Sex
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Organisms of many species are specialized into male and female varieties, each known as a sex. Sexual reproduction involves the combining and mixing of traits, combining to form offspring that inherit traits from each parent. Individual organisms which produce both male and female reproductive traits are termed hermaphroditic. Physical differences are often associated with the different sexes of an organism and can reflect the different reproductive pressures the sexes experience.
Among humans and other mammals, the reproductive functions and outward sexual characteristics are split between the two sexes, though in vary rare occasions an individual may also be intersexed. Other animals have various sex-determination systems, such as the in birds, the insects, and various environmental systems, for example in reptiles and crustaceans. Fungi may also have more complex mating systems, with sexes not accurately described as male, female, or hermaphroditic.
Sexual dimorphism[edit | edit source]
Many animals and some plants have differences between the male and female sexes in size and appearance, a phenomenon called sexual dimorphism. Sex differences in humans include, generally, a larger size and more body hair in men; women have breasts, wider hips, and a higher body fat percentage. In other species, the differences may be more extreme, such as differences in coloration or bodyweight.
Sexual dimorphisms in animals are often associated with sexual selection—the competition between individuals of one sex to mate with the opposite sex. Antlers in male deer, for example, are used in combat between males to win reproductive access to female deer. In many cases the male of a species is larger than the female. Mammal species with extreme sexual size dimorphism tend to have highly polygynous mating systems—presumably due to selection for success in competition with other males—such as the elephant seals. Other examples demonstrate that it is the preference of females that drive sexual dimorphism, such as in the case of the stalk-eyed fly.
Other animals, including most insects and many fish, have larger females. This may be associated with the cost of producing eggs, which requires more nutrition than producing sperm—larger females are able to produce more eggs. For example, female southern black widow spiders are typically twice as long as the males. Occasionally this dimorphism is extreme, with males reduced to living as parasites dependent on the female, such as in the anglerfish. Some plant species also exhibit dimorphism in which the females are significantly larger than the males, such as in the moss Dicranum and the liverwort Sphaerocarpos.
In birds, males often have a more colorful appearance and may have features (like the long tail of male peacocks) that would seem to put the organism at a disadvantage (e.g. bright colors would seem to make a bird more visible to predators).