I spent a little time looking, but I can't find anything better than other people's speculation and assumptions to match ours.
I'm guessing some VERY well trained and well fed (before they faced each other for a camera) animals. A ton of stop and cut editing too, if you re-watch it with that in mind. There much have been hours of that filmed for those 4 minutes of final cut.
Just incredible to think about from the tech side, given this was done in 1988!
....Its even a bit of a touching scene, if one doesn't get lost in those hours of grueling work it had to have taken to make it.
