Ovis aires - Domestic Sheep

Description:

The domestic sheep (Ovis aires) is a very common mammal that most people can associate with due to the widespread occurrence of the species and the fact that it is domesticated and in constant contact with humans.  Sheep are considered ungulates, a group characterized by the presence of hooves, which are specialized variations of claws composed of a large keratin mass.  Sheep are further classified taxonomically into the order Artiodactyla (meaning “even-digited ones”) because they have an even number of toes per foot (Martin, et al., 2001).  Another taxonomically defining characteristic is true horns, which occur only in the family Bovidae (buffaloes, sheep, goats cattle, etc…) (Feldhamer et al., 2004).  True horns do not branch and are permanently fixed on the animal, unlike antlers that are lost annually.  In some species, loss of the horn sheath or layers may occur over time, but the overall form and structural integrity of the horns is kept in tact (O’Gara and Matson, 1975).  Each horn is formed by an extension of the frontal plate to provide a bony central core for the outer layer of true horn, made of keratinized epidermal tissue.  The horns will continue to grow and develop from the base (Martin, et al., 2001).

Generally speaking, the single most concrete method of species characterization is via skull morphology.  Examination of the sheep skull shows distinguishing features of Bovids relative to Cervids and other artiodactylids.  When the skull of a cervid and the skull of a bovid are compared side by side, it is more apparent that the lacrimal bone contacts nasal bone (Kurta, 1995).

Using only the skull features, the skull of the domestic sheep (Ovis aires) can be further distinguished from the skull of a domestic goat (Capra hircus.).  Even though the differences between the two are subtle, directly comparing the skull of the domestic sheep (Ovis aires) with the skull of a domestic goat (Capra hircus.) may simplify the process (Kurta, 1995).  Additionally, skulls of domesticated sheep diverge from the skulls of wild sheep by exhibiting reduced eye sockets and brain case.

Sheep belong to the genus Ovis, which is characterized by glands that give sheep their distinct odor.  The glands are located in several places, such as, a shallow depression of the lacrimal bone, part of the groin, and on the foot between the two main toes.   Sheep also have a narrow snout and vertical cleft that are both entirely covered by short hair except on the nostril and lip borders (Reavill, December 1999).

Sheep were one the first mammals domesticated as livestock, as early as 10,000 years ago (along with goats).  They provided humans with a stable supply of food, milk, wool, and other items (Feldhamer, et al., 2004).  Shortly after domestication, many new varieties were produced through selective breeding efforts to enhance certain traits or by genetic mixing with various subspecies of O. ammon. (Roger Williams Park Zoo and Rhode Island Zoological Society, 2005).  In Iran, an archeological site uncovered a woolen figurine of a sheep with wool, which dated back 6000 years and shows that wool was a valued resource and indicates selective breeding for wool production (Hyams, 1972).  Qualities associated with modern sheep were depicted in artwork and literature as early as 3000 B.C. (Oklahoma State University Board of Regents, 1996).

Numerous theories have been developed about the domestic sheep’s origin and often taxonomists, evolutionary biologists, etc… find difficulty agreeing on the systematics and origins of the domestic sheep species.  Most scientists agree that the wide variety of domestic sheep around the world originated from the mouflon of a specific region in the Middle East or from central and western Asia, the original range of the wild sheep (Ovis ammon) (Roger Williams Park Zoo and Rhode Island Zoological Society, 2005; Rice and Andrews, 1951).  Recent ev­idence from mtDNA analysis reinforces that domestic sheep (Ovis aires) most likely came from early mouflons (Ovis musimon) (Hiendleder et al. 2002).  Currently, two wild populations of mouflons still exist.  The Asiatic mouflon inhabits the mountains in Asia Minor and southern Iran, while the European mouflon is confined to two islands - Sardinia and Corsica (Zeuner, 1963).  Both species are closely related and are very similar in appearance, and the only variance is that the Asiatic mouflon coloration is slightly redder and there is a noticeable difference in the horn arrangements for the two species.  The striking similarities between the two species provide a foundation for other theories that suggest the European mouflon were actually created from domestic sheep released in Europe; consequently, all sheep would be descendants of the Asiatic mouflon (Oklahoma State University Board of Regents, 1996).  Wild sheep have anywhere from 52-58 chromosomes and probably evolved from a species with 60 chromosomes, while most domestic breeds of sheep have 54 chromosomes (Valdez, 1982).

