Femur
Long bone; part of the appendicular skeleton (limbs).
Also known as: Thigh Bone
Classification
Long bone; part of the appendicular skeleton (limbs).
Dimensions
The femur is typically the longest and strongest bone in a mammal's body. It features a straight, cylindrical shaft that widens at both ends to form sturdy joints.
Key Features
The most defining feature is the smooth, round 'ball' at the top end that sits on a distinct 'neck.' Just below this ball, there are usually rough, bony bumps where large leg muscles once attached. The bottom end has two large, rounded rollers that allow it to hinge smoothly against the lower leg bones.
Similar Bones
It can be confused with the Humerus (upper arm bone). However, the femur usually has a much narrower neck supporting its rounded ball and a more deeply grooved joint at the bottom compared to the flatter, spool-shaped bottom of the humerus.
Across Species
The femur shows incredible variation across the animal kingdom. In heavily-built mammals like elephants or rhinos, the femur is massive and pillar-like to support immense weight. In leaping animals like rabbits or deer, it is long and slender to provide leverage for powerful muscles. In birds, the femur is kept close to the body and often stays hidden under feathers, acting more like a stable pivot point for the rest of the leg.
Evolutionary History
This bone can be traced back to the lobe-finned fish of the Devonian period. As vertebrates transitioned from water to land, the femur evolved from a simple pelvic fin support into a load-bearing pillar. Over millions of years, the top end developed a rounded bulb that fits into a deep socket, allowing land animals a wide range of movement for walking, running, and climbing.
Photography Tips
To get the best identification, place the bone on a neutral background next to a ruler or a common object for scale. Take one photo from directly above to show the full length, and a close-up photo concentrated specifically on the rounded ball at the top end.