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But one by one, the berms we've built between ourselves and the beasts are being washed away. Humans are the only animals that use tools, we used to say. But what about the birds and apes that we now know do as well? Humans are the only ones who are empathic and generous, then. But what about the monkeys that practice charity and the elephants that mourn their dead? Humans are the only ones who experience joy and a knowledge of the future. But what about the U.K. study just last month showing that pigs raised in comfortable environments exhibit optimism, moving expectantly toward a new sound instead of retreating warily from it? And as for humans as the only beasts with language? Kanzi himself could tell you that's not true.
All of that is forcing us to look at animals in a new way. With his 1975 book Animal Liberation, bioethicist Peter Singer of Princeton University launched what became known as the animal-rights movement. The ability to suffer, he argued, is a great cross-species leveler, and we should not inflict pain on or cause fear in an animal that we wouldn't want to experience ourselves. This idea has never met with universal agreement, but new studies are giving it more legitimacy than ever. It's not enough to study an animal's brain, scientists now say; we need to know its mind.
There are a lot of obstacles in the way of our understanding animal intelligence not the least being that we can't even agree whether nonhuman species are conscious. We accept that chimps and dolphins experience awareness; we like to think dogs and cats do. But what about mice and newts? What about a fly? Is anything going on there at all? A tiny brain in a simple animal has enough to do just controlling basic bodily functions. Why waste synapses on consciousness if the system can run on autopilot?
There's more than species chauvinism in that question. "Below a certain threshold, it's quite possible there's no subjective experience," says cognitive psychologist Dedre Gentner of Northwestern University. "I don't know that you need to ascribe anything more to the behavior of a cockroach than a set of local reflexes that make it run away from bad things and toward good things."
Where that line should be drawn is impossible to say, since our judgment is clouded by our feelings about any given species. A cockroach likely has no less brainpower than a butterfly, but we're quicker to deny it consciousness because it's a species we dislike. Still, most scientists agree that awareness is probably controlled by a sort of cognitive rheostat, with consciousness burning brightest in humans and other high animals and fading to a flicker and finally blackness in very low ones.
"It would be perverse to deny consciousness to mammals," says Steven Pinker, a Harvard psychologist and the author of The Stuff of Thought. "Birds and other vertebrates are almost certainly conscious too. When it gets down to oysters and spiders, we're on shakier ground."
Among animals aware of their existence, intellect falls on a sliding scale as well, one often seen as a function of brain size. Here humans like to think they're kings. The human brain is a big one about 1,400 g (3 lb.). But the dolphin brain weighs up to 1,700 g (3.75 lb.), and the killer whale carries a monster-size 5,600-g (12.3 lb.) brain. But we're smaller than the dolphin and much smaller than the whale, so correcting for body size, we're back in first, right? Nope. The brain of the Etruscan shrew weighs just 0.1 g (0.0035 oz.), yet relative to its tiny body, its brain is bigger than ours.
While the size of the brain certainly has some relation to smarts, much more can be learned from its structure. Higher thinking takes place in the cerebral cortex, the most evolved region of the brain and one many animals lack. Mammals are members of the cerebral-cortex club, and as a rule, the bigger and more complex that brain region is, the more intelligent the animal. But it's not the only route to creative thinking. Consider tool use. Humans are magicians with tools, apes dabble in them, and otters have mastered the task of smashing mollusks with rocks to get the meat inside which, though primitive, counts. But if creativity lives in the cerebral cortex, why are corvids, the class of birds that includes crows and jays, better tool users than nearly all nonhuman species?