I've even shot a massive beaver that made its way into our pond. I've never seen a beaver before that day and still haven't. I don't know how that thing got so freakishly huge...I thought beavers were small! I thought he was a muskrat when his little head popped out of the water. After he floated to shore I couldn't believe my eyes when I saw that big flat tail! All the animals I've mentioned so far, including the beaver, thrashed around and fought for their lives til the very end. I don't think many humans could survive as long as these animals did with the same number of wounds.
I was under the same impression--I'd thought they were like a woodchuck with a funny tail. Nope! Turns out they never stop growing, so the longer they live.... Even better, there was a prehistoric beaver that was the size of a black bear.
I think the reason that animals die harder than people, in addition to their less sophisticated nervous systems, is that people are heavily trained by movies to be stopped when they are shot--my ultra-wonky pre-med friend, who knows nothing about what gunnies discuss, commented to me the other day on how people are so often "incapacitated" by wounds that shouldn't put them down, even to the point of being knocked down by projectiles that aren't physically knocking them down. Also, I think it's just so much easier to score an actual headshot on a human than on an animal, given how huge our brains are.
People might even tend to let physical pain constrain their behavior, even in a fight-or-flight scenario, to a much greater extent than animals. After all, we're not wild animals, so it's reasonable to think that most of us don't have the same intensity of "flight" response as does a wild animal. People seem to react like wild animals, though, when their adrenaline is already flowing--so much of the time, they don't even know they're shot, which wouldn't be the case if they'd been ambushed while eating a cheeseburger.