What Is Sexual Dimorphism and How Does It Evolve?

Sexual dimorphism describes the condition where the sexes of a species show different physical characteristics not directly involved in reproduction. This phenomenon is widespread, occurring in most animal species and even some plants. These differences can be subtle or highly exaggerated, ranging from size and color to behavior. The variation between males and females is a product of evolutionary pressures that shape each sex in unique ways.

Common Expressions of Sexual Dimorphism

One of the most common expressions is size dimorphism, where one sex is consistently larger than the other. In many mammals, such as gorillas and elephant seals, males are significantly larger. In other animals, like many birds of prey and spiders, the female is the larger sex, a difference often linked to reproductive roles.

Coloration and ornamentation are other prominent forms of dimorphism, known as sexual dichromatism. This involves differences in color and patterning, common in birds and reptiles. Males are often the more brightly colored sex, possessing features like the elaborate plumage of a peacock or the vibrant colors on a male guppy.

Another expression is the presence of specialized weaponry. These are structures used by one sex, usually males, in direct competition for access to mates. Examples include the large antlers of deer, the horns on many hoofed animals, and the enlarged tusks seen in male Asian elephants.

Beyond physical traits, dimorphism can also be behavioral. Males and females may exhibit different courtship rituals, vocalizations, or levels of territoriality. For instance, in many songbird species, only the male produces complex songs to defend territory and attract a partner. There can also be differences in parental care, with one sex taking on the majority of responsibility.

Evolutionary Roots of Sexual Dimorphism

The primary evolutionary driver behind these differences is sexual selection, a concept Charles Darwin identified. This process favors traits that increase an individual’s success in mating. It operates in two main ways: through competition within one sex for access to mates and through the preferences of one sex for particular traits in the other.

Intrasexual selection, or competition, fuels the evolution of traits that provide an advantage in contests. This typically involves male-male competition, where larger body size, strength, or specialized weapons like horns and large canine teeth can determine which individuals get to reproduce. In species where males defend territories or groups of females, the pressure for these competitive traits is strong.

The other mechanism, intersexual selection, involves mate choice, which is usually female choice. In this scenario, females select mates based on certain characteristics, which leads to the evolution of elaborate ornamental traits in males. Bright coloration, complex songs, and courtship displays are thought to be honest signals of a male’s genetic quality or health, making them more attractive.

Natural selection also plays a role in shaping sexual dimorphism, particularly when males and females face different environmental challenges. For example, if the sexes exploit different food sources, they might evolve different beak or jaw structures. When one sex is more involved in caring for young, they may evolve more subdued, cryptic coloration for camouflage, while the other sex remains conspicuous.

Examples Across the Animal Kingdom

In birds, the peafowl is a classic case. The male peacock has a long, iridescent train of tail feathers used in courtship, while the female peahen is a more subdued brown to blend in with her surroundings. Many birds of paradise also feature males with incredibly complex plumage and dance routines not seen in the plainer females.

Among mammals, the size difference between male and female elephant seals is extreme, with males weighing up to three times more than females. This size helps males defend harems of females from rivals. Lions also show clear dimorphism; males are larger and possess a distinctive mane, which signals their fitness to females. In many deer species, only males grow and shed antlers each year for fights over mating opportunities.

Insects also display dramatic differences. Male stag beetles have enormous mandibles that they use to wrestle with other males. Many butterfly species exhibit sexual dichromatism, where males have brightly colored wings to attract mates, while females are duller.

In the deep sea, sexual dimorphism can be present in extreme forms. The female anglerfish is vastly larger than the male. In some species, the male is so small that he attaches to the female as a parasite, fusing with her body and providing sperm in exchange for nutrients.