Evolutionary arms race
The rough-skinned newt, the garter snake and toxin resistance is a well-researched case study. Where the two species live together, the newt produces more toxin, and the snake has evolved to resist it; where they live separately the newt toxin is milder, and the snake has little resistance. The snakes pay a price for their resistance: their digestion and body metabolism is slower than related species. "Really resistant snakes have slower crawl speeds than snakes with little or no resistance".