The horse (Equus ferus caballus) is a familiar animal to all.
Long muzzle. Large size. Single toes on each foot.
They are something that culture has come to see as familiar as any other domesticated animal. So familiar that most overlook its unique anatomy, not understanding how much history it holds.
To learn why the horse’s anatomy looks the way it does we need another creature for comparison. Well, the horse is in the order Perissodactyla, which are hoofed animals with toes arranged in odd numbers on each foot (Clutton-Brock 1992). Rhinoceroses and tapirs are members of this order, but unfortunately they are too distant in anatomy from the horse to make a proper comparison.
It would be best to pick an animal that the horse is more related to, something from its family Equidae which includes zebras, donkeys, and wild asses (Clutton-Brock 1992). But these animals seem to be too similar in anatomy to the horse to make a comparison. It would be better to pick an equine in the family that is long extinct, an ancestor that is similar yet different enough in anatomy to the horse. And there is no better equine for it than the Hyracotherium.
With a short muzzle, small body and four toes on each foot, Hyracotherium seemed more canine than equine (Clutton-Brock 1992). Yet they are the perfect equine to compare and explain the horse’s anatomy.
But before we can begin the comparison we have to understand the environments that Hyracotherium and the horse separately lived in. This is important because it is catalyst that pushed for the anatomy adaptation in the first place.
Eras ago, Hyracotherium lived in an environment of forests and humid air (Budiansky 1997). Eventually however, these forests thinned out and became grasslands. The air also changed, becoming dry. These serious changes in environment required adaption in anatomy or else the horse would never come to fruition as a species. These adaptions in anatomy include but are not limited to:
Hyracotherium’s teeth were adapted to eating whatever fruits and leaves forests grew at the time (Simpson 1951). The soft food meant its teeth didn’t have to put much work into chewing to gain nutrients, so they were short with minimal surface area.
When forests became grasslands, its descendants were suddenly forced to feed on grass to survive. Grass unlike fruits and leaves were not soft and easy to break down into nutrients so required more specialized teeth. This was achieved by increasing the height and surface area.. Now, horses could chew more grass at a time and not have to worry about degrading their teeth down to the point of uselessness.
Being small in size wasn’t a detriment to Hyracotherium at the time. There was no need to be bigger when it didn’t require a lot of digestive work to break down its diet of soft fruits and leaves with its short, minimal surface area teeth (Simpson 1951).
In the grassland however, it payed to be bigger while having the appropriate teeth. Grass compared to fruit and leaves were a poorer source of nutritional value so a compensation needed to be met by eating larger amounts at a time. Larger amounts could only be taken in and digested if body size started to increase.
Four toes per foot fit Hyracotherium appropriately at the time because of its small size. Being small meant its toes could easily handle the weight that it was forced to support (Simpson 1951).
However, when its descendants grew bigger in size it had to compensate for the greater weight forced upon the toes. To do this, they had to start prioritizing a single toe while losing the others over time. This single toe eventually became the horses’s hooves. Large, sturdy and single digited.
Looking back we can see that the horse’s anatomy is entirely due to the diet change it had to undergo as its its environment changed. But the environment, nature itself is not the only catalyst that pushed the horse’s anatomy to look the way it does. Humans also play an important part.
Every feral and domestic horse in the Americas did not originate from there (Clutton-Brock 1992). They actually were brought over from the the Old World by settlers when they needed beasts of burden. By bringing them over, humans exposed them to a variety of environments which once again required adaptions in anatomy. Because of this a variety of horse breeds exist that can practically fit any environment.
Budiansky, Stephan. “The Improbability of the Horse.” The Nature of Horses: Exploring Equine Evolution, Intelligence, and Behavior. NY: Free Press, 1997. 9-38. Print.
Clutton-Brock, Juliet. “Conquest of the Americas.” Horse Power : A History of the Horse and the Donkey in Human Societies. Cambridge, MA: Harvard UP, 1992. 140-53. Print.
Clutton-Brock, Juliet. “The Family Equidae.” Horse Power : A History of the Horse and the Donkey in Human Societies. Cambridge, MA: Harvard UP, 1992. 17-25. Print.
Simpson, George Gaylord. “How Horses Changed: Limbs and Feet.” Orses; the Story of the Horse Family in the Modern World and through Sixty Million Years of History. NY: Oxford UP, 1951. 190-204. Print.
Simpson, George Gaylord. “How Horses Changed: Teeth.” Horses; the Story of the Horse Family in the Modern World and through Sixty Million Years of History. NY: Oxford UP, 1951. 181-89. Print.