Femur
Long bone of the appendicular skeleton (limb)
Also known as: Thigh Bone
Classification
Long bone of the appendicular skeleton (limb)
Dimensions
The femur is generally the longest, heaviest, and most robust bone in a mammal's body. It is characterized by a straight or slightly curved central shaft and thickened ends that form joints with the hip and the lower leg.
Key Features
Look for a distinct rounded 'ball' at the upper end that fits into the hip socket. Opposite this ball is a large, rugged bump that provides an anchor for leg muscles. The shaft is usually smooth and cylindrical, ending at the bottom in two large, rounded rollers that form the hinged part of the knee joint.
Similar Bones
It can be confused with the Humerus (upper arm bone). However, the femur's 'ball' is usually held on a more distinct, elongated neck compared to the humerus. Additionally, the bottom end of a femur has deep twin rollers for the knee, whereas the humerus has a more complex, spool-shaped surface for the elbow.
Across Species
The femur shows remarkable adaptation across the natural world. In heavy mammals like elephants, it is thick and pillar-like to support immense weight. In hopping animals like rabbits or kangaroos, the femur is shortened relative to the lower leg bones to act like a powerful spring. Birds have a hollow femur to aid in flight, while whales have a tiny, hidden version of this bone left over from their ancestors who once walked on land.
Evolutionary History
This bone can be traced back over 300 million years to the first tetrapods that moved from water to land. Originally, it projected outward from the body in a sprawling posture, similar to modern lizards. Over millions of years, in the lineage of mammals and dinosaurs, the femur rotated to sit tucked directly under the body, allowing for more efficient walking and running on dry land.
Photography Tips
To help with identification, take photos from several angles. Always include a top-down view of the rounded ball end and the bottom rollers. Placing a common object like a coin, a ruler, or a pen next to the bone in the photo is essential to show the scale and overall size.