Domestication of sheep was facilitated by many qualities found in the species.  The ability to adapt to changing environmental conditions, along with a passive social structure, and herding behavior, were key components in their rapid domestication (Hafez, 1968). The transition from the wild into a domestic environment occurred through either raising young wild animals from babies or by close human contact during resource shortages (Clutton-Brock 1999).  After only 2000 years, which is relatively short in terms of a species, domesticated stock exhibited several mor­phological adaptation including, reduced body size, shorter limbs, and loss or modification of horns.  However, the most significant change correlated to the domestication of the sheep was that the sheep no longer shed, unlike the wild species that lost it throughout the spring and summer.  This was vital for human survival, because domestic sheep could be sheared, and sufficient amounts of wool could be harvested in a practical manner and used for clothing. (Feldhamer, et al., 2004).

As a result of thousands of years of domestication and selective breeding, a wide range of sheep varieties were established, making a concrete species description difficult.  Adult sheep can weigh anywhere from 20-200g with the female sheep (ewes) being three-quarters to two-thirds the size of male sheep (rams) (Reavill, December 1999).

Distribution:

The domestic sheep is found almost everywhere that people live.  One obvious reason for this type of distribution is the fact that these are essentially not wild animals and can’t freely relocate; instead, they are limited to where they are taken.  Another important factor contributing to their wide abundance is the adjustability to changing environmental conditions; both as the ability tolerate short-term fluctuations and as the ability to accommodate long-term variation in their surroundings by genetic adaptation or evolution (Hafez, 1968).

Domestication of sheep is widespread and only a few areas of the world have not seen sheep.  The first sheep in the United States was brought to Virginia in 1609 and today sheep herds can be seen throughout the country (Hyams, 1972).  In 1987 the United States had 1.5 percent of the world’s sheep (Schraufnagel, et al., 1989).  According to the U.S. Department Agriculture statistics for 2001, the United States raised approximately 4.9 million sheep and lambs on the 65,120 sheep farms and ranches across the country.

The natural evolution of this species coupled with intensive selection for wool color, wool type, social characteristics, herding behavior, etc… over an extensive period of time and huge geographical region with a wide array of ecological regions helps explain why so many variations exist (Hogner, 1945).  Currently there exists over 200 established breeds, with several synthetic breeds from crosses of two or more established breeds; consequently, the species Ovis aires is comprised of over 280 breeds worldwide (Oklahoma State University Board of Regents, 1996).  Some estimates even claim that humans have created over 900 breeds (Valdez, 1982).  The various breeds can be divided up into 6 classes based on their agricultural and economic significance: the meat breeds, fine wool breeds, long wool breeds, dual-purpose breeds, hair and double-coated breeds and minor breeds (American Sheep Industry Association, 2003).  The American Sheep Industry Association explains, “U.S. sheep breeds provide a diverse range of performance for growth, carcass merit, reproduction, milk and wool characteristics. This genetic variability can be used to optimize production under varying climatic conditions” (2003).

