Horses

Horses are in the Perissodactyl mammalian order, along with rhinoceri and tapirs. The evolution of horses is one of the more interesting sequences of mammal evolution. They began as small dog-sized animals that lived in the forest. They gradually adapted to grazing in grasslands and defended themselves from predators by running. Gradual changes in the horse brain size, height, leg structure with loss of toes, increasing size of teeth, and shape of teeth are easy to track in the fossil record (Figure 12‑19); however, some of the evolutionary connections have been difficult to sort out because of the numerous species and taxa.

In the 19^th century, Charles Marsh, presented a schematic diagram of horse evolution; however, the diagram had errors because horse evolution followed a complex path (Figure 12‑20) with many dead ends that were not apparent in the fossil record of the 19^th century. Over the years, opponents of evolution argued that Marsh’s somewhat erroneous diagram was proof that horses did not evolve; however, paleontologists have now resolved the complex of path of equine evolution (Figure 12‑20). Eohippus was dog-sized and appeared in the early Eocene (55 Ma). Mesohippus and Miohippus (37 Ma) lived in the Oligocene and evolved larger middle toes, which eventually became a single hoof. Parahippus (23 Ma) had continually growing teeth, which enabled it to eat grass. Merychhippus appeared in the Miocene and had a very large middle toe and was the first grazing horse. Dinohippus, evolved from Merychhippus. It was initially a three-toed horse (tridactyl) but because of the need to run long distances, it evolved to become a one-toed horse (monodactyl).

The first Equus (modern horse) in the fossil record was the Hagerman horse (Equus simplicidens, Figure 12‑21). Modern horses, zebras, and donkeys (all Equus) evolved in the Pliocene. Horses evolved in North America, but they went extinct in North America when humans arrived, 12,000 years ago, possibly due to humans but possibly due to climate change. Horses survived in the Old World. Christopher Columbus brought horses back to the Americas in 1493, and the Spanish brought 16 horse species to America in the 16th century.

Although horses are thought of as flighty, which is equated with stupidity, this is the innate instinct of flight (running) since that was their primary defense mechanism against predators. Sensory organs are a type of intelligence.[1] Horses have extremely well-developed senses of smell, hearing, touch, taste, and sight. Horses are the most perceptive of the domestic animals. They are also extremely fast learners and have uncanny memory. They are able to desensitize, meaning they can learn to ignore things that do not actually pose a threat. As with dogs, horses rely on body language for communication.

References

[1] Erica Larson. The Equine Mind: Top 10 Things to Know. Accessed on Mar 12, 2019 at <https://thehorse.com/121040/the-equine-mind-top-10-things-to-know/>

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Horses. Credit: Mikal Ortega. Used here per CC BY-SA 2.0