Join Date: Apr 2010
Location: Orange County, California
And some anatomical facts:
Horses don't have clavicles (collar bones).
Horses are members of the group Perissodactyla or "odd toed ungulates". Ungulates are animals with hooves. Other members of the Perissodactyla are tapirs, and rhinos. They are typified by an odd number of toes on their front feet: 5:3:1
The other main group of ungulates is the Artiodactyla or "even toed ungulates." This group includes hippos (contrary to another post), pigs, peccary, camels, deer, giraffe, antelope, sheep, goats, and cows/bison.
Horses (and most Perissodactyls), along with sheep, have both upper and lower incisors. Cows, deer, and bison only have lower incisors. This helps these different grazers "partition their niches" (each have slightly different eating habits). The animals with only lower incisors gather grass by "pinching" it between their lower teeth and the roof of their mouth, so these animals don't tend to graze the grass right to the ground. Sheep/horses pinch the grass between their upper and lower incisors, cutting the grass much more close to the ground (part of the problem in the Sheep/Cow wars of the American West).
Between 20 and 10 Million years ago, most of the adaptations to grazing were taking place in the skull of horses. The high crowned teeth required a deepening of the skull, the cheek bone (zygomatic arch) was strengthened to accommodate strong chewing muscles, and a "walled post orbital" was formed. This is the bone that "closes" the eye socket behind the eye in a modern horse. Humans also have a walled post orbital to keep the masseter muscle from pushing the eye out of the socket.
The strong elastic ligament that runs down the back of a horse's leg also evolved about this time. The ligament stretches as the hoof hits the ground, giving elastic energy to assist move the horse forward.
The modern horse genus, Equus, appeared about 3.5 million years ago, and quickly spread from North America to Europe, Asia, Africa and South America.
Horses (and all perissodactyls) do not ruminate (chew their cud), so they don't digest grasses as fully as artiodactyls. Artiodactyls have a multi-chambered stomach and pass the food through the chambers. Horses have a caecum that is posterior to the stomach that opens between the small intestine and the colon. All of this means that, in horses, food passes relatively quickly through the digestive system. This is also why "road apples" are so different than cow pats. The horse doesn't digest the plant material as well, so the manure has more fibrous material in it, and is more firm. A ruminant, like a cow or bison, keeps the food in its system longer, and digests more of the plant material, making their manure much looser.
The horse tooth is made up of three parts: dentine, enamel, and cementum (just like in humans). Enamel is the hardest of these, but is somewhat brittle. The differences in hardness between these tissues is what makes the surface of a horse's teeth rough, and allows them to grind up the plant material they eat.