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Equus caballushorse

By Christopher Clement

Geographic Range

Due to the domestication of Equus caballus, and its widespread distribution following domestication, the natural geographic range is typically considered the distribution in the Late Glacial period, 9,500-15,000 years ago. During this time, horses were widespread. Wild horse populations existed in the northern tip of Africa adjacent to the Tyrrhenian and Mediterranean Seas, across Europe excluding the most northern regions where the Scandinavian countries currently exist, and as far east in the Palearctic region as China and Mongolia. They extended throughout the northern Palearctic, across Beringia into the Yukon, and as far south in North America as Mexico, as far east as the Mississippi River, and as far west as the Pacific Ocean coastline. It is believed that the wild horses in North America went extinct 8,000 to 10,000 years ago. Domestic horses were then introduced into North America upon European colonization. (Bennett and Hoffmann, 1999)

Habitat

Horses are adaptable and occupy a wide variety of habitats under domestication. Preferred habitats are cool, temperate grasslands, steppes, and savannahs, but they also occupy semi-deserts, swamps, marshes, and woodlands. (Bennett and Hoffmann, 1999)

Physical Description

Horses are ungulates with an unguligrade foot posture, single-digit oval-shaped hooves, long tails, short hair, long slender legs, muscular and deep torso build, long thick necks, and large elongated heads. The mane is a region of coarse hairs, which extends along the dorsal side of the neck in both domestic and wild species. However, wild horses have a mane with short hairs that remain upright while domestic horses have longer mane hairs that do not remain upright. A mid-dorsal stripe, a dark stripe that extends from the mane to the tail, is sometimes present, especially in wild types. The teeth are specialized for grazing, with hypsodont cheekteeth. The dental formula is 3/3, 0-1/0-1, 3-4/3-4, and 3/3. Newborn horses have curly hairs and a finer mane than adults before molting and growing their winter coat. Winter coats start developing in September and October, are fully grown by December. They start shedding in the summer over a period of 56 days for adults or 75 days for newborns. Winter coats are typically thicker and the top of the tail usually develops a region of short hairs, forming a tuft. (Bennett and Hoffmann, 1999; Hansen, 1976)

Domestication of horses has led to wide variation in the characteristics of breeds of horses. Coats vary in color, from white to black and including reds, browns, and yellows, as well as a wide variety of patterns, such as spots and pinto patterns. Size can vary depending on the breed and its intended use, but can range from 227 to 900 kg in mass and 0.9 to 1.7 meters in height, with potential outliers. Leg length, head length, body-build, fur color, metabolism, etc, varies across breeds of domestic horses. Despite the great variation in physical features within horses, they can generally be distinguished from other equids, by having longer manes and tails and generally a lack of zebra-type stripes. (Bennett and Hoffmann, 1999; Ensminger, 1969)

  • Sexual Dimorphism
  • sexes alike
  • Range mass
    227 to 900 kg
    500.00 to 1982.38 lb
  • Range length
    220 to 280 cm
    86.61 to 110.24 in
  • Average basal metabolic rate
    0.11 cm3.O2/g/hr

Reproduction

The horse mating system is similar to that of other equids, which is a polygynous mating system. Males typically herd females during the mating season and defend them against other males, who may be trying to mate with the females. Males do not typically herd and mate with any of his daughters or any females that grew up with him in the same herd. Males fight with other males through kicking and biting, losers submit and retreat. This process is very energy intensive and similar to the rut seen in deer species. Hierarchy in herds is influences by reproductive status and age, with females and their newborn young getting first access to resources, followed by juveniles and non-reproductive females. Alpha males dominate access to resources. (Bennett and Hoffmann, 1999; Linklater, et al., 1999)

