Horses have the same basic physiologic characteristics as people and domestic pets, but they also have some unique features that set them apart. These special features are a result of evolution.
For example, horses have a small stomach in relation to their body size. This makes it easier for them to digest food.
In addition, a horse’s skeletal structure is built for speed and agility. The legs are long and slender, and the lower leg bones (metatarsals and tarsals) are fused to allow for quick movements. The feet are also three-toed, with a central toe that bears the horse’s weight. This is an adaptation for running over rough ground, which could damage feet that were too thick or rigid.
The horse’s sense of smell is extremely important. The nose has a large internal surface area that contains many chemical receptors, and it is a hundred times more sensitive than the nose of a human. The sense of smell allows a horse to identify other horses, and it is also how a stallion assesses a mare’s fertility status. Stallions may nick their mouths with other stallions or sniff the air as they walk by a mare in heat.
Although a horse’s sense of smell is very important, sight remains the most dominant sense in a horse’s life. The equine eye is the largest of any land mammal, and its visual cortex handles one-third of all the horse’s sensory input. In addition, the horse has a large number of rods in its retina that give it good night vision and a reflective tapetum lucidum that gives it some color perception.
When a horse sees something that scares it, its eyes widen and the ears flare as it takes in a great deal of information at once. The horse may flap its mouth (“Flehmen”), expose its teeth (“biting down”) and jig, all of which are ways to communicate that it is feeling fearful or stressed.
Those feelings are not always a cause for alarm, though, as horses have an excellent memory and can use their memories to judge how far away objects are. A horse may even plop down to judge the depth of a puddle or raise its head to judge a fence’s distance.
The next important ancestor of the modern horse was Mesohippus, which evolved during the middle and late Oligocene (which lasted from 33.9 million to 23 million years ago). This animal looked more like a horse than Eohippus. Mesohippus was more grazing than browsing, and its teeth were adapted to grazing as well. The forefeet lost their fourth toe and became three-toed; the hind feet remained two-toed. The ancestor’s neck was longer and more slender, its jaws were less chewing-like, and it had shorter, more rounded cheek teeth.