Lots of convincing dog corpses...

It says in the trivia they were heavily sedated.

Iñárritu, the film director, commented: When they were supposed to be dead, we would inject them with a veterinary anesthesia for 20 minutes — their owner was always there, shouting, "No more than 20 minutes!" I treated the actors worse than the animals!

Yea all of their tongues were sticking out