Pneumatic Bones in Birds

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  • 0:01 Definition of Pneumatic Bones
  • 0:44 Pneumatic Bone Structure
  • 2:24 Pneumatic Bone Evolution
  • 3:02 Pneumatic Bone Function
  • 4:18 Lesson Summary
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Lesson Transcript
Instructor: Taormina Lepore

Taormina has taught advanced high school biology, is a science museum educator, and has a Master's degree in museum paleontology.

In this lesson, we'll discuss pneumatic bones in birds. What are pneumatic bones, how are they structured, and why are they so important to avian anatomy and the process of flight?

Definition of Pneumatic Bones

The term pneumatic may be familiar to you from other contexts outside of anatomy. In a very general sense, the word relates to air under pressure, as in a pneumatic drill. It derives from the Greek 'pneumatikos,' pertaining to wind or the breath.

In biology, the term 'pneumatic' refers to breathing. And in birds, breathing has evolved a peculiar path. Birds have remarkably specialized bones that are pneumatic, because they are full of air sacs that provide a continuous flow of breath throughout their bodies. In short, their lungs are essentially hooked up to their bones. To understand why these pneumatic bones evolved and how they function, let's first look at their structure.

Pneumatic Bone Structure

Pneumatic bones aren't just present in birds. To demonstrate this fact, take a deep breath through your nose. The air you're breathing is passing through a series of pockets and channels within the pneumatic bones of your sinuses. The same pockets and air sacs are present in bird bones. Actually, birds don't just have pneumatic bones in their skull, they have them throughout their skeleton. It is this fact that makes the avian skeleton lightweight and delicate.

Let's zoom in and take a closer look at a cross-section of bird bone. There are many tiny canals through which blood vessels flow, as well as larger, darker pockets that indicate pneumatic bone.

Pneumatic bird bone at 0.1mm magnification
Pneumatic bird bone at 0.1mm magnification

Those big pockets, like the one you see at the lower right side of the image above, are where a bird's extended respiratory system connects the lungs to the pneumatic bone air sacs.

If we zoom out, we can see the overall structure of bird bones is full of hollows, and therefore very lightweight. The outer edges have a thin crust, or cortex, whereas the main body of the bone tends to have a thicker cortex. Most spongy bone has tiny honeycomb pockets called trabeculae, but in pneumatic bird bones, trabeculae widen and become big air sacs.

Pneumatic bird bone in cross section
Pneumatic bird bone in cross section

You can see above a cross-section of an avian long bone, showing trabeculae as well as very broad air sac pockets, in which the respiratory air sacs sit.

Pneumatic bones within a bird's body include: the skull, which has pneumatic bones like ours do; long bones, such as the humerus in the upper arm; parts of the shoulder girdle, such as the keel and the clavicles; and parts of the backbone, including the vertebrae and the pelvic, or hip, girdle.

Pneumatic Bone Evolution

Pneumatic bones have been present in birds for many millions of years. The lightweight structure and abundant air pockets are found in the bones of many carnivorous dinosaurs, such as this theropod dinosaur, Aerosteon riocoloradensis:

Pneumatic bone in the dinosaur Aerosteon riocoloradensis
Pneumatic bone in the dinosaur Aerosteon riocoloradensis

The parallels between non-avian dinosaur bones and avian bones can be seen in the image below, which displays the relationship between the different air sacs within both a dinosaur's and a bird's body.

Pneumatic air sacs in dinosaurs and birds.
Pneumatic air sacs in dinosaurs and birds

Notice how the anterior (in blue) and the posterior (in green) air sacs, as well as the flow of exhalation, are parallel in both ancient carnivorous dinosaurs (Majungasaurus, in this case) and modern birds.

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