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The horse (equus ferus caballus) is an impressive animal which was domesticated more than 4000 years ago and has since migrated and been bred all over the word (National Geographic). It is an animal loved for its majestic looks (see Figure 1) and the ability to be easily trained for the various purposes of mankind due to its intelligence and docility when compared with other animals. Up until the industrial era, the horse was a widely accepted method of land transportation; with great mobility, speed, and reliability. The different roles horses have played in war, such as war mounts, chargers, and gun draughts in the armies of renowned emperors like Alexander (see Figure 2), Napoleon and Genghis Khan, as well as the role they played in World War I (see Figure 3), were crucial to the victories and losses of both sides (Trueman, 2015).       
The Species
The horse belongs to a genus of land mammals referred to as Equus; that includes the modern horse, zebra, and ass, as well as more than 60 species known only from fossils. There are six modern members of this genus, only the races of E. caballus are called horses; three species (E. zebra, E. burchelli, and E. grevyi) are called zebras; and two (E. asinus and E. hemionus) are usually called wild asses (The Editors of Encyclopaedia Britannica, 2017). There is only one species of domestic horse, but around 400 different breeds that specialize in everything from pulling wagons to racing. Horses found in the wild generally gather in groups of 3 to 20 where a stallion (mature male) will lead the group. Groups usually consist of mares (females) and young foals (see Figure 4). When young males become colts (around  two years of age) the stallion drives them away. The colts then roam with other young males until they can gather their own band of females (National Geographic). 

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Horses are known for their varying size and strengths, depending on the specific breed. The standard unit of measurement for the height of horse is known as Hands, one hand being equivalent to four inches or 10 centimetres (see Figure 5). If the height of the animal is above 14.2 hands, it is known as a horse and animals below this height are known as ponies (see Figure 6). However, there are a few breeds that measure below 14.2 hands and are still refereed to as horses instead of ponies (UK Essays). Over the course of their evolution, horses remained relatively small for more than half their history and were much smaller than the modern horses seen today (Shanley, n.d).  

A horse diet mainly consists of grass and shrubbery as they prefer to eat small amounts of food steadily throughout the day. Similar to humans, horses require six main classes of nutrients to survive: water, fats, carbohydrates, protein, vitamins,and minerals. Horses are non-ruminant herbivores and their stomachs are relatively small when compared to their size; an average 1000 lb. horse’s stomach only has a capacity of 2 to 4 gallons. Horses have evolved as grazers so that they can spend up to 16 hours a day grazing pasture grasses. Their stomachs “serve to secrete hydrochloric acid and pepsin to begin the breakdown of food once it enters the stomach” (Williams, 2004). Because horses are unable to regurgitate food, if they overeat or eat something poisonous it is usually fatal since vomiting is not possible.
Life Cycle
From birth to death a horses’s life span is approximately one-third the length of the average human life span. Unlike humans, horses are born after a gestation period of 11 months, and for the next year are referred to as foals. In the first year, horses grow rapidly and will reach 90 percent of their adult height and 80 percent of their adult weight (see Figure 7). By the age of two most horses can be ridden as their growth plates — epiphyses — located in the bones of the legs have closed (Puma, 2017). At the age of four a horse is considered to have reached adulthood and the females are now referred to as mares while the males are referred to as stallions if they can still breed or geldings if they’ve been castrated. By their late teens or early  20s, horses begin to show the signs of aging. Their backs begin to sag, and many develop age-related disorders such as kidney and liver disease. In the wild, these conditions contribute to rapid deterioration and death, but with proper  care, horses can live into their mid 30s (Puma, 2017). 

The evolutionary lineage of the horse is among the best-documented in all palaeontology. Horse species were constantly evolving and branching off the “evolutionary tree” along various unrelated routes over the course of history. Because of this, there is no discernible straight line of horse evolution. Sometimes new species would suddenly split off from their ancestors, other times there were long periods of static where it seems as if no specific horse species was evolving, and occasionally there were enormous bursts of evolution and change, especially when new ecological opportunities arose (Hunt, 1995). The history of the horse family, Equidae (see Figure 8), lasted from about 55.8 million to 33.9 million years. During which the first ancestral horse, a hoofed, browsing mammal designated correctly as Hyracotherium but more commonly called the “dawn horse” (The Editors of Encyclopaedia Britannica, 2017). Many of the evolutionary traits such as, increase in size,  reduction in the number of hooves, loss of footpads, lengthening of legs, fusion of the independent bones of the lower legs, elongation of the muzzle, increase in size and complexity of the brain and development of crested, high crown teeth, have been theorized to have mainly occurred in North America (The Editors of Encyclopaedia Britannica, 2017). Many of the changes that occurred over this period of time, most notably the evolution of grazing dentition, appear abruptly in the fossil records, rather than as the culmination of numerous gradual changes (Cothran & Podhajsky, 2017) . 

Ecological Role
Horses play a big ecological role in environment all over North America. As they share most of their pastures and land environments with animals such as cows, horses have shown to be more ecologically friendly. They have both upper and lower incisors and graze by “clipping the grass” (similar to a lawn mower) allowing the grass to easily grow back, instead  of pulling it up from the roots like cows (see Figure 9). Horses have also proven useful to other species in winter months as they have an instinct to break through deep crusted snow where the grass cannot be seen. They also open up frozen springs and ponds with their powerful hooves, making it possible for smaller animals to drink (American Wild Horse Campaign). Another positive effect of wild horses on biodiversity was  documented in the case of the Coyote Canyon horses in the Anza Borrega National Park. After wild horses were removed from the Park to increase  big horn sheep population, big horn sheep mortality actuality increased drastically: mountain lions (see Figure 10), who were wild horse predators, compensated the loss of one of their prey species by increasing their predation on other species (American Wild Horse Campaign).