Tour of the Penguin Skeleton I: Scapula
Remarkable is a fair way to describe the penguin skeleton. Each bone seems perfectly molded to the complex mission of the penguin – to swim, dive, scramble across rocks, or catch prey as the situation calls. One obvious difference between penguin bones and those of other birds is their density. Pick up box of penguin bones (most species skeletons will pack neatly into the average tennis shoe box) and a box of chicken bones and you’ll instantly notice the weight difference. Increased density and reduction of air space helps penguins maintain negative buoyancy while diving. While almost every bone in the penguin skeleton has undergone increased osteosclerosis for density, the shape of each bone also tells its own story. Some are identical to the comparable element in a “normal” bird, while others would barely be recognized as avian by most non-specialists.
I’d like to review the entire penguin skeleton bone by bone. This will take a long time – there are over one hundred free elements in the adult penguin skeleton (some are formed by multiple bones that fuse together as the penguin grows). Let’s start with the scapula, one of the most unusual.
The scapula is commonly known as the shoulder blade in humans. In fact it is even more blade-shaped in birds. Most living birds have a scapula that looks somewhat like a curved sword. The “blade” extends over the front part of the birds back. The “handle” contacts two other bones, the furcula (wishbone) and coracoid, forming the an opening in between called the triosseal canal. This canal is very important because the tendon that helps birds lift their wings travels through it. Lifting the wing is of course part of the wingbeat cycle – up, down, up ,down and there goes the bird through the air.
In volant (flying) birds, the upstroke is really used only to re-position the wing for the next downstroke, which does all of the actual thrust generation to push the bird through the air. In penguins though, the upstroke is much more important. Because water is so much denser than air, a penguin can push against the water as it lifts its flipper up. Therefore, it can generate thrust on both the upstroke and the downstroke.
This high density of water also creates a problem though – it requires a lot more force to flap a flipper in water than a wing in air. So penguins have maxed out the muscles that lift the wing. One of these, the scapulohumeralis caudalis, attaches to the scapular blade. In most birds, this muscle does not have to be very large, so the thin blade provides more than enough room for it to attach. In penguins, the muscle needs more room and the blade is greatly expanded to accommodate this. A penguin scapula looks almost like a tennis racket, with a normal thin “handle” and a flattened, paddle-like blade region. Interestingly, the blade is only slightly widened in the earliest fossil species, suggesting that penguins gradually improved their wing upstroke strength over time.