sexual dimorphism (redirected from Sexually monomorphic)

sexual dimorphism [′sek·shə·wəl dī′mȯr‚fiz·əm]

(biology)

Diagnostic morphological differences between the sexes.

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Sexual dimorphism

Any difference, morphological or behavioral, between males and females of the same species. In many animals, the sex of an individual can be determined at a glance. For example, roosters have bright plumage, a comb, and an elaborate tail, all of which are lacking in hens. Sexual dimorphism arises as a result of the different reproductive functions of the two sexes and is a consequence of both natural selection and sexual selection. Primary differences such as the structure of the reproductive organs are driven by natural selection and are key to the individual's function as a mother or father. Other differences such as the peacock's (Pavo cristatus) enormous tail are driven by sexual selection and increase the individual's success in acquiring mates. See Organic evolution

A less obvious sexual dimorphism is the difference in size of male and female gametes. In nearly all cases, the sperm (or pollen) are substantially smaller and more numerous than the ova. Eggs are large because they contain nutrients essential for development of the embryo. However, the sole purpose of sperm is to fertilize the egg. Sperm do not contain any nutrients and can therefore be small. For the same investment of nutrients, a male can produce more sperm than a female can produce eggs. Human males, for example, produce about 300 million motile sperm per ejaculate, whereas females normally produce only one egg (30,000 times larger than a single sperm) per month. See Gametogenesis, Reproduction (animal), Reproductive system

In nearly all animal groups (apart from mammals and birds), females are larger than males because larger females tend to produce more eggs and contribute more young to the next generation. In contrast, size does not appear to limit males' ability to produce sperm. However, among mammals and birds males are generally the larger sex. Differences in body size and shape can be caused by factors other than reproductive success. Sexual dimorphism can arise as a consequence of competition between the sexes over resources, or because the sexes use different resources. For example, in many species of snake, males and females use different habitats and eat different food, which has led to differences in their head shape and feeding structures.

Plants also differ in showiness. Many plants bear both male and female flowers (simultaneous hermaphrodites), but male flowers are sometimes larger and more conspicuous. For example, the female catkins of willow are dull gray compared with the bright yellow male catkins, because male flowers compete with each other to attract pollinators. In plant species with separate sexes (dioecious), males tend to produce more flowers than females. For example, males of the American holly (llex opaca) produce seven times as many flowers as females in order to increase their chances of pollen transfer to females. See Flower, Pollination

Animals and plants show marked sexual dimorphism in other traits. Calling, singing, pheromones, and scent marking can all be explained by competition between males and by female mate choice. See Animal communication

Associated with morphological sexual dimorphism are several behavioral differences between males and females. Many of these are related to locating a mate, competition between males, and female choosiness. Animals also show sexual dimorphism relating to their roles as parents. Many parents continue to provide for their young after birth, with the female performing the bulk of the care in most species. Female mammals suckle their young, whereas males cannot because they lack mammary glands. However, some mammals (such as gibbons and prairie voles) and many birds share parental duties, with both males and females feeding and protecting the young.