A wide variety of information can be found about the recognized sheep breeds in the United States, much of which does not overlap.  To accurately represent the actual distributions of these breeds, information from the Domestic Animal Diversity Information System database was combined with information from the American Sheep Industry Association and Oklahoma State University, to give 83 breeds that include the following: Agnis, American Cormo, American Merino, American Miniature Brecknock, Ancon, Barbado, Barbados Blackbelly, Black Welsh Mountain, Blueface Leicester, Booroola Merino, Border Leicester, California Red, California Variegated Mutant , Cheviot , Clun Forest , Columbia , Columbia-Southdale, Coopworth , Cormo, Corriedale , Cotswold , Debouillet , Delaine-Merino , Dorper , Dorset , East Friesian , Finnsheep , Gulf Coast Native , Hampshire , Hawaiian Black Buck, Hog Island, Icelandic , Imperial, Jacob , Karakul , Katahdin , Lincoln , Louisiana Native, Minnesota 100, Minnesota 101, Minnesota 102, Minnesota 104, Minnesota 105, Minnesota 106, Minnesota 107, Monta Khia, Montadale , Morlam, Multinipple, Navajo-Churro , North County Cheviot , North Star Minnesota 103, No-Tail, Oxford , Panama, Panama , Perendale , Polled Rambouillet, Polypay , Rambouillet, Rideau Arcott, Romanov, Romeldale, Romney , Royal White Sheep , Santa Cruz, Scottish Blackface , Sennybridge Welsh Mountain , Shetland , Shropshire , Southdale, Southdown, St. Croix , Suffolk , Targhee , Texel , Thribble Cross, Tunis , Vermont Merino, Warhill, Wensleydale , Willamette, and Wiltshire Horn (Food and Agriculture Organization of the United Nations, 2003; Oklahoma State University Board of Regents, 1996; American Sheep Industry Association, 2003).

Sheep in Wisconsin:

Throughout the 1860’s, Wisconsin ranked second in the nation for wheat production.  Near the end of the 1870’s, several factors devastated the Wisconsin’s wheat market.  To make up for the loss in revenue, the farmer’s of Wisconsin turned to sheep because of their minimal requirements and the continual revenue from year round meat and wool sale (Schraufnagel, et al., 1989).

Wool prices increased until the end of the Civil War when Southern cotton was reintroduced into the market, dropping wool prices from $1.05 to $0.29 per pound (Schraufnagel, et al., 1989).  Following the Civil War, the number of sheep in Wisconsin increased to over a million making it second in the nation.  The increase was attributed to the utilization of sheep for clearing new land and despite the price drop; there was also a strong demand for wool in areas where cotton was inadequate or undesirable (Pope, 2001).  Those who settled in southeast Wisconsin combined sheep production and grain farming, a combination that slowly moved to occupy the least productive land and persisted well after the war due to the lack of other options (Schraufnagel, et al., 1989).  

        Paralleling national trends, the Wisconsin sheep industry began rapidly declin­ing after 1970 and eventually stabilized to a gradual decline (Wisconsin Department of Agriculture, April 1995; Schraufnagel, et al., 1989).  In a ten-year period from 1970 to 1980, sheep numbers fell from 167,000 to 113,000 and then to 83,000 in 1987 (Schraufnagel, et al., 1989).   As far as the actual number of individuals in recent times, Wisconsin ranks 21st in the country with 65,000. (American Sheep Industry Association, Inc., 2002) and holds 0.80% of the sheep in the country.  Today, the sheep industry of Wisconsin makes up only a very small portion of Wisconsin’s total farm revenue (<1%) with the few sheep in Wisconsin typically raised as small side operations in other agricultural endeavors (Schraufnagel, et al., 1989).
In terms of the geographic distribution in the state, sheep “populations” are widespread throughout the state (P. M. Holahan, personal communication, October 23, 2003).  Outside of general specimen information, limited useful information could be obtained from mammalogy collections throughout the state (C. J. Yahnke, personal communication, October 22, 2003; N. E. Kraucunas, personal communication, October 29, 2003).  The literature explains that following the Civil War, the predominant breed in Wisconsin was the Merino (Pope, 2001).  Besides that, however, little documentation exists about the distribution of various sheep breeds in Wisconsin; consequently, the occurrence of a given breed in Wisconsin was derived from breeder lists along with information from “breed-based” organizations (Wood Violet Farm, 2003; Carlson, 2003; Blackbelly Barbados Sheep Association, 2003; North American Barbados Blackbelly Sheep Registry, 2003; Columbia Sheep Breeders' Association of America, 2003; Katahdin Hair Sheep International, 2003).  Some of the more common Wisconsin breeds are described below.

