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.