So Why Do They Do It?
On the surface, it seems like animal sacrifice is a pretty pointless, even counterproductive ritual for ancient humans to develop. If we've just managed to domesticate some walking milk factories that are great for pulling plows, then what's the point of killing a bunch of them?
Well, apparently it's a holdover from when humans first learned the art of selective breeding. If you're an upstart farming civilization, you can't just get a bunch of animals together and let them breed willy-nilly. To maintain a strong herd, you need to weed out the undesirables and thin the herd so that they don't overrun the place. Academics who study this kind of thing note that communities that practice animal sacrifice usually do it at the time of year when it's ideal to cull the herd (that is, to kill off some of the male animals so you don't have to feed them through the winter).
"Tons of sex, or death? Can't we compromise?"
As another expert explains, it also forced ancient herders to get good at selective breeding. One thing these ceremonies all have in common is that they demand a perfect animal -- which requires figuring out how to consistently make them. Which is to say, it's easier just to make a ritual out of something like this than it is to exhaustively explain to each new generation of illiterate farmers how herd culling works.