Horns are often associated with male animals, serving as weapons for competition over mates. This common perception of sexual dimorphism—where males and females look physically different—overlooks the numerous species where females also possess horns. In many groups of ungulates, particularly within the family Bovidae, female horn possession is either the rule or a significant variable. Understanding the biological reasons behind female horn development offers insight into the varied selection pressures that shape evolution.
Horns Versus Antlers
Scientifically, a true horn describes a specific cranial appendage found primarily in the Bovidae family, which includes cattle, sheep, goats, and antelopes. True horns consist of a permanent bony core (os cornu) that grows from the skull and is covered by a sheath of keratin. Unlike antlers, horns are never naturally shed and grow continuously throughout the animal’s life. Antlers, found exclusively in the deer family (Cervidae), are fundamentally different because they are pure bone structures that are shed and regenerated each year. A third type of headgear, the ossicone, is found on giraffes and okapis, consisting of bone covered by skin and hair.
Species Where Females Always Possess Horns
In roughly half of all bovid species, females possess horns, though they are typically smaller and less robust than those of males. The presence of horns in both sexes is standard in many larger bovid groups, such as domestic cattle and the African Cape buffalo. Many antelope species also exhibit this trait; both sexes of the Greater Kudu and Oryx species, like the Scimitar-Horned Oryx, have long horns. Within wild sheep, such as the Bighorn sheep, females possess horns that are much shorter and less coiled than the males’ headgear. Similarly, both sexes of domestic goats and the Bharal (Blue Sheep) are horned, with the female’s horns being noticeably thinner and shorter.
The Purpose of Female Horns
The evolutionary drivers for horns in females are primarily linked to natural selection and survival, rather than the sexual selection that drives male headgear. A primary function is defense against predators, especially in species that live in open habitats. Females of large species, such as the wildebeest and African buffalo, use their horns to protect themselves and their young from threats like lions and hyenas. Horns also play a role in intrasexual competition among females, particularly for resources. For example, territorial duikers use their horns in contests to defend feeding territories. In Bighorn sheep, longer female horns correlate with greater reproductive success and signal social status or dominance within the female hierarchy.
Variability in Female Horn Presence
While some species have universally horned females, the trait is far more variable in others, often presenting as smaller or thinner structures. In the mountain gazelle, females consistently have horns, but they are significantly shorter than the males’ horns and lack pronounced rings. This difference in size represents a form of sexual dimorphism even when the trait is present in both sexes. In species like Thomson’s gazelle, female horns are notably short and spiky, often showing high variability. This suggests that in smaller species where the primary defense is flight, the selective pressure for large female horns is reduced. The presence of horns in domestic sheep can also be polymorphic, meaning some individuals are horned, some are polled (hornless), and some have small, vestigial scurs.