Some female sheep grow horns, some grow small bony stumps called scurs, and some have no horns at all. The outcome depends almost entirely on breed and genetics, with a single gene region playing the starring role. What makes this question interesting is that even when a ewe carries the genetic instructions for full horns, her biology often interprets those instructions differently than a ram’s would, producing smaller horns or none at all from the same underlying code.
Breed Matters More Than Sex
In wild sheep species and several primitive domestic breeds, both sexes grow horns. Female bighorn sheep, Dall sheep, and mouflon all develop horns, though theirs tend to be shorter, thinner, and less dramatically curved than those of rams. Among domestic breeds, the picture is more varied. Soay sheep, an ancient breed from the Scottish island of St Kilda, produce horned ewes alongside horned rams. Dorset Horn and its polled variant, Scottish Blackface, Icelandic, and Jacob sheep all include females that regularly grow some form of horn. At the other end of the spectrum, many commercial breeds like Suffolk, Hampshire, and Texel have been selectively bred so that neither sex grows horns. These are called polled breeds, and in them, the question is moot for both ewes and rams.
Between horned and polled breeds sit the breeds where things get complicated. In Merino flocks, for instance, rams are frequently horned while ewes range from fully horned to polled within the same flock. That unevenness is not random variation but a predictable consequence of how horn genetics interact with sex.
The RXFP2 Gene and Sex-Dependent Expression
The genetics of sheep horns center on a gene called RXFP2, located on chromosome 10. A specific insertion in this gene’s regulatory region is strongly associated with polledness: animals that carry this insertion on both copies of the chromosome tend to be polled, while those without it tend to be horned.1PubMed Central. A 1.8-kb insertion in the 3′-UTR of RXFP2 is associated with polledness in sheep But what happens in between, when an animal carries one copy of the horned variant and one copy of the polled variant, depends heavily on whether the animal is male or female.
In rams, the horned allele tends to behave in a dominant fashion. A ram with one horned copy and one polled copy will usually grow horns. In ewes, the same genetic combination often produces scurs or no horns at all. This is what geneticists call sex-dependent dominance: the same gene variants produce different physical outcomes depending on the animal’s sex.2PubMed Central. Genomic prediction of the polled and horned phenotypes in Merino sheep Even ewes that carry two copies of the horned allele do not always grow full horns. Research in Merino sheep found that females homozygous for the horned allele showed a horned phenotype only about 22% of the time, with the remaining 78% developing knobs or scurs instead.3PubMed Central. Segregation GWAS to linearize a non-additive locus with incomplete penetrance: an example of horn status in sheep Rams with the same genotype showed nearly complete penetrance, meaning almost all of them grew full horns.
This incomplete penetrance in females explains much of the confusion surrounding the topic. A ewe can be genetically “horned” but physically look polled or sport only small bumps. The genetics say one thing; the phenotype says something subtler.
Hormones Modulate What Genetics Initiate
The sex difference in horn expression is not purely a genetic toggle. Hormonal signaling, particularly the INSL3/RXFP2 pathway and its interactions with testosterone, plays a central role in shaping horn growth.4PubMed. Regulation of Horn Development in Hu Sheep Involves RXFP2 Expression and Hormonal Signaling Independent of the 1.78-kb Insertion Genotype Rams have substantially higher testosterone levels, which partly explains why their horns grow faster, thicker, and longer even when the underlying genetic blueprint is shared between the sexes.
In wild Iberian ibex, a related bovid, researchers found that the rate of horn growth before the autumn mating season was related to springtime testosterone levels, though the seasonal spike in testosterone during mating itself did not directly trigger a stop in horn growth.5PubMed. Role of testosterone and photoperiod on seasonal changes in horn growth and sperm variables in the Iberian ibex: a model for polygynous wild bovids The hormonal picture is not simply “more testosterone equals bigger horns,” but testosterone clearly steers horn development in a direction that favors larger structures in males. Since ewes produce far less testosterone, the hormonal push toward full horn growth is weaker, which is why even genetically horned ewes often end up with smaller structures or mere scurs.
Why Would Ewes Need Horns at All?
Ram horns make intuitive sense from an evolutionary standpoint: males use them to fight for mating access. But why would natural selection maintain horns in females, especially when growing bone and keratin costs metabolic energy? Researchers have identified two main explanations, and they apply to different situations.
