Given that animals and humans who don't produce (or are insensitive to) myostatin appear to be otherwise healthy, I think it's a safe bet that myostatin's main

Given that animals and humans who don't produce (or are insensitive to) myostatin appear to be otherwise healthy, I think it's a safe bet that myostatin's main function in mammals is to limit muscle growth. Muscle is metabolically expensive to build and maintain, so inhibiting unnecessary muscle growth would be advantageous when high-calorie/high-protein food is scarce.