Males can reproduce year-round but are most active from April to June. Females are polyestrous during the breeding season and go into an estrous period on average every 21 days, each lasting an average of 6.5 days. Females have spontaneous ovulation one to two days before the estrous period is over, which is when they are most fertile. In autumn, females no longer go through estrous periods until nearing the next breeding season. Average gestation period is 335 days, but ranges from 287 to 419 days. This broad gestation range means that birth can be in either spring or autumn of the next year. If the foal is not born by the time in which the mare would go into estrous during the breeding season, then she cannot participate that year. Usually only one foal is produced annually, and it is rare to see twin foals. In domestic horses, only 14% of twin foals survive two weeks, and either one or both twins are born underdeveloped. (Bennett and Hoffmann, 1999; Ensminger, 1969)

Births occur at night and in a quiet location. Wild horse foals are usually born at a mass of 25 to 30 kg, but the average birth weight of domesticated foals is 40 kg. Foals are born precocial and well-developed, usually being able to stand within an hour of birth and walk within four to five hours to follow their mother. When the mother returns to the herd, she goes through postpartum estrus for usually seven to nine days, which is significantly less than regular estrus periods. In domesticated horses who do not have a herd, postpartum estrus usually happens five to twelve days after giving birth. Foals are able to eat solid food within a week of being born. For their first month, the young stay close to their mother and nurse for brief and frequent periods before their second month, when they start to forage on their own. When they start to forage independently, they begin the weaning process which could take up to two years for wild foals. In domesticated horses, foals are often weaned between four and six months old. (Bennett and Hoffmann, 1999; Ensminger, 1969)

Foal weight doubles every week for four weeks, then gradually declines as they reach maturity. It takes females four to five years, and males six to seven years, to reach full reproductive maturity. Although females first go into estrus at either 11 or 12 months, they are not usually fertile until their second year. Even then, females have difficulty developing the fetus of a foal and giving birth, without severe health complications to herself or the foal, until they are four years of age. (Bennett and Hoffmann, 1999; Ensminger, 1969)

  • Breeding interval
    Horses may breed up to once each year.
  • Breeding season
    Horses generally breed from April to June.
  • Range number of offspring
    1 to 2
  • Average number of offspring
    1
    AnAge
  • Range gestation period
    287 to 419 days
  • Average gestation period
    335 days
  • Range weaning age
    24 (high) months
  • Range time to independence
    2 to 3 years
  • Range age at sexual or reproductive maturity (female)
    11 to 48 months
  • Average age at sexual or reproductive maturity (female)
    36 months
  • Average age at sexual or reproductive maturity (male)
    6 years
  • Average age at sexual or reproductive maturity (male)
    Sex: male
    973 days
    AnAge

Foals are precocial at birth and able to walk on their own shortly after birth, but still need parental assistance. Although they are precocial, they rely heavily on their mother for protection against predators and food until they begin to forage on their own. Being recently born, foals get access to food resources second to the alpha male of the herd. As they mature into juveniles, they eventually get access to food after recently born foals, but still before single and non-reproductive females. Although young primarily learn by observation of their mother in the first couple months, the herd also provides a learning environment and protection against predators. Sometimes, females will nip their young for discipline as well. Herd hierarchy is also taught through kicking, biting, vocalization, and facial gestures between individuals for communication. Although young are precocial and grow relatively quickly, they do rely on the protection, group care, and learning experiences offered by the herd on their mother for development in wild horses. Studies show that feral horses are no longer dependent on their natal herd and tend to disperse to other herds in their 2nd or 3rd year of age. Gender is not a factor in the age at which hoses disperse from their natal herd. Domesticated foals are usually separated from their mother either during or immediately after the weaning process is complete. (Bennett and Hoffmann, 1999; Cameron, et al., 2003; Ensminger, 1969)

  • Parental Investment
  • precocial
  • female parental care
  • pre-fertilization
    • provisioning
    • protecting
      • male
  • pre-hatching/birth
    • provisioning
      • female
    • protecting
      • female
  • pre-weaning/fledging
    • provisioning
      • female
    • protecting
      • female
  • pre-independence
    • provisioning
      • female
    • protecting
      • female
  • maternal position in the dominance hierarchy affects status of young

