Sheep (Ovis aries) often exhibit sexual dimorphism, with horns typically restricted to males (rams). Most domestic sheep breeds worldwide are naturally polled, meaning both sexes are born without horns. When horns appear, they are primarily a male feature used for dominance displays and fighting. However, horns on female sheep (ewes) are a notable exception found in specific breeds where the genetic expression bypasses the usual male-only limitation.
The Genetic Mechanism for Horn Development
Horn development in sheep is governed by a genetic principle known as sex-influenced inheritance. This means the expression of a single gene is controlled or modified by the sex hormones present in the animal. The primary gene responsible for the horned or polled phenotype is located on an autosome, but its dominance changes between males and females.
The allele for horns acts as a dominant trait in rams but a recessive trait in ewes. A ram needs only one copy of the horned allele to develop full horns. Conversely, a ewe must inherit two copies of the horned allele, making her homozygous, to express the trait and grow horns. If a ewe possesses only one horned allele, the hornless allele is dominant in her system, resulting in a polled phenotype.
Research has narrowed the genetic location influencing this trait to a region on sheep chromosome 10, with the RXFP2 gene being a strong candidate. The difference in how this gene is regulated by androgens (male hormones) in rams versus the lower levels in ewes determines the final outcome. This sex-influenced pattern explains why ewes are generally polled, but also how a small number of females carry the necessary genetics to develop horns.
Specific Breeds Where Ewes Are Horned
The existence of horned ewes is primarily concentrated among primitive, ancient, and specialized domestic breeds. These animals have retained the ancestral genetic traits for horn growth in both sexes, unlike many modern commercial breeds that were selectively bred to be naturally polled for easier management.
Examples of Horned Ewe Breeds
The Jacob sheep is a well-known example, distinguished by its multi-horned appearance, where both rams and ewes commonly have two, four, or even six horns.
The Scottish Blackface is a hardy, mountain-dwelling breed where most ewes are horned, though their horns are noticeably smaller than those of the rams. The Navajo-Churro sheep, an ancient North American breed, also frequently features horned ewes, sometimes with four horns, reflecting their heritage.
The Icelandic sheep, a northern European short-tailed breed, is also commonly horned in both sexes. The Wiltshire Horn, an ancient English breed, is another instance where both rams and ewes consistently develop horns. These breeds represent a genetic link to the wild ancestors of domestic sheep.
Physical Characteristics of Ewe Horns
When ewes grow horns, their appearance and structure are distinct from the massive headgear of a ram. Ewe horns are consistently more slender, shorter, and lighter than those found on males of the same breed. Ram horns are characterized by their thick base and tight, heavy spiral that can complete multiple turns as the animal matures.
In contrast, ewe horns often exhibit a simpler, more open curvature, sometimes described as a sickle or crescent shape that sweeps back and slightly outward. The circumference of the horn base is significantly smaller in ewes. In some cases, the female may only develop “scurs,” which are small, incomplete, or deformed horn growths that are not fully attached to the skull.
The size difference is directly related to the influence of sex hormones during growth. The high androgen levels in rams stimulate the production of much larger, denser horn material. The ewe’s horns are functional but do not possess the bulk required for the violent head-clashing behavior seen in rams.