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Human vs. Canine Skeletal Structures

Dog owners may grow to look like their pets on the outside over time, but the similarities do not go down to the bone. Since both species are mammals, they share the same basic body structure: spine, four limbs, skull. The differences largely derive from method of locomotion -- bipedal versus quadrupedal -- and the behavioral habits, such as diet and giving birth, but they can be found on a microscopic level as well. Yet dogs remain man's best friend.

Skull
- Human skulls are generally more rounded than dog skulls.
  
  A human skull is oriented on a vertical axis, with a short face and a large, rounded cranial vault -- where the brain is housed. Dog skulls have the opposite morphology, oriented on a horizontal axis with a small, curved cranial vault and a long face, even in the short-faced breeds like pugs and pekinese. A dog's skull is also likely to have a top ridge. Inside, a human's brain case is generally smooth, while a dog's is more complex.
Teeth
- Humans have lost the large canines other apes retain.
  
  Dog mandibles are V-shaped, with a separation between the two jawbone pieces. Human mandibles are U-shaped, with a less-evident division. A dog's teeth are specialized for catching prey and tearing meat from bones, but a human's teeth are more generalized, with flat rear molars made to grind nuts and plant material. Both species eat a varied diet that may include meat and plants, but the dog's teeth reflect its more-carnivorous habits.
Torso
- The human skull sits atop a pillar of vertebrae.
  
  The human spine has slight, opposing curvatures designed as a support column compared to the dog's straight spine, which behaves like a suspension bridge. The human pelvis is broad and short rather than narrow and elongated. A dog's vertebrae have spiny protrusions and are concave and convex, while a human's are large, broad and flat. Both species have fused sacral vertebrae, but a dog has fewer, which are longer and narrower. And most dogs, of course, have tails.
Limbs
- Dog shoulder blades are perpendicular to the spine.
  
  The bones of the quadrupedal dog's forelimbs are typically more robust than the bones of a human's arms. The radius and ulna in a dog's forelimbs and the tibia and fibula of its legs are often fused, while a human's are not. Some dogs completely lack a fibula. The femur is the longest bone in the human body, but a dog's femur is comparable to the length of its other limb bones. The shoulder blades of the two species are also oriented differently.
Bones
- Human and dog bones differ even microscopically.
  
  The bones of dogs are denser than those of humans because canine bones are less porous. When viewed in cross-section, the compact bone that surrounds the marrow appears thicker in a dog bone than in that of a human. However, the bones of a dog's skull are more compact and therefore thinner than those of a human skull. The microscopic structures within the bones themselves differ in their distribution, giving scientists a way to differentiate between human and canine bone fragments.

Animals & Science