Lifespan/Longevity

Horse lifespan is dependent on several factors, including breed variations and environment. Typically, domestic horses have a lifespan of 25 to 30 years, although a maximum of 61 years has been attained. The longest living horse in wild as of 1974 was 36 years. Factors on lifespan of E. caballus include: nutrition, activity, number of reproduction cycles, reproductive status, disease, dental health, and physical activity. (Bennett and Hoffmann, 1999; Ensminger, 1969)

Behavior

Horses are social mammals. In wild or feral populations they form herds with a social hierarchy. These herds, also known as bands, can have up to 26 mares, 5 stallions, and various ages of pre-dispersed young. Typical herd sizes will have only 5 to 11 mares and 1 to 4 stallions, in addition to their offspring, but these numbers vary among scholarly articles and may vary with food availability, environmental conditions, and other factors. This hierarchy is comprised of the alpha male being dominant over the herd, males dominant over females, females with young dominant over non-reproductive females, and adults being dominant over juveniles. The dominant or alpha male is significantly more active than other members of the herd. Studies show that, in feral horses, where regular members of the herd usually spend less than 10% of their time moving around, dominant males move from 25 to 45% of the time and sleep only 5 to 6% of their time, when the other herd members usually spend 20 to 27% of the time throughout the day sleeping. Alpha males also expend large amounts of energy rounding up females and defending them against other males during the breeding season. Alpha males usually move in the back of the herd, but will move to the front in the presence of a threat and signal to the herd in the event that the herd needs to flee. The alpha may also attack a threat if necessary to defend the herd. (Bennett and Hoffmann, 1999; Ensminger, 1969; Linklater, et al., 1999)

Horses are crepuscular and have an interrupted sleeping pattern that compliments this. During the summer, horses graze in morning or evenings to avoid mid-day high temperatures, and can be seen visiting water sources at night as well. To adapt to this feeding pattern, they sleep in segments throughout the day, which are usually not more than two hours long. They also avoid laying down for more than an hour at a time if possible, and can be seen sleeping while standing up. (Bennett and Hoffmann, 1999)

  • Range territory size
    0.59 to 17.68 km^2

Home Range

Wild horses generally stay close to water sources but, in transition between spring and summer when food is most abundant, horses can travel far from their water access points. One study in a feral population in New Zealand showed that home range sizes were between 0.96 to 17.68 square kilometers, with a density of 0.48 to 3.22 individuals per square kilometer. Herd size and home range are positively correlated, with a larger herd occupying a larger home range. Some studies on wild or feral horses show that there are seasonal movements and variation in the home range. These seasonal movements are typically associated with regular seasonal precipitation changes, food availability, temperatures, and topography. In winter, populations move away from lower elevations that were exposed to unusually cooler temperatures due to frost inversion layers, but would return prior to foaling or the mating season. Winter home ranges in feral populations are larger than summer home ranges on average, but these ranges vary significantly. True wild horse population are restricted to various reserves and reintroduction sites for their protection. (Bennett and Hoffmann, 1999; Boyd and King, 2014; Linklater, et al., 1999; Linklater, et al., 2000)

Communication and Perception

In horses, the nostrils, muzzle, whiskers, and cheeks all have whiskers that are used to perceive the environment through touch. Vision is the primary means of perceiving the environment in horses. Ears are long, slender, and upright, which aid in auditory perception. Although their sense of smell is important, it is not the chief means of perception and provides a smaller role than vision or the sensitive receptors on the nostrils, muzzle, whiskers, or cheeks. (Bennett and Hoffmann, 1999)

