strength, not aim to kill their opponent? This case is slightly different, because the foe is conspecific. While humans understand the concept of mutually-assured destruction, animals seem to understand this too, and that it's evolutionarily folly when fighting one's own species.

Sorry for an incomprehensive answer, but I hope my insight spurs the ultimate answer.