But the growing body of research on cetacean brains, for one, shows that these brains are as complex as human brains but in a different way. While the human brain is somewhat more layered, cetacean brains are less layered but with more 'vertical' connections through layers. They are clearly intelligent, perhaps more so as a class than any other kind of mammal, when we consider only mammalian intelligence.
Farther afield, we come across the anomalous cases of corvid intelligence — ravens for example — where birds show strong evidence of forward planning, co-opting other animals as partners in an enterprise, and facial recognition with emotional response. And as we metaphorically walk out even farther from home, we meet the cephalopoda; an octopus keeps two-thirds of its neurons in its arms, shows strong evidence of structured memory and observational learning, and comes from the only class of invertebrates which uses tools — in 1986, the octopus became an honorary vertebrate according to UK law, and enjoys the same protections as vertebrates as far as animal rights are concerned.
What is it, then, to willingly (and intentionally, and deliberately) terminate the life of a crow, an octopus, or dolphin? It is the termination of a thinking, emotion-bearing, mind-using organism. These creatures had hopes, fears, ambitions — until they were killed. It is nothing less than murder, but it is not murder yet — for humans still define murder in terms of human-termination.