The primary means of communicating with other horses is through vocalizations and gestures, particularly facial gestures. During the breeding season, males may grunt or scream at one another, in addition to stomping and pawing at the ground in an aggressive manner. Males may neigh to females in estrous and females may squeal and kick if they are refusing a rutting male. Grunting and aggressive gestures, including biting, shoving, and kicking, may occur among herd members to establish or reinforce the hierarchy structure and express dominance. Submissive individuals will let the winner lay their head on the loser’s back-end, which is seen in equids. (Bennett and Hoffmann, 1999)

In addition to body gestures, horses have an array of facial gestures. Positive reactions include raising of the lips to expose upper teeth, similar to a smile, and head bobbing or pointing the ears forward and erect. Aggressive facial gestures include the ears being laid back and the nostrils closed while exposing the same teeth. (Bennett and Hoffmann, 1999)

Food Habits

Primarily folivores by nature, horses primarily graze on true grasses. Domestic horses are often fed various amounts of grains, including oats, barley, corn, wheat, flax, soybeans, alfalfa, clover, Timothy hay, and Johnson grass. In addition to this diet, they may also be provided vitamin and mineral supplements. Feral horses eat a similar diet, including Russian thistle, dropseed grasses, mesquite, prairie junegrass, green sprangletop, and saltbush, and likely varies depending on location and season. (Bennett and Hoffmann, 1999; Ensminger, 1969; Hansen, 1976)

  • Plant Foods
  • leaves
  • wood, bark, or stems
  • seeds, grains, and nuts

Predation

Natural predators of wild horses are likely to have been mainly wolves, coyotes, and mountains lions. Predators mainly prey on elderly, ill, or young animals. When a herd is threatened by a predator, the alpha male may attack it through biting and kicking with the hooves. Females protect their young in a similar manner. Humans are also predators of horses, both historically and currently. Wild populations were probably hunted by humans and horse meat is consumed in some places. (Bennett and Hoffmann, 1999)

Ecosystem Roles

The domestication of horses contributed to the development of agricultural societies and changed the mobility and political relationships between different human populations. As grazing animals, horses influence diversity and structure of the ecosystems in which they live. In some places horses have been important in seed dispersal of certain trees. There are over 150 species of parasites documented in horses. (Bennett and Hoffmann, 1999; Hardin, 1997)

Mutualist Species

Economic Importance for Humans: Positive

Horses are economically very important to humans now and historically. They have been used as a food source, have been critical in the transportation of people and goods, have played important roles in military campaigns, in sport and recreation, and in agricultural development. Horses are also beloved companion animals and widely used in therapeutic approaches. In agriculture, horses are used to pull plows and carriages and their manure is an important fertilizer. Horse hairs are used in various products. (Ensminger, 1969)

  • Positive Impacts
  • pet trade
  • food
  • body parts are source of valuable material
  • research and education
  • produces fertilizer

Economic Importance for Humans: Negative

Feral horse populations may be disruptive to ecosystems that are not adapted to the presence of large equid grazers. They can compete with other grazing animals for resources and cause excessive damage to native vegetation. Studies show horses successfully outcompete other grazing species. Wild, feral, and domestic horses can also transmit zoonotic diseases, which can cause negative impacts to local animal species, agriculture, and people. ("Pest animal risk assessment: Feral horse (Equus caballus)", 2009; Bennett and Hoffmann, 1999)

Conservation Status

Domesticated horses are abundant in many areas around the world. Their closest relatives, Przewalski's wild horses were listed as endangered on the IUCN Red List, endangered under the U.S. Endangered Species Act, and on Appendix I of CITES. Some older studies state that domestication and release of domestic horses led to this decline in Przewalski's wild horse populations and they were thought to no longer exist in the wild. Current conservation efforts in Przewalski's wild horses include legal protection of the species in Mongolia as well as reintroduction efforts in several areas. (Bennett and Hoffmann, 1999; Boyd and King, 2014)

