Sexual Dimorphism

Sexual dimorphism is the presence of nongenital physical differences between males and females of the same species. These differences may be in coloration, body size, or physical structures, and can be quite striking. Male and female mallard ducks (Anas platyrhynchus ) are so different in appearance that they were originally classified as different species. Mule deer bucks have antlers while females do not, male peacocks display an elaborate tail not found in females, and male elephant seals may weigh 2.5 tons (2.25 metric tons) more than their mates. All these differences are examples of sexual dimorphism.

The word "dimorphic" means "having two forms." Species in which males and females appear identical are called sexually monomorphic. Sexual dimorphism generally results from different reproductive roles and selective pressures for males and females. Scientists have proposed several hypotheses to explain the existence of sexual dimorphism, including niche differentiation , sexual selection through intrasexual competition, and sexual selection through female choice.

Niche differentiation

The niche differentiation hypothesis states that the differences between males and females of a species allow them to exploit different food resources and thus reduce competition for food. For example, male raptors (hawks, falcons, eagles, and owls) are considerably smaller than females. This difference is especially pronounced in raptors that prey on birds, such as the Cooper's hawk (Accipiter cooperii ). The small male Cooper's hawks are better than their larger mates at catching small, fast birds. The male catches these often-abundant small birds to feed to his mate as she incubates the eggs and later while the chicks are young. Once the young hawks are bigger, the female hunts for larger birds in the area to feed the growing chicks.

Intrasexual Competition

In 1871 Charles Darwin suggested that larger, more aggressive males would be more successful in competing for females. Intrasexual competition results in males that are larger, stronger, or equipped with different physical traits than females because they must compete with other males to win access to mates. American elk (Cervus elaphus ) bulls have large antlers and use them aggressively to fight for harems of females. A harem is a group of adult females defended by one male from other males in the area. This dominant male mates with most if not all females in the group. Bulls that have large antlers and outweigh their opponents are able to dominate their opponents and pass on their genes for size to their offspring. Elk cows are not involved in such battles and have no need for antlers or such massive bulk.

Finally, the female choice hypothesis states that females of dimorphic species prefer to mate with males that are colorful, large, or ornamented. If this is the case, the flashier, larger, or more-ornamented males are more likely to sire offspring and pass on their genes than their dull, small, plain competitors. Why females might prefer to mate with elaborate males is a matter of debate. It may be that the traits advertise the males' ability to find food or avoid predators.

Sexual Selection

By mating with obviously successful males, females increase the chances that their offspring will inherit their father's traits and also be successful. It may also be that some sexually selected traits, such as the peacock's unwieldy tail, advertise the male's ability to escape predators in spite of a severe handicap. Longer tails make it harder for the males to run or fly away, so males that have particularly long tails and yet manage to escape predators must be particularly strong and fast. Competition between males, and female choice among males are both forms of sexual selection. This means that they cause differences in mating success of individuals and therefore result in adaptations for obtaining mates.

In species that are sexually dimorphic, the male is usually the bigger, more brightly colored, or more elaborately ornamented sex. However, there are several cases where this pattern is reversed, such as in raptors and toads. The female is often the larger sex in polyandrous birds such as phalaropes, where brightly colored females compete to attract male mates. This unexpected situation, sometimes called reverse sexual dimorphism, led the American ornithologist and painter John James Audubon (1785-1851) to incorrectly label the male and female in all his phalarope paintings. The plumage patterns of phalaropes make sense in light of sexual selection, but also highlights the importance of selection for dull plumage. Male phalaropes incubate the eggs and take care of the young with no help from the female. Their cryptic (meaning blending into the surroundings) brown plumage makes them difficult to see on the nest and helps them avoid predators. In general, the sex that cares for the young is under strong selection for dull coloration.

The hypotheses discussed here are not mutually exclusive, and different species may be more or less affected by various selective pressures. Why some species are sexually dimorphic and others are not is an active area of research in evolutionary biology.

Emily DuVal

Bibliography

Darwin, Charles. The Descent of Man, and Selection in Relation to Sex. London: John Murray, Albermarle Street, 1871.

Ehrlich, Paul R., David S. Dobkin, and Darryl Wheye, eds. The Birder's Handbook: A Field Guide to the Natural History of North American Birds. New York: Simon and Shuster, Inc., 1988.

Owens, Ian P. F., and Ian R. Hartley. "Sexual Dimorphism in Birds: Why Are There So Many Different Forms of Dimorphism?" Proceedings of the Royal Society of London, series B. 265 (1998):397-407.

Pough, F. Harvey, Christine M. Janis, and John B. Heiser. Vertebrate Life, 5th ed. Upper Saddle River, NJ: Prentice Hall, 1999.

Zahavi, Amotz, and Avishag Zahavi. The Handicap Principle: A Missing Piece of Darwin's Puzzle. New York: Oxford University Press, 1997.

