The dying of a field mouse, at the jaws of an amiable household cat, is a spectacle I have beheld several times. It used to make me wince. Early in life I gave up throwing sticks at the cat to make him drop the mouse, because the dropped mouse regularly went ahead and died anyway, but I always shouted unaffections at the cat to let him know the sort of animal he had become. Nature, I thought, was an abomination.
There are now some plausible reasons for thinking it is not like that at all, and you can make up an entirely different story about the mouse and his dying if you like. At the instant of being trapped and penetrated by teeth, peptide hormones are released by cells in the hypothalamus and the pituitary gland; instantly these substances, called endorphins, are attached to the surfaces of other cells responsible for pain perception; the hormones have the pharmacologic properties of opium; there is no pain. Thus it is that the mouse always seems to dangle so languidly from the jaws, lies there so quietly when dropped, dies of his injuries without a struggle. If a mouse could shrug, he’d shrug.
Pain is useful for avoidance, for getting away when there’s time to get away, but when it’s end game, and no way back, pain is likely to be turned off, and mechanisms for this are wonderfully precise and quick. If I had to design an ecosystem in which creatures had to live off each other and in which dying was an indispensable part of living, I could not think of a better way to manage.

On Natural Death, form The Medusa and the Snail: More Notes of a Biology Watcher, by Lewis Thomas +