No other animal could have had the same impact or meaning in the film.
Dogs can be loyal, faithful, humble, gentle, and generous.
But, at the same time, they can kill if threatened. Actually, our animal nature is worse than theirs because we kill for money, ambition, and power, while dogs kill only to survive. It is important to note that Gonz lez I rritu constructs his amazingly realistic scenes of canine savagery through artful and careful filmmaking. The film's animal trainer is very respected in animal welfare circles and he used his own dogs, so he genuinely cared about them.