Double Muscled Animals.

Animals are described a "double-muscled" when they have a myostatin mutation that caused them to have much greater muscle mass ("muscle hypertrophy) and somewhat smaller organs and bones.

These animals do not actually have two muscles, but each muscle is significantly larger than normal. Double-muscled animals have an incredibly muscular look even if they do not exercise. This is because they do not produce myostatin, also called growth differentiation factor 8, which controlls and limits the rate of muscle growth.

This mutation is of particular interest in species farmed for their meat (like cattle) but can occur in any mammal.

Some specific examples of animals with the myostatin mutation are discussed below.

The myostatin mutation is particulalrly valued in cattle and is a recessive trait of the "Belgian Blue" and Marchigiana breeds.

Sheep

Double-muscling is also present in some lines of sheep including those of the Texel breed and East Friesian.

Wendy the whippetis a spontaneous mutant from a breed generally known for being extremely lean. She quickly became an internet sensation and even appeared on the Today Show.

Whippets with this mutation are referred to colloquilly as "bully whippets". The tend to occur because a single copy of the gene makes a whippet a fast racer and so it is present in many dogs raised for racing.

Whippet breeder consider double-muscling an undesireable trait and it appears to be occurring with increasing frequency and several lines. They are working on developing a test that would identify carriers, to avoid breeding two carriers together.

Myostatin mutation is studied in a line of mice called the Mighty Mouse. These mice were developed as a side effect of a line of research at the Johns Hopkins School of Medicine when they inactivated or "knocked out" a gene repsonsilbe for producing myostatin.

So unlike the dogs and cattle they these mice did not arise from a spontaneous mutation. The potential for developing similar mutations in animals raised for meat was immediately apparent.

Trout have been produced by genetically modifying speciment to suppress myostatin. thus producing the equivalent of double-muscling in fish.

The purpose behind this research is to more efficiently raise farmed fish for food.

Human examples of muscle hypertrophy are also known including a child in America (Liam Hoekstra, 2006) and Germany (2004).

An interesting parallel is apparent to the occurance of animals used for racing such as whippets, in that the parents of children with muscle hypertrophy are often accomplished athletes.

• R. H. S. Bellinge, D. A. Liberles, S. P. A. Iaschi, P. A. O’Brien and G. K. Tay. (2005). Myostatin and its implications on animal breeding: a review. Animal Genetics, 36, 1–6 [pdf]
• Cinzia Marchitelli, Maria Carmela Savarese, Alessandra Crisà, Alessandro Nardone, Paolo Ajmone Marsan and Alessio Valentini (2003). Double muscling in Marchigiana beef breed is caused by a stop codon in the third exon of myostatin gene . Mammalian Genome Volume 14, Number 6, 392-395. [Abstract]
• Schuelke, M. (2004) Myostatin Mutation Associated with Gross Muscle Hypertrophy in a Child. The New England Journal of Medicine. [Full Text]