Many of you will have eaten turkey over the Thanksgiving holidays.  Some people prefer white meat while others prefer dark meat.  What is the difference?

Dark meat is composed of red muscle fibers (cells).  The red color comes from a high concentration of myoglobin in the fibers.  Myoglobin, like the hemoglobin in our red blood cells, binds oxygen that can be released as needed to the muscle fibers to allow them to contract.  The myoglobin increases the entry of oxygen to the muscle fibers.  Contraction of muscles allows for such useful activities as walking, flying and capturing food. Red muscle also has an abundance of capillaries to help provide oxygen to the fibers.

Red muscle fibers are quite narrow and so have a very high surface area relative to their volume.  As a result, oxygen does not have to move very far as it diffuses into the muscle cells.  The flight muscles of small songbirds (and small bats) have the highest aerobic capacity of any vertebrate species.

Red muscle fibers are often referred to as slow twitch fibers.  The fibers contract but at a relatively slow rate.  The fibers require lots of oxygen to do their work but his oxygen is provided by the blood and stored in the myoglobin until needed by the muscles.  As a result, red muscles can do slow but steady work; they do not tend to fatigue.  Red muscles are excellent for sustained flight.

The breast of a turkey, on the other hand, is made up of white muscle fibers.  These muscles are not well supplied with capillaries and do not contain much myoglobin to help store oxygen.  White fibers are often referred to as fast twitch muscles. They are capable of very rapid contraction.  However, these contractions occur in the absence of oxygen.   After a short period of time, a waste product called lactic acid accumulates in the cells, causing the fast twitch muscle fibers to cease contracting.  White fibers are therefore capable of a few very powerful, very strong contractions but tire quickly.

Let’s return to our turkey.  The muscles of the thigh and drumsticks are composed of red muscle fibers.  These muscles are used for walking and scratching the ground.  The muscles do not have to act particularly quickly.  Because they are slow twitch muscles, they do not tend to fatigue.  A turkey can walk around all day without experiencing muscle fatigue.

The flight muscles of the turkey (the breast muscles) are white, fast twitch fibers.  When alarmed, a turkey can use those fast twitch fibers to take off explosively.  However, the flight must be a short one because those white fibers quickly fatigue as lactic acid builds up in the muscles.

In most birds, muscles are not made up of only white fibers or only red fibers.  In turkey breast muscles, there are some red fibers among the many white fibers.  Similarly, the red breast muscles of a pigeon have a few white fibers scattered throughout.  Exceptions to the rule occur in the breast muscles of sparrows and hummingbirds that only have red fibers.

The relative quantity of slow versus fast twitch muscles is related to the particular life style of a bird.  For long distance migrants like tanagers, sandpipers or warblers, their flight muscles must be able to sustain long periods of use.  White fibers would be poorly suited to the task so it comes as no surprise that the breast or flight muscles of these birds are mostly red muscle.

On the other hand, the great power but short duration of white fiber contractions makes white breast muscles suitable for birds that need to take evasive action in flight to avoid predators or to fly through thickly forested habitats.

The size of the breast muscles in a bird is related to its flying ability.  In birds that are powerful fliers, over 20% of the bird’s weight is breast muscle.  In birds that do not fly, less than 10% of the body weight comes from the breast muscles.  The very large breast muscles of domestic turkeys would indicate they are powerful fliers.  However, the size of the domestic turkey’s breast muscles is a result of selective breeding.

The breast muscles of a bird belong to the group of muscles called voluntary or skeletal muscles.  A bird can consciously cause those muscles to contract to allow movement.

The skeletal muscles play another important role in the winter.  When the temperature falls below a critical level, the skeletal muscles begin to shiver.  These rapid, involuntary contractions release heat to help the bird maintain its body temperature.

[Originally published on December 26, 2010]