Pneumatic Bones: Bones that Breathe
Pneumatic Bones in Birds and You
As a birder and naturalist I love it when I learn new information that not only helps me understand the avian world but also my own, thus the post this week. Most of us learned long ago that birds have hollow bones. We learned in grade school that it helps the birds be lighter so that they can fly. However, there's something more to unpack in the story that I think is pretty neat, they can breathe with those hollow bones.
Let's start with the term that scientists use, pneumatic (knew-mat-ick) bones. You've probably heard of pneumatic brakes on trucks, essentially it's air in a closed system that is used for braking. The term pneumatic originated from 17th century France and pneumatique, but goes farther back to Greece pneuma meaning "wind," and pnein "breathe." (Isn't etymology great?) Essentially pneumatic bones are "breathing bones." Pneumatic bones have air spaces in them they're structurally more like corrugated cardboard than solid bone. In birds this system is "open" for air to move through, unlike in car brakes where the system is "closed."
Now here's where things get interesting, it's not just birds that have pneumatic or hollow bones, you do too. Human facial bones are pneumatic, just like bird skulls. Pneumatic bones can be found around your inner eyebrow, under your eyes, around the nose, and around your lower cheeks, essentially they are your sinuses. You've been carrying them around your whole life and probably didn't even know it.
Pneumatic bones in your face support the nasal sinuses, are light weight, thought to provide some impact protection, and they help provide resonance for your voice because of their hollow chambers.
Your true sinuses are hollow openings in the pneumatic bones that are lined by epithelial (skin) cells. Why we have sinuses and their function is being debated. It's thought that the spaces help condition air before it reaches your lungs. This means getting the air to a warmed or cooled temperature before it goes into your lungs (picture ice-cream brain-freeze but in your lungs, not pretty...this is what sinuses prevent). When you get a cold or have allergies, your sinus tissues swell, which is what causes many of us seasonal misery.
BIRD BREATHING BONES
Did you know that dinosaurs had pneumatic bones too? It makes sense if you think about it. Birds are the ancestors of dinosaurs. Sauropodomorphs were the first dinosaurs to demonstrate pneumatic bones in their backbones, specifically Thecodontosaurus caducus and theropod Coelophysis bauri. Sauropods are some of the longest neck dinosaurs that were around, and to have hollow bones in their necks made a lot of sense, it avoided the "pain in the neck" of heavy bones, and allowed oxygen to flow more freely into their bodies. It's estimated that pneumatic bones made their necks so ultra light that their bones were 60-89% air volume (specifically Sauroposeidons).
While most animals have just facial pneumatic bones, birds have them in their skulls and throughout their bodies. These include their clavicle, humerus, keel (big chest bone we like for white meat), pelvic girdle, lumbar and sacral vertebrae. Now I want you to sit and think about this before reading on. How could a bird breathe through its bones? I'll give you a hint, think didgeridoo playing.
Birds are what I call the original digeridoo players. They have circular breathing. To play a digeridoo you have to master circular breathing, that is breathing in while breathing out. For humans that's not an easy task, for birds it's necessary. A bird's respiratory system is set up very differently than ours, and it's great for flight and getting as much oxygen out of air as possible to feed their large flight muscles.
Birds have huge lungs compared to their body mass, about 50% larger than ours are by comparison. The thing is, they don't have a diaphragm like we do so their lungs don't expand and contract like ours either. All this gets even stranger. Bird lungs are tucked up under their backbone, and unlike ours that have little sacks much like cul-de-sacs or dead end chambers in the lungs, birds have spongy lungs that allow air to flow through (like water through a sponge).
Attached to a bird's lungs are eight or nine (up to eleven) air sacs, just hollow balloon like sacs that hold air and act like bellows. There are usually four or five around the front and underbelly of the bird and four at the back. These air sacs allow air to move one way, which is super efficient. Think about our breathing, it's multi-directional, air goes in and air goes out the same way. Old air mixes with new air and it's pretty inefficient. In birds the air is moving around in one direction, and they're always getting a "fresh breath of air" when they breathe in (no old air moving out past it and mixing).
I've been lucky enough to hold birds in my hands, ranging from warblers to ospreys. One thing that I always noticed was that when the birds were in my hands I felt their whole body move while they were breathing. At first I chalked this up to the idea that they must be scared, and really "heaving" for breath. However, I learned that bird breathing, and its circular pattern, is a whole body affair. Since birds don't have a diaphram like we do, they have to move their chest muscles to breathe in and out, and to pump the "bellows" of their air sacs. You can literally suffocate a bird to death by holding it too tight, because they need to move their chests to breathe!
There are four key steps to a bird's circular breathing, and it's pretty complex (you can read all about it here). I'm not going to explain it all here (that's for another post), the best thing to do is watch this great video:
SO WHERE DO PNEUMATIC BONES COME INTO ALL THIS?
A bird's pneumatic bones are attached to the air sacs. As you can see from the diagram below, a bird's anterior bones are connected to the anterior air sacs, and the posterior bones to the posterior air sacs.
The function of this connection is still being studied, but it's thought that by having air sacs attached to the bones that their breath/air provides air pressure and strength to the skeleton, helps cool the bird by diffusing heated air, and helps the bird remain light for flight. If you think about it, a bird is always breathing through its lungs, air sacs, and bones all the time. How amazing is that!?
THOUGHTS ON EARLY DINOSAURS AND BIRD AIR SACS
As a closing thought, consider this, early dinosaurs and birds have striking similarities in the location of their air sacs and bones. The similarity has reinforced the idea that birds came from dinosaurs.
Some researchers this type of adaptation is what allowed dinosaurs to walk upright too.
Pneumatic bones are a wonderful innovation of nature, and provide people and animals with the hollows they need to breathe and extract oxygen. Just think, you, dinosaurs, and birds all have hollow bones. Even though we use them differently it's a connection none the less, and quite fascinating!