The fundamental form of the vertebrate foot has five digits, however some animals have fused digits, giving them less, and some early tetrapods had more; Acanthostega had eight toes. Feet have evolved many forms depending on the animal's needs. One key variation is where on the foot the animal's weight is placed. Some vertebrates: amphibians, reptiles, and some mammals such as humans, bears, and rodents, are plantigrade. This means the weight of the body is placed on the heel of the foot, giving it strength and stability. Most mammals, such as cats and dogs are digitigrade, walking on their toes, giving them what many people mistake as a “backward knee”, which is really their ankle. The extension of the joint helps store momentum and acts as a spring, allowing digitigrade creatures more speed. Digitigrade mammals are also often adept at quiet movement. Birds are also digitigrade. Hooved mammals are known as ungulates, walking on the fused tips of their fingers and toes. This can vary from odd-toed ungulates, such as horses, pigs, and a few wild African ungulates, to even-toed ungulates, such as cows, deer, and goats.