because animals have more bones and they produce more
chicken
In a four-legged animal, the weight of the body would be distributed between the pelvic and pectoral girdles. In humans, the pelvis has to support the weight of the whole trunk.
ilium
Humans have to carry all of their weight upright and on just two legs. Animals that have four legs to support their weight and their pelvic bones are less massive. We "pay" for this as we are more prone to fractures and other problems of the upper leg bones. See link for more information:
The reason the pelvic bones, or the hip bones, of a four legged animal are less massive than those of a human is because in humans all of their weight is concentrated onto that one area of the body, and only onto two legs. But in four legged animals there are four legs to distribute the weight onto, thus the hip bone does not need to be as strong.
Sphinx/ Σφύγξ is the name of the creature that has the body of lion and a female human head.
mammals
A sphinx. Most often, the animal's body is that of a lion. Another, similar to the sphinx is the manticore, which was reputed to have poisonous spikes on its tail, which could kill instantly. There is also the centaur, but this has not only the face of a human, but also the torso.
The shape of the pelvic bones and the frontal bones differ.
Des pattes are 'animal legs' in French. Horses are the only animals with 'jambes' (the name for human legs), all other legged animals have 'pattes'.
The pelvic fin
Most four legged animals