Musculoskeletal System

The musculoskeletal system consists of the muscles, bones, ligaments, and tendons. In marine mammals, one of the first noticeable differences during a necropsy is the dark red color of the muscles compared with terrestrial animals.

This color is a result of a high concentration of myoglobin (the oxygen carrying protein in muscle). The high myoglobin load acts as a SCUBA tank for these diving animals by storing oxygen in the muscles. Notice in the photos of the young animals below, the muscles are more light pink than dark red. This is because there is a developmental period required for young animals to fully develop their diving capacity which can last up to three years in some species.

In cetaceans, the fluke, or the propeller of the animal, is powered by two sets of powerful muscles that run nearly the length of the body. The continuous muscle that inserts on the fluke and which sits on top of the transverse processes of the vertebrae is the EPAXIAL muscle. Remember that “epi” is “on top of” and “axial” is referring to the long axis of the animal. The right and left muscle run nearly the entire length of the animal’s body and when contracted, pull the fluke up. Similarly, the HYPAXIAL muscles (left and right) lie below the transverse processes of the vertebrae. The hypaxial muscles insert on the fluke and along the dorsal surface of the rib cage; when contracted, they pull the fluke down.

There are also special features of the skeletal system of marine mammals as well. For example, you will notice when dissecting the ribs that there is a joint in most of the ribs which allows the rib cage to collapse as the animal dives and is exposed to increasing amounts of pressure. NECROPSY TRICK: When dissecting the ribs, locate the joint with your finger and cut through the rib at this point – it will be easier on YOUR musculoskeletal system!

You can also learn a lot about the animal by looking at the teeth. Porpoise teeth are SPATULATE meaning they are flat on top while dolphin teeth are CONICAL or pointy on top. Even if you only find a skull you should be able to identify the animal down to the porpoise or dolphin level. Teeth can also indicate relative age. Are the teeth very worn? This is likely an indication that the animal was older.

For more information about the musculoskeletal system of odontocetes see Pabst 1990 and 1993, Long et al., 1997, Dearolf et al., 2000, Noren et al., 2000, Etnier et al., 2004, and Cotton et al., 2008.

Detailed annotated images of the musculoskeletal system of a harbor porpoise are shown below.  CLICK on an image to see an enlarged view.