Sexual+Dimorphism

Kelly Gilbert

** Sexual Dimorphism **

**Overview of Sexual Dimorphism** Sexual Dimorphism is a representation of the differences that exist between males and females of the same species. The differences may be represented through variations in color, structure, shape, and size (Hedrick et al., 1989). Sexual dimorphism is a result of genetic inheritance (Hedrick et al., 1989). Darwin hypothesized that it evolved through sexual selection and is therefore intrinsically linked to intersexual and intrasexual selection (Darwin, 1874). In fact, Darwin’s book from over 100 years ago, __The Descent of Man,__ contains a comprehensive overview of sexual dimorphism. The differentiation of traits is valuable in terms of selecting, recognizing, and choosing a mate. Rather than increasing a species’ ability to obtain food or survive, sexual dimorphism favors characteristics that enhance an individual’s likelihood of finding, attracting, and fighting for access to mates (Darwin, 1874).

**Mating Systems** The amount of dimorphism depends on the type of mating system in place. Sexual dimorphism is important in determining a species’ likelihood of practicing monogamy versus polygyny. For example, species that are more similar in terms of sexual dimorphism are more likely to be monogamous (Hendrick et al., 1989). Monogamy is the sexual bonding of an individual with one mate; it is the practice of choosing and maintaining one sexual partner (Davies, 1988). Conversely, species that display high levels of sexual dimorphism and have males who are brightly colored, dangerous, or are large are more likely to be polygynous (Hendrick et al., 1989). In polygynous cultures, multiple females mate with a single male (Davies, 1988). Males with sexually dimorphic traits can breed with more females and have more access to them, specifically in polygynous species, through competition or female choice (Davies, 1988). This is likely due to sexual selection.

**Role of Sexual Selection in Sexual Dimorphism** High levels of sexual dimorphism, as often seen in polyandrous groups, are useful in terms of intrasexual selection. Intrasexual selection determines mating success through interactions between members of a species that are the same sex (Buss, 1988). Examples of this include male-male combat, female competition in polyandrous societies, and sperm competition (Buss, 1988). Dimorphism gives individuals a competitive advantage in obtaining a mate. For example, male elephant seals participate in bloody fights in order to mate with females. Since male elephant seals weigh three times as much as females, their increased size gives them an advantage when fighting. On the other hand, intersexual selection competition between sexes of a species for mating success, such as female choice of males, or epigamic selection (Buss, 1988).

**Ornamentation** One of the most famous examples of sexual dimorphism is peacocks (Zahavi, 1975). The male peacock flaunts extravagant and colorful feathered tails while the females are short-tailed in comparison and are a grey-brown color. Sexual dimorphism may seem confusing and unnecessary since the ornamental tails are costly for males to develop and maintain (Hendrick et al., 1989). However, the dimorphism may help communicate to a potential partner how fit and genetically healthy an individual is (Hendrick et al., 1989). Zahavi’s “handicap” hypothesis explains this by saying that only males that are in good condition and have a good set of genes will be able to produce and maintain the ornamental tails (Zahavi, 1975). Thus, the tails serve as an indicator to a female that a male is of high genetic quality and is worthy of producing offspring with (Zahavi, 1975). Another explanation for choosing traits that do not improve survival of offspring, and may even reduce it, is explained by the sexy son hypothesis. Essentially, it explicates that females must choose a mate that has a trait that is generally perceived to be attractive (Weatherhead et al., 1979). If the female does not choose the attractive trait, her son will not be attractive and will therefore not be chosen to mate with selective females in the future (Weatherhead et al., 1979). In order to produce a son that will result in the most grandchildren, the female must choose a mate that has a trait, such as a peacock tail, that is considered attractive regardless of its value for survival (Weatherhead et al., 1979).

**Natural vs. Sexual Selection in Determining Sexual Dimorphism** Sexual dimorphism may result from natural or sexual selection. (Lande, 1980). Although sexual dimorphism is often linked with sexual selection, natural selection can also differ between sexes (Zahavi, 1975). An example of natural selection causing sexual dimorphism is that of fecundity selection (Lande, 1980). Whereas mating success is defined as the number of mates, female fecundity is the number of offspring produced. Since females have far fewer eggs than males do sperm, and must take the time to gestate and give birth to the offspring, fecundity often causes females to be more selective when choosing mates (Lande, 1980). This has been theorized to lead to larger females in some species since larger females can produce more eggs. Another example of natural selection causing sexual dimorphism is niche divergence. In niche divergence, males and females fill different roles in an ecological niche and evolve to reduce competition for limited resources (Lande, 1980).

**Sexual Dimorphism Examples** Some noteworthy examples of sexual dimorphism can be seen in peacocks and elephant seals as previously discussed, in addition to ducks, spiders, gorillas, and orangutans. For example, most people recognize male mallards from female mallards. The male ducks are more recognizable for their bright green heads and light bodies; whereas the females are monochromatically brown (Ralls, 1977). Spiders are a great example of sexual dimorphism producing females that are significantly larger than their male counterparts. In fact, most insects, in general, have larger females than males. On the contrary, gorillas and orangutans are both examples of species whose males are notably larger than the females (Ralls, 1977). Female orangutans weigh about half as much as males and are resultantly able to stay in trees (Ralls, 1977). Additionally, it is easy to determine the gender of an orangutan due to the fleshy pads surrounding the faces of males. Similarly, male gorillas weigh about 400 pounds while females weigh on average 155 pounds (Ralls, 1977).

References Buss, D. M. (1988). The Evolution of Human Intrasexual Competition: Tactics of Mate Attraction. //Journal of Personal and Social Psychology, 54//, 616-628. Darwin, C. (1874). //The Descent of Man. London. // Davies, N. B. (1989, August). Sexual Conflict and the Polygamy Threshold. //Animal Behaviour, 38//(2), 226-234. Hedrick, A. V., & Temeles, E. J. (1989, May). The Evolution of Sexual Dimorphism in Animals: Hypotheses and Tests. //Trends in Ecology and Evolution, 4//(5), 136-138. Lande, R. (1980, March). Sexual Dimorphism, Sexual Selection, and Adaptation in Polygenic Characters. //Evolution, 34//, 292-305. Ralls, K. (1977, September). Sexual Dimorphism in Mamals: Avian Models and Unanswered Questions. //The American Naturalist, 111//(981). Weatherhead, P. J. (1979, February). Offspring Quality and the Polygyny Threshold: “The Sexy Son Hypothesis”. //The American Naturalist, 113//(3). Zahavi, A. (1975). Mate Selection - a Selection for a Handicap. //Journal of Theoretical Biology, 53//, 205-214.