Merino-

The Merino breed is divided into three types: “A” type, “B” type, and “C” type (Plumb, 1906).  "A" and "B" type Merinos, are frequently called "heavy types" due to their heavy fleece, comprise only a small percentage the breed as they are not suited for most conditions.  The "C" type or Delaine has a smooth body, free of wrinkles and is the most suitable Merino for most farm applications and is found throughout the country at nearly all elevations (Oklahoma State University Board of Regents, 1996).   The productive life of the Merino is much greater than any other breed, with productive ewes 10 to 12 years of age.  The continuous line of breeding over the last twelve hundred years has resulted in the production of the best wool in the world.   The adaptability and refined characteristics make the Merino one of the best all around breeds in the world (Oklahoma State University Board of Regents, 1996).

Polypay-

The Polypay was fairly recently developed in the United States at the U.S. Sheep Experimentation Station in Dubois, Idaho.  Development of this breed was fueled by the realization that sheep productivity must be increased to make any profit. Scientists set out to develop a sheep breed that could produce two lambs per year and still produce one crop of wool per year. The Polypay breed began mainly with Targhee x Dorset and Rambouillet x Finnsheep crosses and proceeded until a group of very prolific sheep were produced (most with twins or triplets in the first pregnancy) that became officially recognized as a new breed in 1976 (Wood Violet Farm, 2003).

Rambouillet-

The Rambouillet breed originated over two centuries ago from the Spanish Merino breed, which were known to produce the highest quality wool.  Because of the coveted wool, the Spanish government prohibited the export of Merino sheep until October 12, 1786 when the King of Spain ordered that 359 of the finest Merinos be sent to the Rambouillet farm as a gift to his cousin Louis.  This small flock was essentially the basis for Rambouillet breed that was established in 1801(Carlson, 2003).  In the United States, Rambouillet sheep are usually thought of as a range breed of Texas and the Western states, but has also been established as a popular farm breed (Carlson, 2003).

Navajo-Churro Sheep-

The Navajo-Churro breed originated from the Churra (transformed by the settlers to “Churro”), an Iberian breed with wool second only to the Merino.  About four hundred years ago, Don Juan Onate brought the first Churro sheep into the Southwest United States, making the Churro the first domesticated sheep breed in the New World.  The breed became well established and formed a stable population until the mid 1800’s.  The California Gold Rush and the Civil War demanded more sheep resources.  As a result, many Churros were crossed with other breeds, while most of remaining Churros were wiped out by the US Army as revenge for Native American attacks.  Today these sheep still exist because a few isolated individuals enabled the Navajo-Churro breed to be preserved; however, because of its devastating history, it is still considered a “rare” breed.  (Navajo-Churro Sheep Association, 2003)

Targhee-

In response to the demand for a sheep that was highly adaptable to the various living/farming conditions and could produce apparel quality wool, scientists at the U.S. Sheep Experiment station in Dubois, Idaho, developed the Targhee breed in 1926 by initially breeding 3/4 Rambouillet and 1/4 long wool cross.  The success of this breed is evident by the flocks inhabiting ranges at high elevations in mountainous regions, low elevations in the plains, grasslands, and even on farms throughout the country (U.S. Targhee Sheep Association).

Tunis-

The Tunis breed became established in the United States in 1799 when 10 individuals were sent as a gift to the United States, making the Tunis one of the oldest breeds in the United States (The National Tunis Sheep Registry, Inc. 2003).  Only two survived the long boat ride, but more were imported allowing the breed to expand until the civil war, when all but one flock was completely wiped out.  From that one flock, the breed has made a comeback in modern times (Southeast Tunis Breeders 2002).  In the Tunis breed the females make excellent mothers that produce 2 or 3 young and have good milk production (Roger Williams Park Zoo and Rhode Island Zoological Society, 2005).

Blackbelly Barbados-

Blackbelly Barbados sheep are visually identified by unique, well-defined markings that are characteristic of the breed.  The belly and inside of the legs are black, with some black wrap around the legs (front legs are black in front from the knees down).  Two black bars go down the center of the face and the chin and the insides of the ears are black  (Blackbelly Barbados Sheep Association, 2003).  In general, Blackbelly Barbados sheep are prolific breeders, excellent mothers, low maintenance, adaptable, versatile eaters, hardy and healthy, mild flavored, and easy to market (Blackbelly Barbados Sheep Association, 2003).