Sexual Dimorphism

The natural occurrence of physical differences between males and females is referred to as sexual dimorphism. Often, these physical differences are quite striking and obvious, such as the differences seen in humans where external genitalia at birth can usually be used to tell boys from girls unambiguously. As children develop and mature into adulthood, a whole host of other physical traits emerge such as body hair patterns, breast development, and a wide array of other growth characteristics. While it might seem self-evident that physical differences exist between males and females, some species, such as Quaker parrots, do not exhibit any outward differences, and cannot be sexed externally. Even avian experts must rely on genetic testing to sex these birds. Quaker parrots could, therefore, be described as sexually monomorphic.

The sex of an animal can oftentimes be determined by external physical traits such as overt differences in the appearance of the external genitalia. Dogs, for example, are sexually dimorphic at this level as one can easily determine gender from observation of external genitalia, even at a distance. Other species, such as hamsters, exhibit differences in external genitalia that are more subtle and require careful examination. Many birds may be sexed by the scientist, despite a lack of observable differences in external genitalia, because of striking differences in coloration patterns in the feathers.

In fruit flies (Drosophila melanogaster ), it has long been recognized that the ratio of X-chromosomes to Y-chromosomes is the primary determinant of whether the embryo will develop as a male or as a female. Males can easily be discriminated from females based on external differences such as length and coloration patterns of the abdomen, and the presence of sex combs being limited to the foreleg of males. There are indeed differences in external genitalia of fruit flies, but these differences are far more subtle, making discrimination of gender on the basis of external genitalia impractical. There has recently been a revolution in scientific understanding of how sex-determination in fruit flies (Drosophila ) generates sexual dimorphism in somatic tissues at the molecular level. The mechanisms for sex determination alter the activities of various signaling molecules and transcription factors within cells to direct various sex-specific elements of growth and differentiation.

In flowering plants, there are two dimorphic breeding systems that are fairly widespread among species that develop seeds within an ovary. The first system, called dioecy, involves males and females. Male expression in plants involves stamen and pollen production. Female expression involves production of the pistil and ovaries. The second and more common system, called gynodioecy, involves females and hermaphrodites (plants which express both male and female components). Hermaphrodites are individuals that produce both male and female sexual parts. Hermaphrodites are very common among plants. Conditions within the environment such as the availability of water or soil nutrients can alter the sexual expression in hermaphroditic plants, resulting in differences in the balance of male to female flowers over time.

The concept of sexual dimorphism can be applied at many different levels. Thus, while one might ask at the most basic level whether males and females are physically different and therefore distinguishable from one another, the question of sexual dimorphism can be applied toward specific traits, both internal and external. Differences in hormone levels betweens males and females constitute a kind of sexual dimorphism of their own at the biochemical level. Genetic differences betweens males and females, even prior to the rise of hormonal differences, can give rise to differences in both structure and function in the brains of vertebrate animals when comparing males and females. Even in cell culture , response to hormonal supplementation can be different in male and female neurons even when the neurons in culture are taken from the embryo prior to time that the testosterone surge masculinizes the male embryonic brain. This leads to differences in structural development as well as differences in the biochemical environment. One can even consider behavioral traits to be sexual dimorphisms if the patterns of behavior are consistently different between males and females.

Evidence of sexual dimorphism may be seen even in the circadian rhythms (daily physical patterns) of males compared with females in many species. Careful study of the development and the differences in circadian rhythms in male and female rodents shows that differences arise after the onset of puberty and require the presence of hormones produced by the testes or ovaries. Removal of the testes or ovaries in animals prior to the onset of puberty prevents the development of distinctive changes in circadian rhythms normally seen shortly after puberty, even when sex-specific hormones are applied.

In the most general sense, any aspect of physical structure, coloration, gene expression, physiology , biochemistry, or behavior that shows evidence of differences between males and females can be described as a sexual dimorphism. The existence of sexually dimorphic traits at so many different levels of function and development provides researchers with insights into the meaning of sex within nature.

see also Anthropology; Biometrics; Gene; Physiology.

sexual dimorphism Phenomenon of morphological differences (besides primary sexual characters) that distinguish the males from the females of a species. For example, male deer often have larger antlers than females, and the males of many birds have differing plumage (often more brightly coloured). Sexual dimorphism is known to have been common in ammonites (Ammonoidea), and many fossils originally thought to have represented separate species are now recognized as dimorphs within one species (e.g. the Jurassic Kosmoceras jason and K. gulielmi are dimorphs within K. jason). See also DIMORPHISM.

sexual dimorphism The occurrence of morphological differences (other than primary sexual characters) that distinguish males from females of a species of organism (e.g. male deer often have larger antlers than females, and the males of many birds have differing (often more brightly coloured) plumage).

sexual dimorphism The occurrence of morphological differences (other than primary sexual characters) that distinguish males from females of a species of organism (e.g. male deer often have larger antlers than females, and the males of many birds have differing (often more brightly coloured) plumage).