the cattle version of Arnold Schwarzenegger! This is a prime example of the genetic power that selective breeding holds.
The Belgian Blue’s sculpted, heavily muscled, bodybuilder-like appearance is known as “double-muscling”. The condition is heritable and results from an increased number of muscle fibers (hyperplasia) rather than the normal enlargement of individual muscle cells (hypertrophy).
The increased muscle tissue is due to a natural mutation in a gene that is involved in muscular hypertrophy. This gene is responsible for the expression of a protein called myostatin (“myo” meaning muscle and “statin” meaning stop). You probably guessed it right, myostatin inhibits muscle growth. Essentially, the breed produces less myostatin and thus produces more muscle tissue than the average cattle. Furthermore, the same mutation apparently interferes with fat deposition, resulting in very lean meat.