California Red sheep-

The California Red Sheep has its origin in the early 1970's when Dr. Glenn Spurlock of the Department of Animal Science at the University of California - Davis started a side project to create a wool-less breed with quality meat, but without the usual health and reproduction issues.   The breed began with the crossing of Tunis and Barbados sheep.  The wool of the breed is highly sought after, and the meat is very high quality.  Also, the breed demonstrates resistance to many health problems and successfully breeds year round with little complication (Altomare).

Columbia Sheep-

Columbia Sheep are the largest white-faced sheep in the world (Gerber).   The United States Department of Agriculture developed the Columbia sheep as a true breeding animal in an effort to eliminate cross breeding on the range.  In 1912, rams of the long wool breeds were crossed with high quality Rambouillet ewes to produce large ewes with more pounds of wool and lamb. The first attempted cross was a Lincoln-Rambouillet, which produced the line that went on to make the Columbia breed.   While they were originally developed for range conditions, they have proved admirably adaptable to the lush grasses and farm flock management of the Midwest, east, north and south (Columbia Sheep Breeders' Association of America, 2003).

St. Croix-

St Croix Sheep are a rare breed of hair sheep that have been bred for meat instead of wool.  St. Croix sheep are thought to have originated in Africa, and were transported to the Virgin Islands via slave ships.  In terms of modern agriculture, the breed possesses many desirable traits that benefit today’s farmers (St. Croix Sheep Breeders Association, 28 February 2003).

Katahdin-

The desire to combine the wool-less coat, prolificacy and the hardiness of the Virgin Island sheep, with the meat, conformation and rate of growth of the wooled breeds initiated the creation of the Katahdin breed by Michael Piel in the late 1950’s. (Katahdin Hair Sheep International, 01 November 2003b).  The breed was founded on the theory that by selectively breeding for meat would be greatly promoted by, selecting only for meat and no longer selecting for wool and meat (Piel, 1966). Piel experimented by making crosses of wool-less sheep and variety of British breeds, particularly the Suffolk.  When he had expended almost every possible combination nearly 20 years later, he selected a flock of ewes he named "Katahdin" after Mount Katahdin in Maine (Katahdin Hair Sheep International, March 1998).   The Katahdin breed today is a heavy muscled, medium sized, easy-care meat type sheep, with a calm disposition, and no required shearing (Canadian Katahdin Sheep Association, 2003). Katahdin hair breeds are considerably more heat-tolerant than wool breeds and generally have much greater parasite resistance than wooled breeds, especially after one year of age (Kirchhoff, 2003).  Katahdin sheep are efficient and adaptable for utility and production in many different types of management systems, but is ideal for pasture lambing and grass/forage-based management systems. (Katahdin Hair Sheep International, October 2002).

Ontogeny and Reproduction:

In terms of breeding behavior, sheep are classified as seasonally polyestrous and most breed only during fall and winter. Sheep exhibit seasonal breeding activity because many breeds were developed in northern climates where it was favorable to breed in the fall and give birth in the spring under mild weather conditions and ample food supply (Neary, 1992).  The lack of seasonality of hair sheep in the tropics does not seem to totally inhibit the response of ewes to ram exposure (Godfrey, et al., 1998b).   Photoperiodism is an important abiotic control mechanism in sheep reproduction, which is why sheep have been labeled as “short day” breeders (Neary, 1992).         

The hypothalamus, pituitary gland and ovaries regulate sheep reproduction.  The hypothalamus detects the reduced photoperiod and uses gonadotrophin-releasing hormone to signal the pituitary gland, which stimulates the ovaries by emitting follicle-stimulating hormone and luteinizing hormone.  The chain reaction makes the ovary secrete estrogen, resulting in a heat cycle.  After the first cycle, ewes continue to come into estrus every 15 to 17 days until they are bred or until hit the anestrous phase (Neary, 1992).  Different types of mating stimuli do not influence estrus initiation and duration (Godfrey, et al., 2001).  Once fertilized, embryonic implantation does not occur until after 21 to 25 days.   Depending on sheep breed and seasonal factors, gestation typically lasts for 144 to 152 days (Neary, 1992).

