Anatomy Development
Eohippus was the ancestor of both domestic and wild horse species.Aleaf-eatingmammalas small as a fox, Eohippus lived during the Eocene epoch and had several toes on its feet. Descendants gradually grew larger, lost toes, and became grass grazers. Przewalski's horse, indigenous to Mongolia but not seen in the wild since 1968, links ancient and modern horses. Because it has sixtysix chromosomes and domestic horses have sixtyfour, Przewalski's horse is a separate wild horse species and not an ancestor of modern horses. A hybrid produced by a horse and a Przewalski has sixty-five chromosomes and is fertile. Modern horses and zebras are distinguished by a single hoof on each foot. Vestigial remains of prehistoric toes are located above hooves. Horses and zebras have long skulls and jaws that hold approximately forty to forty-four permanent teeth, including incisors to bite grass and molars to chew roughage. The teeth have long crowns that slope with age and can be examined to determine how old an animal is. Horses use their teeth to eat, groom, and fight. The horse's digestive system is bigger and more efficient than that of carnivores, and has approximately 40 meters (131 feet) of intestines with a meter-long cecum attached to the colon. Horses and zebras live on grasslands ranging between sea level and mountains and eat mostly fibrous food such as hay, grain, and oats. Food ferments in the cecum, which can hold as much as thirty-eight liters (ten gallons). Horses eat an average of sixteen hours per day to sustain their systems. The average horse's heart weighs about 3 kilograms (6.6 pounds), and the surface area of a horse's lungs measures approximately 2,500 square meters (8,200 square feet). Horses' weight varies from minimums of nearly 500 kilograms (1,000 pounds) to maximums of more than 908 kilograms (1 ton). Fifty-one vertebrae are in horses' spines from the top of the skull to the base of the tail. Horses' eyes are placed on the sides of their heads, enabling them to see the horizon without moving their head. Their eyesight is better than that of dogs. Horses depend on hearing more than sight, but rely most on the sense of smell. The flehmen response is when the horse raises its upper lip to indicate it has smelled something interesting.
Social Behavior
Depending on the age, female horses are called fillies and mares, and males are colts, stallions, and geldings. Horses attain sexual maturity at two to three years old and usually produce a single foal after eleven months of gestation. Multiple births are rare, with twins occurring on average once per 1,500 births. Foals are able to stand within an hour of birth. Stallions fight while competing for mares, and one stallion may mate with several mares during a season. Mares usually come into heat one week after giving birth. Foals are weaned before they are one year old. Horses are herd animals and rely on this social relationship, as well as on speed and endurance, as protection from predators. Domesticated horses enjoy the companionship of horses and other animals.Wild horses form a family group of one stallion, several mares, and their offspring. Young stallions sometimes form a bachelor group led by an older stallion. These groups forage in an area where they can find food, water, and shelter, and a pecking order maintains a hierarchy of rank within the group. Several groups often share the same range. Horses communicate by whinnying and nickering. Vulnerable to wild animals, horses can run an average speed of forty-five to sixty kilometers (twenty-eight to thirty-seven miles) per hour. Their legs are slender and angled forward so that their weight is carried forward to enhance quick motion. Horses' shoulders absorb shock, while their powerful hindquarters provide impulsion. Horses naturally move at the gaits of walk (four beat), trot and pace (two beat), canter (three beat), and gallop (an extended canter), with several other natural gaits including pacing and the running walk.
Appearance
Domesticated horses are described by three types: heavy or draft, light, and pony. Humans genetically developed specific breeds for different tasks and degrees of hardiness, stamina, and versatility. Most horse breeds are derivatives of Arabians and thoroughbreds. Draft horses (often referred to as cold-blooded, which is not physiologically accurate), usually stand over sixteen hands high at the withers (the ridge between the shoulder blades) and have sturdy, thick dimensions, useful for pulling loads. Light horses, sometimes called hot bloods, are at least 14.2 hands high and tend to be streamlined. Ponies are shorter than 14.2 hands and vary in conformation according to breed.Welsh ponies tend to be more delicate, while Shetland ponies are more rotund. Miniature horses are extremely small ponies that are often less than one meter tall. Coat colors range from shades of brown and red to solid black and white, with some horses, such as Appaloosas and pintos, having spots and others being gray or roan. Spotted patterns are linked to genes that also often produce mottled noses, eyelids, and genitalia, blue eyes, and striped hooves. Lipizzans are born black and turn completely white by age two years. Some horses have dorsal stripes and zebra stripes on their legs. Various white facial and leg markings also distinguish horses.
African Cousins
Zebras belong to the horse family and are represented by three species, the plains, Grevy's, and mountain zebras. Several subspecies exist according to geographic range, herd behavioral differences, and physical variations such as dewlaps. Zebras are smaller than horses and have erect manes, longer ears, and thinner tails. The average zebra stands 140 centimeters (55 inches) high and weighs 300 kilograms (660 pounds). These striped animals exist in herds that graze in southeastern Africa's grasslands and also live in nearby deserts. Each zebra species is determined by a specific stripe pattern of black, brown, and white markings. Individual zebras have unique striping somewhat like the uniqueness of human fingerprints. Scientists have determined that zebras, fromthemomentof birth, are drawn to striped objects, a type of imprinting which has led to speculation that stripes might be a factor in herd cohesiveness and sociability. Previous hypotheses that stripes are for camouflage and to confuse predators have been discredited. The exact purpose of stripes remains a scientific mystery. Researchers are aware that abnormally striped zebras are often shunned by herds, which threatens their survival. Herds can include several zebras, often a male and females with their offspring or a group of young males. Herds can expand to hundreds of zebras. Typically passive animals, male zebras occasionally fight for females during breeding season by kicking, biting, and shoving each other. Female zebras attain sexual maturity at age three and usually reproduce annually throughout their lives, while males reach breeding age at five years. Gestation lasts approximately eleven to thirteen months, and twins are rarely foaled. The life span of wild zebras can extend to twenty-two years, and zebras kept in captivity can live longer. Zebra foals weigh approximately thirty-two to thirty-six kilograms (seventy to eighty pounds) at birth. Able to stand soon after being born, zebra foals eat grass which adds about 0.45 kilograms (1 pound) of body weight per day until they reach physical maturity. Zebra foals develop more quickly than horse foals and become independent sooner.
Defense Mechanisms
The herd protects zebras from predators, primarily large cats, such as lions and cheetahs, in addition to hyenas. Humans also hunt zebras for their hides. While other zebras sleep, a guard zebra watches for potential hazards and is aided by its keen night vision, comparable to an owl's, and tall ears which can rotate to pick up sounds. Zebras' primary defense mechanism is to run away from danger, and they can reach speeds as high as sixtyfive kilometers (forty miles) per hour, which is much slower than most of their enemies. Zebras also are at risk from reduced water resources and grasslands due to ranching and farming and compete with livestock for basic nutritional needs. While the common zebra remains abundant, the Grevy's zebra and the mountain zebra are considered endangered species and another type of zebra, the quagga, became extinct in 1883.Acentury later, the Quagga Breeding Project attempted to revive the quagga because genetic researchers determined that quagga and plains zebra deoxyribonucleic acid (DNA) was similar, and hypothesized that the quagga was not a separate species but rather a variation of the plains zebra. Zebroids are horse-zebra hybrids that are sterile.
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