On the Feet of Penguins
Penguin feet are very distinct. Perhaps the most important bone in terms of giving penguins their cachet with the public is the tarsometatarsus (the ever-present foot bone). Thanks to their almost comically short feet, penguins move on land with an endearing waddling gait rather than with the more serious step of birds with longer, more gracile legs. Aside from length, another major difference between the feet of penguins and those of most other birds is that penguins have a very tiny hallux, or first toe. Most birds have four toes, instead of the five typical of mammals like ourselves. A few have only three toes (for example, some kingfishers) and the ostrich is unique in having only two. In most living birds, the first toe quite large and is modified for perching. It is reversed, facing the opposite direction as the remaining three toes, to help grasp branches more firmly. In some aquatic birds, the first toe is connected to the second toe by a web which makes the foot a more efficient paddle.
Penguins certainly don’t perch. They also don’t paddle with their feet, instead using their flippers to propel themselves through the water. Penguin feet are made for walking and steering. These birds, although much more graceful at sea, are quite capable of marching across challenging terrain. Adelie Penguins can march up to 100 kilometers (about 60 miles) across sea ice to get to their breeding grounds. Penguins can also jump surprising distances. The aptly named rockhopper penguins can maneuver dangerously jagged stacks of wave-beaten rocks on windswept islands by bounding from one to another with striking skill. Penguin feet may not be good for running or perching in trees, but they are well designed for this type of workload. The tarsometatarsus and phalanges (small bones of the toes) are wrapped in a thick layer of blubbery fat to cushion them, and covered with rough, thick scales to stand up to wear and help gain purchase on slippery surfaces. Steering is the other locomotory task of the penguin foot. If you have a chance to observe penguins swimming through a glass divider at an aquarium or zoo, pay attention to their feet. They employ their feet like little rudders, angling them to help control their dive direction.
The fossil record shows us that penguins have developed a more compact foot over time. Waimanu, the oldest known fossil penguin, has a relatively short tarsometatarsus by the standards of flying birds, but it cuts an elegant figure compared to the squat tarsometatarsus of living penguins. We see a more square shape and some shortening in the Eocene penguins from Seymour Island. By the Oligocene (25 million years ago) most of the penguins preserved in the fossil record have a foot of modern proportions. So far, no fossil has preserve the hallux. This suggests that even the oldest penguins already had a very small hallux, as such a tiny bone can easily be lost during the fossilization process or even go unrecognized during fossil collection and preparation. Aside from being a bit of trivia, many ornithologists consider the fact that both Procellariiformes (tubenose seabirds like albatrosses and petrels) and penguins have a miniscule hallux to be strong evidence linking these two groups of birds to a common ancestor.