The modern horse, Equus caballus, evolved from the leaf eating rabbit-like animal, Hyracotherium which lived in South America hundreds of millions of years ago. Hyracotherium lived on succulent leaves and had simple short crowned, or brachydont teeth. Climatic changes caused great changes in vegetation, particularly the replacement of forests by vast areas of coarse grassland. Some descendants of Hyracotherium evolved that could survive on a coarse grass diet which required prolonged mastication. Evolutionary developments included an increase in body size as well as dental modifications, to meet the high rate of wear associated with this abrasive diet. The anatomical tooth changes included an increase in the length of the teeth (hypsodonty) which in contrast to the permanent teeth of brachydonts, such as dogs, erupt continually throughout most of the horse’s adult life (Gorrel 1997).
Due to the changes that take place to the teeth during the horses lifetime it is possible to estimate the age of a horse by assessment of numerous dental criteria, such as the angle and markings of the teeth.
Understanding the function and anatomy of equine teeth is essential in order to establish correct feeding methods and dental care. This then helps maximise the efficacy and life span of the modern horse by enabling the horse to masticate efficiently and painlessly.
The Functional Anatomy of Equine Teeth
The horse has incisor teeth at the front of the mouth, which are efficient at cropping grass close to the ground. Since they meet each other directly, they are much more effective cutters than human teeth, but the horse does not have the ability to advance and retract the lower jaw as humans do. Like all mammals, the horse has both milk teeth and permanent teeth (Wanless 1997).
Both humans and horses have cheek teeth; the premolars and molars situated further back in the jaw, which grind food. The molars of the horse lie in a caudal aspect towards the temporomandibular joint. Unlike humans, who have looser, more mobile cheeks, the cheek teeth of the horse are very close to the sides of the mouth, and the flesh of the cheek is stretched tightly over them, particularly in Thoroughbreds (Wanless 1997).
Canine and Wolf Teeth
In the space between the horse’s incisors and molars, stallions, geldings and somemares have a single tooth called the canine or tush (Wanless 1997). Wolf teeth are vestigial remnants from the Eocene period, and are commonly found in front of the premolars in the maxilla and very occasionally in the mandible.
The first and sixth equine cheek teeth are angled towards each other, in a caudal and rostral direction respectively. This results in the occlusal surfaces of the six cheek teeth being squeezed into close proximity, acting as a single grinding unit and preventing food from accumulating between the teeth (Tremaine 1997).
Also like the human, the mandibular cheek teeth are narrower in the lateral plane than the maxillary cheek teeth. Therefore the mandible in the horse is always narrower than the maxilla, which means that the lateral aspect of the maxillary cheek teeth and the medial aspect of the mandibular cheek teeth do not cover each other (Wanless 1997).