White meat or dark? This question will be asked thousands of times as families and friends gather around the Thanksgiving turkey.

But why are there two kinds of muscles? We’ll need to explore some muscle physiology to answer the question. Muscles are made of many elongate cells called muscle fibers. Each fiber is capable of contracting, causing the muscle to shorten. The muscles are attached to bones via a piece of connective tissue called tendons. When all the fibers of a muscle contract, the muscle is capable of remarkable force, causing movement in the part of the body to which the muscle inserts.

All muscles contain a mixture of two types of fibers: white and dark (or red). They differ in their metabolism and their contractile properties. Dark fibers are sometimes referred to as slow-twitch muscles or aerobic muscles. As the name implies, aerobic fibers require a constant supply of oxygen to continue to function. This oxygen supply is enabled by the myoglobin molecules in the muscle fibers.

Like the related hemoglobin in the blood, myoglobin is a molecule that readily binds to oxygen. Oxygenated blood courses through muscles and oxygen is transferred from the hemoglobin in the red blood cells to the myoglobin molecules in the muscle fibers. The more myoglobin within the muscle fibers of a muscle, the darker the muscle appears.

Dark fibers are great for activities involving endurance. Walking and running are generally powered by the contraction of aerobic fibers.

White fibers, also called fast-twitch fibers or anaerobic fibers, are used for rapid, short-term activities like fleeing from danger. These fast-twitch muscles are able to contract more quickly than the dark, slow-twitch muscles. However, they fatigue very quickly. These fibers operate in an anaerobic mode, a mode not requiring the continuous input of oxygen. To fuel their contraction, white fibers take up the starch glygogen, which is stored in the muscle fibers. The glycogen stores are quickly depleted so the rapid contraction of the white fibers is limited in duration.

In life, muscles made mostly of white fibers appear translucent and glossy. When cooked, the proteins in the muscle fibers denature and coagulate, resulting in the white, opaque appearance we associate with a chicken or turkey breast.

The myoglobin in the dark muscles also breaks down during cooking, imparting the brownish color to the meat. The breakdown of myoglobin also makes a steak brown when it is cooked.

Birds that migrate long distances have breast muscles made mostly of dark muscle fibers to enable long bouts of strenuous flight. Ducks and geese have breast muscles made of aerobic fibers and are dark when cooked. Wild turkeys do not fly for great distances. These birds have breast muscles that contain fewer dark fibers than a duck but more dark fibers than a domesticated turkey.

Domesticated turkeys have much larger breast muscles than wild turkeys. Selective breeding by turkey farmers has led to the increase in these muscles. The breast muscles of a male turkey are so massive that the tom turkeys are incapable of getting close enough to a hen turkey to mate. Domesticated turkeys are produced by artificial insemination.

In a cooked turkey or chicken, you can see two distinct muscles in the breast: the smaller supracoracoideus (closer to the base of the breastbone) and the much larger pectoralis muscle. Both attach to the upper wing bone, the humerus. The pectoralis pulls the wing down, providing the power for flight. The supracoracoideus muscle pulls the wing back up in preparation for the next power stroke.

How does a muscle below the wing raise the wing? The supracoracoideus runs through a canal between the humerus, the scapula and the coracoid bone to attach on the upper side of the humerus. Thus, with a downward tug, the wing is raised.

Herb Wilson teaches ornithology and other biology courses at Colby College. He welcomes reader comments and questions at:

whwilson@colby.edu