If we decide – and this is our decision; it's not imposed on us from above – if we decide that reducing the amount of suffering in the world is a good ethical principle to live by, then it becomes entirely unjustified and arbitrary to extend this principle to human beings but not also to extend it to other animals capable of suffering. Why should the suffering of nonhumans be less important than that of humans? Surely a universe with less suffering is better than one with more, regardless of whether the locus of suffering is a human being or not, a rational being or not, a member of the moral community or not. Suffering is suffering, and these other variables are morally irrelevant.

(p. 274) - Darwin, God and the Meaning of Life: How Evolutionary Theory Undermines Everything You Think You Know (2010)