Other Comments

It is important to note that there have been some taxonomic changes that may affect the validity of information in the older literature on horses. Many older literature sources consider Przewalski’s wild horses as the wild subspecies of horses, and refer to it as E. c. przewalskii; however, recent sources and the IUCN Red List consider Przewalski's wild horses as Equus ferus przewalskii and suggests that E. f. przewalskii is not the ancestor of E. caballus due to a difference in chromosome numbers. It is thought that they descended from different lineages, but this taxonomic debate is ongoing. It is difficult to make this clarification due to the near extinction of the Przewalski's wild horses, producing a bottleneck in genetic diversity. The current population of Przewalski's wild horses is descended from 13 captive individuals, one being a domestic horse and one being a hybrid between a domestic horse and a Przewalski's wild horse. In the meantime, much older literature does not distinguish between wild E. caballus and Przewalski's wild horses. Much information available might actually be for both subspecies. (Bennett and Hoffmann, 1999; Boyd and King, 2014; Wallner, et al., 2003)

Contributors

Christopher Clement (author), University of Alaska Fairbanks, Laura Prugh (editor), University of Washington, Tanya Dewey (editor), University of Michigan-Ann Arbor.

Glossary

Australian

Living in Australia, New Zealand, Tasmania, New Guinea and associated islands.

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Ethiopian

living in sub-Saharan Africa (south of 30 degrees north) and Madagascar.

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Nearctic

living in the Nearctic biogeographic province, the northern part of the New World. This includes Greenland, the Canadian Arctic islands, and all of the North American as far south as the highlands of central Mexico.

World Map

Neotropical

living in the southern part of the New World. In other words, Central and South America.

World Map

Palearctic

living in the northern part of the Old World. In otherwords, Europe and Asia and northern Africa.

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acoustic

uses sound to communicate

agricultural

living in landscapes dominated by human agriculture.

bilateral symmetry

having body symmetry such that the animal can be divided in one plane into two mirror-image halves. Animals with bilateral symmetry have dorsal and ventral sides, as well as anterior and posterior ends. Synapomorphy of the Bilateria.

causes or carries domestic animal disease

either directly causes, or indirectly transmits, a disease to a domestic animal

chemical

uses smells or other chemicals to communicate

cosmopolitan

having a worldwide distribution. Found on all continents (except maybe Antarctica) and in all biogeographic provinces; or in all the major oceans (Atlantic, Indian, and Pacific.

crepuscular

active at dawn and dusk

dominance hierarchies

ranking system or pecking order among members of a long-term social group, where dominance status affects access to resources or mates

endothermic

animals that use metabolically generated heat to regulate body temperature independently of ambient temperature. Endothermy is a synapomorphy of the Mammalia, although it may have arisen in a (now extinct) synapsid ancestor; the fossil record does not distinguish these possibilities. Convergent in birds.

female parental care

parental care is carried out by females

folivore

an animal that mainly eats leaves.

food

A substance that provides both nutrients and energy to a living thing.

forest

forest biomes are dominated by trees, otherwise forest biomes can vary widely in amount of precipitation and seasonality.

herbivore

An animal that eats mainly plants or parts of plants.

holarctic

a distribution that more or less circles the Arctic, so occurring in both the Nearctic and Palearctic biogeographic regions.

World Map

Found in northern North America and northern Europe or Asia.

introduced

referring to animal species that have been transported to and established populations in regions outside of their natural range, usually through human action.

iteroparous

offspring are produced in more than one group (litters, clutches, etc.) and across multiple seasons (or other periods hospitable to reproduction). Iteroparous animals must, by definition, survive over multiple seasons (or periodic condition changes).

marsh

marshes are wetland areas often dominated by grasses and reeds.

motile

having the capacity to move from one place to another.

native range

the area in which the animal is naturally found, the region in which it is endemic.

nomadic

generally wanders from place to place, usually within a well-defined range.

oceanic islands

islands that are not part of continental shelf areas, they are not, and have never been, connected to a continental land mass, most typically these are volcanic islands.

oriental

found in the oriental region of the world. In other words, India and southeast Asia.

