Tour of the Penguin Skeleton VI: The Mandible
Mandible is the scientific term for the lower jaw. Whereas we humans have a lower jaw made of only a single bone (the dentary), penguins have a more complicated mandible made up of half a dozen different elements (the dentary, splenial, articular, prearticular, angular and surangular). These bones are all connected in penguins, though some of the joints are rather loose, which allows the jaws to flex a bit This process is called kinesis. At the front tip of the mandible, the left and right sides of the jaw meet and connect at the symphysis. This region is often a very firm connection, with no movement possible. One of the many unique things about living penguins is that they have a very short, flexible connection at the symphysis. This allows for more “play” in the jaws, which may be helpful when a bird has a mouthful of thrashing prey. Not all penguins have a short symphysis though. The spear-beaked fossil species Icadyptes salasi, for example, has a long, firm connection which is probably related to a different style of prey capture (e.g., spearing versus biting).
Different types of penguins exhibit different mandible shapes. The depth of the mandible can be an important clue to the type of food penguins eat. Many fish and squid specialists have low, slender mandibles like most other birds. Krill-loving species like the Adélie Penguin often show much deeper jaws. This difference is interpreted as an accommodation for the larger, spikier tongue of those penguins, which helps them capture shoaling prey. For this reason, I am always eager to measure the jaw dimensions of fossil penguin specimens that I stumble upon in the field (or in museum drawers). Its quite interesting to note that so far, none of the ancient penguin species that have been discovered had deep jaws. This suggests they had not yet adapted specializations to catching krill. Perhaps this feeding strategy was acquired only recently in penguin evolution, as Antarctic ice sheets spread and gave rise to new ecosystems.