They seem to react like that when they get nervous or scared. For a few seconds they fall on the ground and stay paralized. I don't know how the species would survive in nature if a predator enters and half of them just allow themselves to be caught.
Google Video - Via
Google Video - Via
...or coyotes, wolves, stray dogs, and hungry bums... ;-)
--TwoDragons
Which is kinda sad, if you ask me. I'd prefer a flock of healthy herbivores and a few well-trained sheepdogs, thanks. Something about deliberately breeding neurological defects into an animal strikes me as cruel.
--TwoDragons