Across bovids more broadly, the leading explanation for female horns is defense against predators. A phylogenetic analysis of the bovid family found that an inability to rely on hiding or taking refuge in dense vegetation drove the evolution of horns in females. Species that live in open habitats where they cannot easily flee or conceal themselves are more likely to have horned females.6PubMed Central. Evolution of weaponry in female bovids Pointed horns can deter a predator just enough to matter when escape is not an option.
The second explanation involves competition among females themselves. In Soay sheep, researchers demonstrated that horned ewes displace individuals with lesser horn development when competing for food, and this relationship held even after accounting for age differences. Aggressive encounters increased when local density was high, suggesting the horns function as weapons in scrambles over limited resources.7PubMed Central. Function of weaponry in females: the use of horns in intrasexual competition for resources in female Soay sheep Male and female Soay sheep produce horns of different shapes, with the broader male horns suited to head-on clashing and the thinner female horns acting more like spikes for jostling competitors aside.
These two drivers, predator defense and female-on-female competition, are not mutually exclusive. They can both be at work in the same species or even the same population depending on ecological pressures.
Horns and Social Rank in Ewes
Given that Soay ewes use their horns to displace competitors, you might expect horn size to be a reliable predictor of social rank in sheep generally. But in bighorn sheep, the relationship is weaker than you would guess. Studies of bighorn ewes found that horn length and body weight were not consistently correlated with social status.8Animal Behaviour. Correlates and consequences of social status in female bighorn sheep A more detailed analysis confirmed that once age was accounted for, ewe dominance increased with body mass but not with horn size.9Animal Behaviour. Determinants and life-history consequences of social dominance in bighorn ewes
This makes sense when you consider the difference in how males and females use horns. Rams compete by crashing into each other head-on, so horn mass and curl shape directly determine the outcome of a fight. Ewes do not typically settle hierarchies through head-butting bouts. Their ranking systems lean more on body condition and age, which reflect overall competitive ability rather than weapon size alone. So while horns give a Soay ewe a useful edge in a feeding scrum, they are not the currency of female social power in every sheep species.
Scurs, Knobs, and the In-Between
The intermediate horn structures that many ewes develop deserve their own explanation because they cause real confusion. Scurs are small, loosely attached horn-like growths that often look lopsided or oddly shaped. They are not simply “small horns.” True horns grow from a bony core that is fused to the skull and are covered in a permanent keratin sheath. Scurs grow from a smaller base, are sometimes movable, and can break off more easily.
In Soay sheep, the inheritance pattern is relatively clean: polledness is recessive, and heterozygous females (one horned allele, one polled allele) typically grow scurs rather than full horns.10PubMed Central. Analysis of genetic variants for different horn phenotypes and their inheritance in Icelandic sheep But Icelandic sheep complicate this picture. In Icelandic flocks, scurs were observed in males as well as females, and they appeared in offspring from every parental combination: both parents polled, both horned, or one of each. That contradicts the cleaner Soay model and suggests that the inheritance of these intermediate structures is not identical across breeds.
For farmers, scurs can be a practical headache. They are not large enough to cause the injuries that full-sized horns sometimes do, but they can grow at irregular angles that catch on fencing or feeders. They are difficult to predict from parentage alone, which complicates breeding decisions.
Variation Beyond RXFP2
While RXFP2 dominates the horn-or-no-horn conversation, deeper sequencing has revealed that multiple specific variants within and around this gene fine-tune the outcome. A study that sequenced three Chinese sheep breeds identified several significant variant positions. One variant contributed to larger horn size in both sexes, while another showed a sex-specific effect, influencing horn development differently in males and females.11PubMed. RXFP2 gene variation drives horn sexual differences and polledness in sheep A third variant had strong explanatory power for polledness in Plateau-type Tibetan sheep specifically, suggesting that different populations may rely on different genetic levers even within the same gene region.
Then there is the entirely separate genetics of polyceraty, or the growth of four (or more) horns. Jacob sheep and Navajo-Churro sheep both include four-horned individuals, and genome-wide analysis mapped this trait to a single region on chromosome 2, distinct from the RXFP2 locus on chromosome 10.12PubMed. Genome-wide association reveals the locus responsible for four-horned ruminant Further work identified a small deletion in the HOXD1 gene as the causal mutation, and a corresponding structural variant was found in four-horned goats, with a perfect association between the mutation and the extra horns in both species.13Molecular Biology and Evolution. Analysis of Polycerate Mutants Reveals the Evolutionary Co-option of HOXD1 for Horn Patterning in Bovidae Four-horned ewes do exist, so this is another route by which female sheep end up with conspicuous headgear.