World Map

pet trade

the business of buying and selling animals for people to keep in their homes as pets.

polar

the regions of the earth that surround the north and south poles, from the north pole to 60 degrees north and from the south pole to 60 degrees south.

polygynous

having more than one female as a mate at one time

riparian

Referring to something living or located adjacent to a waterbody (usually, but not always, a river or stream).

seasonal breeding

breeding is confined to a particular season

sexual

reproduction that includes combining the genetic contribution of two individuals, a male and a female

social

associates with others of its species; forms social groups.

swamp

a wetland area that may be permanently or intermittently covered in water, often dominated by woody vegetation.

tactile

uses touch to communicate

temperate

that region of the Earth between 23.5 degrees North and 60 degrees North (between the Tropic of Cancer and the Arctic Circle) and between 23.5 degrees South and 60 degrees South (between the Tropic of Capricorn and the Antarctic Circle).

terrestrial

Living on the ground.

tropical savanna and grassland

A terrestrial biome. Savannas are grasslands with scattered individual trees that do not form a closed canopy. Extensive savannas are found in parts of subtropical and tropical Africa and South America, and in Australia.

savanna

A grassland with scattered trees or scattered clumps of trees, a type of community intermediate between grassland and forest. See also Tropical savanna and grassland biome.

temperate grassland

A terrestrial biome found in temperate latitudes (>23.5° N or S latitude). Vegetation is made up mostly of grasses, the height and species diversity of which depend largely on the amount of moisture available. Fire and grazing are important in the long-term maintenance of grasslands.

tundra

A terrestrial biome with low, shrubby or mat-like vegetation found at extremely high latitudes or elevations, near the limit of plant growth. Soils usually subject to permafrost. Plant diversity is typically low and the growing season is short.

visual

uses sight to communicate

viviparous

reproduction in which fertilization and development take place within the female body and the developing embryo derives nourishment from the female.

young precocial

young are relatively well-developed when born

References

Biosecurity Queensland. Pest animal risk assessment: Feral horse (Equus caballus). PR09-4511. Brisbane: The State of Queensland, Department of Employment, Economic Development and Innovation. 2009.

Bennett, D., R. Hoffmann. 1999. Equus caballus Linnaeus, 1758. Mammalian Species, No. 628: 1-14.

Boyd, L., S. King. 2014. "Equus ferus ssp. przewalskii" (On-line). The IUCN Red List of Threatened Species. Accessed November 02, 2014 at www.iucnredlist.org/details/7961/0.

Cameron, E., W. Linklater, K. Stafford, E. Minot. 2003. Social grouping and maternal behaviour in feral horses (Equus caballus): the influence of males on maternal protectiveness. Behavioral Ecology and Sociobiology, 53(2): 92-101.

Ensminger, M. 1969. Horses and Horsemanship. Danville, Illinois: The Interstate Printers & Publishers, Inc.

Hansen, R. 1976. Foods of Free-Roaming Horses in Southern New Mexico. Journal of Range Management, 29(4): 347.

Hardin, D. 1997. "Controlling Internal Parasites of Horses" (On-line pdf). Accessed December 09, 2014 at extension.missouri.edu/p/g2854.

Linklater, W., E. Cameron, E. Minot, K. Stafford. 1999. Stallion harassement and the mating system of horses. Animal Behaviour, 58(2): 295-306.

Linklater, W., E. Cameron, K. Stafford, C. Veltman. 2000. Social and spatial structure and range use by Kaimanawa wild horses (Equus caballus: Equidae). New Zealand Journal of Ecology, 24(2): 139-152.

Wallner, B., G. Brem, M. Müller, R. Achmann. 2003. Fixed nucleotide differences on the Y chromosome indicate clear divergence between Equus przewalskii and Equus caballus. Animal Genetics, 34: 453-456.

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