Sexual dimorphism
Sexual dimorphism refers to the distinct physical differences between male and female members of a species, which can manifest in variations of size, coloration, and secondary sex characteristics. These differences can range from minor disparities, such as slight size variations, to more dramatic distinctions, like the male peacock's elaborate tail compared to the simpler appearance of the female peahen. In many species, including humans, males are typically larger than females, though this is not universal, as seen in certain birds of prey and spiders where females are the larger sex. Secondary sex characteristics, such as mane in male lions or the colorful plumage of male birds, serve to enhance these distinctions and often play a role in mating preferences guided by sexual selection.
Examples of sexual dimorphism can be found across various species, including mandrills, elk, and elephant seals, each exhibiting unique traits that help attract mates. The phenomenon is not limited to mammals; many bird species also display significant sexual dimorphism, often with males showcasing more vibrant colors or unique features that influence female choice. Notably, the triplewart seadevil illustrates extreme sexual dimorphism, where females are much larger than their male counterparts, which are significantly smaller and live parasitically attached to females. Understanding sexual dimorphism provides insight into the evolutionary pressures that shape the physical traits of species and their mating behaviors.
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Sexual dimorphism
Sexual dimorphism is a condition involving physical differences between a species' male and female members. These differences may include variation in size, coloration, and/or secondary sex characteristics. Furthermore, these differences can be minor or extreme. A slight difference in size between the males and females of a certain species is an example of a minor case of sexual dimorphism. An extreme case of sexual dimorphism is a drastic difference in size between males and females and obvious differences in secondary sex characteristics. Sexual dimorphism is related to a theory called sexual selection.
Species that exhibit sexual dimorphism include humans, mandrills, elk, orangutans, African lions (lions and lionesses), elephant seals, orange-tip butterflies, peafowl (peacocks and peahens), pheasants, birds of paradise, Mandarin ducks, and triplewart seadevils. For example, in humans, mandrills, and pheasants, males typically are larger than females. Other examples of sexual dimorphism include different coloration of the plumage between the male and female members of many bird species, the presence of a mane in male African lions, and the existence of a train of tail feathers in male peacocks.
Background
Variation in size between males and females is one of the main characteristics of sexual dimorphism. In many species, the male is generally larger than the female. In some species, however, the female is the bigger sex. This is the case with owls, birds of prey, and spiders, among others.
Difference in coloration between males and females is another common feature of sexual dimorphism. The male and female either have different colors, or one of the sexes has less vibrant colors than the other. For example, in certain species of birds, the color of the plumage of the male differs from the color of the plumage of the female.
Sexual dimorphism can display itself through secondary sex characteristics. Secondary sex characteristics are features unrelated to the reproductive system that one of the sexes possesses that help to distinguish it from the other sex. For example, the horns of cattle that are present in male cattle but not female cattle are secondary sex characteristics. Secondary sex characteristics differ from primary sex characteristics, which are features tied to the reproductive system. Such features include an organism's genitals.
In many species, sexual dimorphism is related to sexual selection. Sexual selection is a theory hypothesizing that certain traits help an organism to successfully mate with members of the opposite sex. In other words, one of the sexes prefers to mate with members of the opposite sex that possess these traits. Taking sexual dimorphism into consideration, this means that size differences, coloration variations, and differences in secondary sex characteristics help members of the opposite sex to mate with one another. Furthermore, the members that have more pronounced traits than other members usually are more successful at mating.
Overview
Many mammalian species exhibit sexual dimorphism, including humans. Males typically are larger than females. Females have breasts, while males do not. Males generally have more hair on their bodies than females. There are several other instances of sexual dimorphism in humans, but these three differences are perhaps the most obvious ones.
Some mammalian species display sexual dimorphism that is extreme. The mandrill, which is a primate, is one such species. Male mandrills are much bigger than female mandrills—sometimes three times the size. Males also have much more colorful faces and buttocks than females.
The elk, orangutan, African lion, and elephant seal are other mammalian species that exhibit sexual dimorphism. The male elk possesses antlers, and the female elk does not. The adult male orangutan has large cheek flaps, which the female orangutan does not possess. These cheek flaps are related to the male's dominance, as males with more prominent cheek flaps generally are more dominant than males with less prominent ones. Male African lions have a large mane, while female African lions, called lionesses, lack this feature. Furthermore, this mane is a prime example of sexual selection. Lionesses tend to choose mates based on the condition of their manes. Male elephant seals have a large, bulbous nose that looks slightly like an elephant's trunk. This nose is another example of sexual selection, as the males are able to inflate the nose for mating purposes.
Sexual dimorphism is found in many other species, including orange-tip butterflies, peafowl, pheasants, birds of paradise, Mandarin ducks, and triplewart seadevils. Male orange-tip butterflies feature orange coloration on the tips of their forewings, while female orange-tip butterflies do not. Peafowl, which include the male peacock and the female peahen, offer one of the more stunning examples of sexual dimorphism. The peacock has large, colorful tail feathers, collectively referred to as a train. The peahen does not possess a train. Like a lion's mane and a male elephant seal's nose, the peacock's train is an example of sexual selection. The peacock fans out this train to display it during mating rituals. The peahen tends to pick mates based on the features of the train. Pheasants, birds of paradise, and Mandarin ducks display similar sexual dimorphism to one another, as the males of each of these species have more ornate plumage than the females. Male pheasants are larger than female pheasants and generally have longer tails as well. Male Mandarin ducks boast red bills, but the bills of female Mandarin ducks typically lack this color. One of the most extreme displays of sexual dimorphism is found in the triplewart seadevil, which is an anglerfish. Female triplewart seadevils are drastically bigger than male triplewart seadevils. While females are about a foot long, males are only about half an inch long. Furthermore, the tiny males attach to the females as parasites.
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