Under idealized conditions, an ewe has intercourse only once during the year, whereas, the ram may have intercourse 50 to 100 times in that same time period.  (Ideal conditions are those that minimize reproductive complications (Jordan, 1990).)  Although, one ram can breed a whole flock, they are still a significant part of sheep reproduction and quite unique.  Rams have the most concentrated semen and largest testicular tissue per kg of body weight of any other livestock species.  The ram lambs generally attain puberty around 5 to 7 months (Neary, 1992).

A recent study of hair sheep rams shows that sexual behavior does not seem to be a negatively influenced by an elevated ambient temperature (Godfrey, et al., 1998a).  However, even though sexual behavior remains unaffected high body temperatures prior to breeding can lead to temporary sterility for 6 to 8 weeks.  In terms of reproduction, sheep health and nutrition are also important components, especially during critical periods of the production cycle, such as, pre-breeding, late gestation and lactation (Neary, 1992).   Nutrition may also be more important during certain seasonal periods (Godfrey and Dodson, 2003; Godfrey, et. al., 2003).

Ecology and Behavior:

In terms of domestication, most sheep thrive in both upland and lowland pasturage as well as under farm conditions.  Sheep sustain themselves on a wide variety of grasses, and other plants.  Breeds that have been out-crossed with the mouflon (0. ammom musimon) may even be able to consume plants that are known to be toxic and fatal to some species.  In addition to herbaceous food items, the sheep will also eat buds and shoots of woody plants.  In severe conditions some breeds have been known to strip the bark from trees and eat it (Roger Williams Park Zoo and Rhode Island Zoological Society, 2005).  Because sheep eat woody plants, broadleaf plants, weeds, grasses and other plants that most animals avoid or find toxic, monitored sheep grazing can be an effective biological control program that can benefit the environment and wildlife.  In the United States and Canada, sheep grazing has helped regenerate conifer forests, enhance wildlife habitats, improve biological diversity, control noxious weeds, and prevent the spread of forest fires (American Sheep Industry Association, Inc.).  Additional benefits to wildlife come from, information gained from research on domestic sheep can be used for predicting behavior of similar species that have little known about them (Leslie, 1980; Maehr, et al., 2001).   

Relative to other farm animals, sheep are highly sensitive copper and most susceptible to copper toxicosis.  Ironically, copper is a required mineral for health of all farm animals, but is potentially toxic in excess.  In sheep copper nutrition can be quite complicated because it is a required mineral for sheep, yet highly toxic and the effect of the copper in sheep is determined by breed or crosses of the breed, age, health, as well as, by mineral and other chemical levels in the body (Neary, 2002).

Remarks:

The domestic sheep (Ovis aires) has experienced some recent fame in the field of genetics (Wilmut, et al., 2000).  A domestic sheep was used in the creation of a genetic clone known to the world as “Dolly” (Kolata, 1998).  The technological advancement offered huge medical potential and but at the cost of human moral conflicts (Rantala, and Milgram, 1999). The ethical aspect of this event continues to be an issue of intense debate around the world (Ruse and Sheppard. 2001). 

On a slightly humorous note, if a wool sweater shrinks after being put into a washing machine why sheep don’t shrink when it rains?   Although, it seems like a funny question, it has a scientific explanation.  Wool is made up of keratinized protein with flat, overlapping scales.  While on the sheep, the scales point in the same direction and can slide back and forth, but after processing, the fibers have mixed orientations with the scales pointing in random directions.  When wool fabrics are put into a washing machine, the fibers slide past each other, but the scales do not allow the fiber to slide back, making the shrink.  Free movement of the fibers is also afforded by the production of lanolin, which coats the wool and acts as a “lubricant” and water repellant (Padula, 2003).

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Reference written by Matthew Schmidt, Biol 378: Edited by Chris Yahnke. Page last updated 4-29-04.