**The Evolution of Sexual Dimorphism: Understanding Mechanisms of Sexual Shape Differences**

Chelsea M. Berns

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/55154

### **1. Introduction**

Understanding the origin of biodiversity has been a major focus in evolutionary and ecological biology for well over a century and several patterns and mechanisms have been proposed to explain this diversity. Particularly intriguing is the pattern of sexual dimorphism, in which males and females of the same species differ in some trait. Sexual dimorphism (SD) is a pattern that is seen throughout the animal kingdom and is exhibited in a myriad of ways. For example, differences between the sexes in coloration are common in many organisms [1] ranging from poeciliid fishes [2] to dragon flies [3] to eclectus parrots (see Figure 1).

**Figure 1.** A) Male Eclectus (© Stijn De Win/Birding2asia) B) Female Eclectus (© James Eaton/Birdtour Asia)

© 2013 Berns, licensee InTech. This is an open access chapter distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2013 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Sexual dimorphism is also exhibited in ornamentation, such as the horns of dung beetles [4], the antlers of cervids [5], and the tail of peacocks [6]. Many species also exhibit sexual differences in foraging behavior such as the Russian agamid lizard [7], and parental behavior and territoriality can be dimorphic in species such as hummingbirds [8, 9]. Another common pattern is that of sexual size dimorphism, such as is observed in snakes [10] and monk seals [11].

The Evolution of Sexual Dimorphism: Understanding Mechanisms of Sexual Shape Differences 3

Sexual size dimorphism is a frequent phenomenon where the size of males and females of the same species differ (see Figure 2), driven by one or more of the mechanisms mentioned above. When these processes occur in closely related species, distinct patterns of amongspecies size dimorphism can result, one of which is termed 'Rensch's Rule' [31]. Rensch's rule is a pattern wherein the degree of sexual size dimorphism increases with body size in species where males are the larger sex, and conversely decreases in those species where

**2. Processes and patterns of sexual size dimorphism** 

Photograph by: http://www.joshsfrogs.com/catalog/blog/category/poison-dart-frog-care

**Figure 3.** Rensch's Rule, where in species above the broken line (broken line denoting where female and male sizes are equal) females are larger than males and below, males are larger than females. From

**Figure 2.** Sexual size dimorphism in poison dart frog.

R. Colwell, Am. Nat., 2000.

females are the larger sex (see Figure 3).

There are many mechanisms that drive the evolution of SD, the most accepted mechanism being sexual selection [12-14], which enhances fitness of each sex exclusively in relation to reproduction [15, 16]. This states that SD evolves in a direction such that each sex (especially males, see 17) maximizes reproductive success in two ways: by becoming more attractive to the other sex (inter-sexual dimorphism) or by enhancing the ability to defeat same-sex rivals (intra-sexual dimorphism), in both cases such that each sex increases the chances to mate and pass genes on to the next generation. Many researchers have argued that competition for mates is at the very heart of sexual selection because these rivalries greatly influence mating and fertilization success. Indeed, competition for mates has been shown to be the major factor impacting SD in several taxa [18]. However the complexity of SD cannot be explained by a single mechanism.

Mate choice is an important proximate mechanism of sexual selection. Often the sex with the higher reproductive investment is the 'choosy' sex. Patterns then emerge, such as those consistent with the 'sexy son' hypothesis [19], where females prefer mates with phenotypes signifying fitness. The females prefer males that are phenotypically 'sexy' to ensure that the genes of their offspring will produce males that will have the most breeding success, propagating her genes successfully [16, 20]. Taken further, sometimes females prefer males that exhibit very extreme phenotypes within a population. Over evolutionary time these traits become increasingly exaggerated despite the potential fitness costs to the males themselves, termed Fisherian runaway sexual selection [19]. Examples include the tails of male peacocks, plumage in birds of paradise and male insect genitalia [14, 21, 22].

Alternatively, ecological mechanisms, such as competition for resources, may exert distinct selective forces on the sexes resulting in the evolution of SD [23]. Here, intraspecific competition in species-poor communities may allow divergent selection between the sexes (rather than between species), resulting in sexual niche segregatation [12, 24-26]. In this case morphological traits often change to minimize this intersexual competition. Other ecological hypotheses have been proposed to explain patterns of SD, such as the influence of sexspecific divergence in response to environmental gradients (i.e., intersexual niche packing: sensu 27]. For example, both sexes of fruit flies *Drosophila subobscura* increase in body size with latitude, however in South America these size increases are less steep and weaker in males as compared to females [28]. Another study found weaker latitudinal clines in males as compared to females in houseflies *Musca domestica* [29], and yet another study found geographical variation in climate that corresponded to a change in the magnitude of sexual size dimorphism between males and females [30]. Hypotheses continue to be proposed and the explanations for the evolution of SD may not be mutually exclusive but instead, may operate in a synergistic or antagonist fashion to shape these patterns.
