Equus (genus) - Characteristics

Characteristics

Equines are medium to large mammals, with long heads and necks with a mane. Their legs are slender and end in a single, unguligrade toe, protected by a horny hoof. They have long, slender tails, either ending in a tuft, or entirely covered in flowing hair. They are adapted to generally open terrain, from plains and savannas, to mountains or deserts.

The pinnae (outer ears) of equines are mobile, enabling them to easily localise the origin of sounds. They have two-color, or dichromatic vision. Their eyes are set back far on the head, giving them a wide angle of view, without entirely losing binocular vision. Equines also have a vomeronasal organ, that allows males to use the flehmen, or 'lip-curling' response to assess the sexual state of potential mates. Equines are one of only two mammals (the other is the human) capable of producing copious sweat perspiration for thermoregulatory cooling, enabling fast running over long distances.

Equines are herbivores, and feed predominantly on tough, fibrous food, such as grasses and sedges. When in need, they will also eat other vegetable matter, such as leaves, fruits, or bark, but are normally grazers, not browsers. Unlike ruminants, with their complex stomachs, equines break down cellulose in the "hindgut" or caecum, a part of the colon. Their dentition is almost complete, with cutting incisors to crop food, and grinding molars set well back behind a diastema. The dental formula for equines is:

Equines are social animals, living in herds or bands. Horses, along with Plains and Mountain Zebras, have permanent herds generally consisting of a single male and a band of females, with the remaining males forming small "bachelor" herds. The remaining species have temporary herds, lasting only a few months, which may be either single-sexed or mixed. In either case, there are clear hierarchies established amongst the individuals, usually with a dominant female controlling access to food and water resources and the lead male controlling mating opportunities.

Females, usually called mares in horses and zebras, or, in the case of asses and donkeys, jennies, usually bear a single foal, after a gestation period of approximately 11 months. Young equines are able to walk within an hour of birth, and are weaned after four to thirteen months (animals living in the wild naturally wean foals at a later date than those under domestication). Depending on species, living conditions and other factors, females in the wild may give birth every year or every other year.

Equines which are not in foal generally have a seasonal estrous cycle, from early spring into autumn. Most females enter an anestrus period during the winter and thus do not cycle in this period. The reproductive cycle is controlled by the photoperiod (length of the day), with estrus triggered when the days begin to lengthen. Anestrus prevents the female from conceiving in the winter months, as that would result in her foaling during the harshest part of the year, a time when it would be more difficult for the foal to survive. However, equines who live near the equator, where there is less change in length of day from season to season, have no anestrus period, at least in theory. Further, for reasons that are not clear, about twenty percent of domestic mares in the Northern Hemisphere will cycle the year round.