McTeague is a massive, mentally slow, and psychologically primitive man who has evolved as far as he possibly can. He manages to control his animal-like tendencies until Trina enters his life. Then the animal inside of him surfaces, and he can no longer cope with his environment or Trina's obsessive hoarding. He becomes an animal, and the end of the story supports this. He finds himself handcuffed to Marcus's corpse in the middle of Death Valley. In the end, he's like an animal who has been caught in a trap and would probably chop off his hand to save himself if he could. The very last words of the book show his disintegration: "McTeague remained stupidly looking around him, now at the distant horizon, now at the ground, now at the half-dead canary chittering feebly in its little gilt prison." McTeague is the canary, and he is imprisoned by the handcuffs connecting him to Marcus.
Trina starts as a thrifty, nervous character who disintegrates into an obsessive hoarder of her precious gold. Her greed brings out the violent animal instincts of McTeague, but it also shows her as little more than an animal, defending her gold as a wolf might defend its kill.
Marcus's jealousy comes out only after Trina wins the lottery, and he becomes bitter at McTeague's good fortune. Marcus attacks McTeague twice, and then he strips McTeague of his profession as a dentist. Marcus's greed and jealousy leads him to his death in Death Valley. Marcus is an animal on the prowl for his prey.