Horns as Radiators
Horn cores are not solid bone; they contain a highly vascularized network of blood vessels. This turns them into heat exchangers. A study measuring horn core surface area relative to body mass across wild sheep species found a striking environmental gradient. Thinhorn sheep living in subarctic climates had the smallest horn cores relative to body mass, while desert bighorn subspecies had values more than double.14Canadian Journal of Zoology. The thermoregulatory potential of Ovis horn cores In cold environments, small horn cores conserve heat. In hot environments, large horn cores help dump excess heat into the air.
This thermoregulatory dimension adds a layer to the question of female horns. If horns help regulate body temperature, then even modest-sized ewe horns could provide a physiological benefit in warm climates that has nothing to do with fighting or predator defense. It also helps explain why selection has not simply eliminated female horns entirely in wild populations: they may be doing useful work beyond their obvious weapon function.
Breeding for Polledness and Animal Welfare
In commercial sheep farming, horns are often more trouble than they are worth. Horned sheep can injure each other, damage equipment, and get tangled in fencing. When horned lambs are born in otherwise polled flocks, farmers sometimes resort to disbudding or dehorning, both of which are painful procedures that raise welfare concerns.15In Practice. Assessing and minimising the distress caused by painful husbandry procedures in ruminants
This is one reason breeders have worked for generations to select for naturally polled animals. Genotype-based targeted selection for polledness is a widely used strategy across sheep and cattle populations, offering a way to eliminate the need for dehorning without surgical intervention.16PubMed Central. The Complex and Diverse Genetic Architecture of the Absence of Horns (Polledness) in Domestic Ruminants, including Goats and Sheep The challenge is that the sex-dependent dominance described earlier makes this trickier than it sounds. A ewe that looks polled might still carry a hidden horned allele and pass it to her sons, who are much more likely to express it as full horns. Genomic testing can identify carriers, but uptake varies across the industry, and smaller breeders often rely on visual assessment, which is unreliable in ewes.
Sexual Dimorphism in Wild Populations
In wild bighorn sheep, the gap between male and female horns is dramatic. Rams can grow massive curling horns weighing over ten kilograms, while ewes produce short, gently curved spikes. This dimorphism has a genetic basis that researchers have tried to quantify. A study of wild bighorn sheep estimated the genetic correlation between males and females for horn volume at roughly 0.24, meaning the genetic architecture underlying horn size overlaps only modestly between the sexes.17PubMed Central. Quantitative genetics and sex-specific selection on sexually dimorphic traits in bighorn sheep For comparison, the genetic correlation for body mass between the sexes was much higher, around 0.63. Horns are among the most sexually dimorphic structures in sheep, to a degree that even the genes shaping horn size operate somewhat independently in males versus females.
This low genetic correlation means that selection for larger horns in rams does not automatically drag ewe horn size along with it, at least not to the same degree. It also means that the evolutionary pressures acting on male and female horns can diverge without as much genetic constraint pulling them back together. Female horns can shrink or disappear in a population even while male horns remain large, which is essentially what has happened in many domestic breeds.
How Domestication Changed the Picture
Wild ancestors of domestic sheep, the mouflon, have horned females in most populations. The shift toward polled ewes happened gradually through domestication, driven partly by deliberate breeding choices and partly by what researchers call unconscious selection: the accumulated effects of choosing calmer, more manageable animals for breeding without specifically targeting horn status. Traits like reduced aggression, smaller body size, and diminished sexual dimorphism tend to cluster together under domestication, and the loss of female horns fits neatly into that pattern.
Some heritage breeds retain the ancestral horned-ewe condition precisely because they have been less intensively selected. Soay sheep, stranded on their island for thousands of years with minimal human management, still look much like their wild forebears. Modern commercial breeds, shaped by centuries of targeted breeding for wool, meat, and docility, have drifted much further from that baseline. The result is a spectrum across today’s breeds, from ewes with fully functional horns to ewes with no trace of them, representing different distances from the wild starting point.