There's a picture of an animal's life that's just about standard, and even favored by many animal advocates: an animal's life is all choppy. Your dog lives moment to moment, without the moments being connected together into "wholes." By contrast, there is lots of connection in the life of a human being. This diffierence (people assume) has relevance to the value of animal lives, the badness of animal deaths, and the ethics of killing.
To wit: this sort of contrast is made especially starkly in Jeff McMahan's book The Ethics of Killing. He has a rich notion of the "wholes" that matter in the lives of people. For one, there's the whole formed when you anticipate a later time and wish it to be a certain way--you want to lie on the beach in Hawaii in three months. All signs are that animals don't have thoughts like that. But that's not the only sort of continuity that counts.
McMahan attaches importance to the "complex narrative unity" of a life (or parts of a life). That can be tragically ruined by death. The bride dies right before the wedding, the student is killed in a car accident on the way to graduation, the author doesn't get to see her book posthumously published. He writes--
As an animal continues to live, goods may continue to accumulate in sequence, but the effect is merely additive. There is no scope for tragedy--for hopes passing unrealized, projects unwillingly aborted, mistakes or misunderstandings left uncorrected, or apologies left unmade.But surely the lives of animals are full of premature endings. For example, a year ago Eight Belles collapsed moments after coming in second at the Kentucky Derby, because of two broken ankles. That broke off a story before it was over. Are we really to think that a horse that races madly to a finish line is not engaged in a "project," that no project has been "aborted" if the horse falls to the ground?
I have the feeling we spend too much time around denatured pets and farm animals to realize that animal lives don't just consist of a series of moments. Beavers work for months to build dams. Rutting season doesn't end as it's supposed to if the animals are shot by hunters before there's any mating. Emperor penguins spend weeks trudging back from the sea to feed their young--an effort that ends badly if the chicks have died in the meantime.
I know what some people are going to say. The animals don't think about the future--Eight Belles wasn't looking forward to her victory lap; the deer aren't thinking about copulating; the penguins don't desire a reunion with their young. But narrative unity is supposed to be a further factor affecting the significance of a death, one that goes beyond the issue whether death prevents desires from being fulfilled. In the human case, it does not seem true that an incomplete project is only tragic to the extent that the agent had a particular set of desires and thoughts. All that adds to the tragedy, but isn't all there is to it.
Thinking of an animal's life as a series of discrete moments makes its death matter less, and so makes it easier for us to kill with a clear conscience. We need to think about the lives of animals without so much eagerness to find the sharpest possible differences.