Well, I don't exactly have figures, is there a good way to survey it?
To be honest, you've got me on this one. It's a claim that seems uncontroversial so I've never seriously questioned it. Just the sort of thing that needs some hard facts.
"If fighting is sure to result in victory, then you must fight! Sun Tzu said that, and I'd say he knows a little bit more about fighting than you do, pal, because he invented it, and then he perfected it so that no living man could best him in the ring of honor. Then, he used his fight money to buy two of every animal on earth, and then he herded them onto a boat, and then he beat the crap out of every single one. And from that day forward any time a bunch of animals are together in one place it's called a zoo! (Beat) Unless it's a farm!"