Sexual Dimorphism

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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.

Sexual Dimorphism

 

in dioecious species, the differences between male and female characteristics.

In animals. In unicellular organisms, such as coccidians and haemosporidians, the large nonmotile macrogamete copulates with the small motile microgamete. During conjugation in the infusorian Vorticella, the “female” individuals are sessile, while the “males” are motile. Sexual dimorphism in gametes is associated with a separation of functions; the large nonmotile female gamete provides the developing zygote with nutrients, while the small motile male ensures that the gametes meet.

In multicellular animals, sexual dimorphism refers to both secondary sex characteristics and the structural differences between copulative organs that carry out internal insemination. Sexual dimorphism is completely developed before the onset of sexual maturity. Permanent and seasonal sexual dimorphism are distinguished. Permanent sexual dimorphism can be nearly or completely independent of seasonal conditions and is characteristic of many worms, arthropods, and vertebrates. The males are much smaller than the females, for example, in nematodes, daph-nids, and ixodid ticks. In males, sexual dimorphism is associated with adaptations for holding the female during copulation, such as suckers on the anterior legs of the diving beetle. Females have adaptations for feeding their young and depositing their eggs, such as ovipositors in horntails and mammary glands in mammals. In insects, such as winter moths and coccidians, the females frequently have wings that are reduced, while the males have wings that are fully developed. The reverse can also be true (for example, in Blastophaga grossarum) but is less common. Males are often more brightly colored than females, for example, the males of many species of lycaenid butterflies and galliform birds. Because the females are more concerned with the offspring and are less mobile than the males, their duller coloration serves as protection. Other manifestations of sexual dimorphism are such secondary sex characteristics as the “horn” of a stag beetle, the tooth of a male narwhal, the tusks of a male elephant, and the antlers of many male deer, which are all used in combat for possession of the female.

Seasonal sexual dimorphism occurs only during the reproductive period and is known as breeding coloration in many fish and amphibians. During this period, the male minnow becomes brightly colored and the male newt develops a pecten and also becomes brightly colored.

In man. In males, sexual dimorphism is manifested by such characteristics as facial hair and a powerful skeleton and musculature. Females are characterized by the development of breasts and wider hips.

In plants. In plants, permanent sexual dimorphism is predominant. For example, the male hemp (staminate) differs from the female hemp (pistillate) by having a shorter stem, sparser foliage, and greater fiber yield. Sexual dimorphism is manifested in dioecious plants, such as the willow and Eucom-mia, by structural differences between male and female flowers (seeds develop only on female specimens during the fruiting period). In all organisms, sexual dimorphism develops as a result of natural and sexual selection.

M. S. GILIAROV

sexual dimorphism

[′sek·shə·wəl dī′mȯr‚fiz·əm]
(biology)
Diagnostic morphological differences between the sexes.
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