The Hip Region

Glycogen Body

Within the fused vertebrae that make up the synsacrum in the hip, the neural tube is greatly enlarged to accommodate a poorly understood structure known as the glycogen body. Early paleontologists were surprised at the small brains in dinosaurs and a comparable structure in the hip region of dinosaurs was once thought to be an auxiliary brain to control the long tail. Its physiological processes in birds are very unusual and at one time it was thought to be responsible for the production of the myelin sheath that insulates the nerves. Modern research suggests that it is an organ of balance. It seems appropriate for bipedal animals to have a large organ of balance associated with their hind limbs.

Sacrum-Back Interface or Lumbar Region

Birds must carry two large muscle masses, one for the wings and one for the legs. Because these locomotory modules function more or less independently of each other, they create stress in the skeleton where they meet. The same is true in humans where the zone of interaction is marked by distinctive lumbar vertebrae. The design of the lumbar vertebrae in humans is notoriously inadequate and contributes to lower back pain and other medical problems.

Birds do not have a lumbar zone in the sense of a series of vertebrae with distinctive characteristics but the word seems an appropriate name for the interface between the thoracic zone and the sacral zone. The vertebrae in front of the interface typically carry ribs that are attached to the sternum and link the thoracic vertebrae to the pectoral girdle. The vertebrae immediately behind the interface are fused to the hipbones and sacral vertebrae as part of the posterior locomotory module. The precise location of this interface varies from group to group. It is furthest forward in pelicans and frigate birds because an exceptionally large number of vertebrae are fused to the hips.

As in humans, the lumbar interface may represent an “achilles heel” in the skeletal design of birds. It includes the largest and most robust vertebrae but size alone may not be sufficient to prevent injury. Grosbeaks and other forest birds are often killed in large numbers along the sides of highways. They are not actually struck by vehicles but the wind from passing semi-trailers whips them into the air and subjects them to forces not normally experienced in the shelter of the forest. Their bodies often show massive internal bleeding near the kidneys. It appears that the wings have twisted the forward part of the body in one direction while the tail and legs have twisted the rear section in another until the spine has broken. Dislocated and damaged vertebrae rupture the large blood vessels that pass along the spine, killing the bird. Consequently aerobatic species, large species, and both wing-propelled and foot-propelled diving birds exhibit a variety of specialized structures that re-enforce the lumbar area.

Type 1: The long face

Some birds seem to depend on size alone to stabilize the lumbar area but others have greatly extended dorsal, ventral, or lateral processes on the vertebrae. Ligaments connect the tips of processes on adjacent vertebrae and twisting is simply resisted by the leverage offered by the elongated processes. Cormorants have carried the strategy to an extreme and have exceptional tall vertebrae in the lumbar area that give the sacral vertebra a very long and narrow face with a rather small centrum.

Cormorant 1

Cormorant 2

Type 2: Hypapophyses

In some diving birds, the series of hypapophyses on the thoracic vertebrae extends onto the synsacrum and its ligaments help stabilize the lumbar joint.

Loon

Auklet 1

Auklet 2

Auklet 3

Type 3: Dorsal development and extension of the hipbones.

Some birds have greatly exaggerated dorsal or lateral development to stabilize the lumbar joint.

Duck

Cuckoo

Type 4: Fingers

New-World Vultures and owls are two types of birds with large wing surfaces, long tails, and large, rather heavy, legs that share similar adaptations of their lumbar interface. The aerobatic activities of owls may exert stress on their lumbar vertebrae. They regularly make sudden turns and perform other maneuvers to capture their prey. Vultures seem less active but they may need to stabilize their lumbar zone when they are attempting to take off or land with a heavy load of food in their gut. In both cases, the edges of the saddle shaped centra are exaggerated so that a pair of stubby fingers extend from the lower edge into grooves in the previous vertebrae to lock the two bones in place.

Vulture

Owl 1

Owl 2

Owl 3

The shape of the interface between the thorax and the hips varies from group to group: see Gallery of the Lumbar Interface.