We’re all familiar with the major bones of a human body, although many folks don’t know their official names. Far less well-known are some small bones, most of which are really tiny, roughly the size of a sesame seed. They are therefore called sesamoid bones, and they form within tendons that connect muscles to bones; most of them do not articulate directly with other bones.
Humans sometimes have dozens of these little bones, especially at the joints in hands and feet, although the number of them varies with genetics and usage. Like other bones, they ossify gradually, as we grow. Most of these little bones are tiny, but slightly bigger ones (a centimeter or so) are associated with our big toes and thumbs, and the biggest of all is the knee-cap or patella.
The patella develops within the tendon that runs from the quadriceps muscle of the thigh (thighbone = femur) to the shinbone (tibia); that muscle is involved in extending or straightening the leg. The patella protects the joint between femur and tibia and, like other sesamoid bones, it is thought to provide some mechanical advantage or leverage to the muscles that move the jointed appendage.
Other animals have sesamoid bones too, of various sizes. They occur in most lizards and in bird feet, legs, and wings. Mammals usually have patellas and other smaller sesamoid bones. Interestingly, patellas have disappeared in some mammalian lineages (e.g. Monotremes—the echidnas and platypus); they have come and gone in a few other lineages too.
In some cases, in the course of evolution, a sesamoid bone has enlarged and acquired a new function. According to most classifications, pandas are bears, but they occupy a separate evolutionary lineage within that group. Giant pandas are the well-known black and white bamboo-eaters of China, popularized as children’s toys. They have developed a special sesamoid bone of the wrist, associated with the arm bone called the radius, so it’s called a radial sesamoid bone. It’s an addition to the normal five toes that bears usually have, and, unlike human thumbs, is not fully opposable. Nevertheless, it functions like a thumb and enables a panda to hold a bamboo stalk so it can strip off bamboo leaves and eat them, along with new shoots. The panda’s “thumb” has a pad beneath it, like the other hand bones do, and there is a groove between the pads, where the critter holds the stalk. Most of their diet is bamboo, and they eat a lot of it.
Red pandas of the eastern Himalayas are also highly herbivorous, eating lots of bamboo, although they snack on fruits and bugs and other things too. They also have a radial sesamoid bone that aids in manipulating bamboo stalks as well as helping them climb small branches. They are not very closely related to giant pandas, and their enlarged radial sesamoid bone has evolved independently.
Moles are burrowers and they too have a large radial sesamoid bone that’s like the panda’s thumb. In the case of moles, it increases the size of the front foot, probably for better digging of the burrows. Peruvian cotton rats are reported to have an enlarged radial sesamoid bone that may assist them in grasping small branches and twigs, as they may sometimes climb in search of food, but this needs further documentation, because they apparently live mostly in arid, grassy areas.
I’ve also read that elephants can develop a so-called sixth toe, which may increase stability. It develops in a cartilaginous and fatty lump behind the other, forward-pointing toes and points towards the back of the foot, ossifying as the critter ages. Elephants actually walk on tiptoe, although that is not evident externally, so the “sixth toe” provides another sturdy point of ground contact. It may have originated when elephant ancestors walked more on land and became very large.
Although this topic may seem rather arcane for Juneau natural history, I hope readers will think of it broadly in terms of what unusual twists can sometimes occur in the course of evolution.
• Mary F. Willson is a retired professor of ecology. “On The Trails” appears every Wednesday in the Juneau Empire.
