The presence of horns in female sheep, known as ewes, depends on genetics and breed history. Horns are not universal across all sheep breeds, nor are they strictly limited to males (rams). Whether an individual ewe grows horns, or is naturally hornless (polled), depends almost entirely on its specific breed lineage. This results in variations from completely polled breeds to those where both sexes are typically horned.
Horn Presence Varies by Breed
Many modern sheep breeds developed for commercial farming have been selectively bred to be naturally hornless (polled). This makes polled ewes the most common sight in agriculture. Breeds like the Suffolk, Dorset Horn (polled strain), and Cheviot generally have ewes born without horns. Selecting for polled animals simplifies management and reduces the risk of injury within the flock.
Older, primitive, or wild sheep breeds often show a different pattern, where horns are common in both sexes. For example, ewes in breeds such as the Scottish Blackface, Jacob sheep, and the wild Mouflon routinely develop horns. The Bighorn sheep, a North American species, also has horned females, though their horns are much smaller than the rams’.
Understanding Sexual Dimorphism in Sheep Horns
Even in breeds where both sexes are horned, a significant physical difference, known as sexual dimorphism, exists between the horns of the ram and the ewe. Ram horns are massive structures that grow in tight, outward-coiling spirals. These substantial horns are used primarily for combat during the breeding season to establish dominance and secure mating rights.
Ewe horns are typically much more modest in size and structure. If present, they are generally shorter, thinner, lighter, and may form a simple, upward-curving or sickle shape, unlike the ram’s tight spiral. Some females may develop only small, rudimentary, unattached growths known as “scurs.” While their function is less clear, ewe horns may offer defense against predators or serve in social signaling.
The Genetics of Horn Growth
The presence of horns is determined by a single, sex-influenced autosomal gene locus, often called the Horns (Ho) locus, located on chromosome 10. This trait is sex-influenced because the gene behaves differently depending on the animal’s sex. The gene coding for horn growth is dominant in males but recessive in females.
This difference in dominance means that a ram needs only one copy of the horn-producing allele to grow large horns. Conversely, a ewe typically needs to inherit two copies of the allele to grow full, well-developed horns. The influence of male sex hormones, such as testosterone, further enhances the growth and size of horns in rams, explaining the vast difference in horn mass between the two sexes. This genetic mechanism ensures that fully horned ewes are less common than fully horned rams.