Tour of the Penguin Skeleton III: The Pygostyle

It’s time to continue our tour of the penguin skeleton.  Today, we will look at the pygostyle.  This is a special element that is formed by multiple caudal (tail) vertebrae that fuse together as birds reach adulthood.  Whereas the dinosaurian ancestors of modern birds had long bony tails with dozens of individual vertebrae, living birds only have a few individual “normal” caudal vertebrae with a pygostyle at the end.  This structure is usually somewhat plate shaped – that is,  flattened in the vertical plane. The pygostyle is very important in flight because it serves as the attachment site for muscles the raise and lower the tail, and those that move the tail from side to side.  This allows volant birds to change the angle of the tail feather fan, which is critical in landing and turning.

A typical avian pygostyle (left) compared to a penguin pygostyle (right). The grey shapes show the cross-section of the bone at the plane of the red line.

Penguins have a pygostyle, but it is quite different in shape from the standard avian pygostyle.  In penguins, the element is more elongated and less flattened. Rather than being plate-like, it is almost triangular in cross-section with a flattened base.  Penguins also have a very different set of tail feathers.  Rather than forming a fan, penguin tail feathers are very stiff and quill-like, and stick out somewhat like the bristles of a broom.  This is especially true of penguins from the genus Pygoscelis – the Linnean name Pygoscelis actually translates to “stiff tail”.  These penguins are prone to be caught slouching around, partially propped up on their tail feathers.  It seems that without the necessity of maintaining a  “fan” of tail feathers, penguins have gone ahead and modified their pygostyle to a shape more suited to supporting themselves on land than steering in flight.

A gentoo penguin (Pygoscelis papua) propping itself on its tail feathers.