**Author details**

10 Sexual Dimorphism

attention.

**4. Conclusion** 

of which are still unknown.

sex-linked genes determine the allocation of traits differently in males and females. Any variation of these genes due to plasticity may then determine the genetic relationship of sexual shape dimorphism and differing conditions. Many genetic hypotheses continue to examine sexual size dimorphism and just recently is sexual shape dimorphism receiving

Although studies are currently underway, many questions about sexual shape dimorphism still remain. For instance, how frequently is sexual shape dimorphism exhibited and how is this related to ontogenetic and biomechanical influences? Worthington *et al*. [83] propose that the sexually dimorphic patterns seen in the stalk-eyed fly are due to sexual selection, but also to biomechanical and possibly ontogenetic constraints. However, more information is necessary before a conclusion can be made about the actual process underlying the pattern of sexual shape dimorphism. Kaliontzopoulou *et al.* [82] suggest that a lack of sexual differences in cranial development of *Podarcis* species may indicate an ontogenetic limitation on both sexes, but also note that the habitat appears free of head constraint. The examination of ontogenetic development as well as biomechanical constraints on sexual shape dimorphism may reveal much about the causes and selective forces of these patterns, many

Does sexual shape dimorphism follow well-known patterns of sexual size dimorphism, such as Rensch's Rule? How much impact does allometry have in driving the evolution of sexual shape dimorphism? Although patterns such as these have been suggested as a component of sexual shape dimorphism, only recently have researchers begun to investigate these patterns. Is allometry in sexual shape dimorphism common? Berns and Adams [58] did not find a significant effect of allometry, whereas Worthington *et al.* did [83]. In species of *Drosophila melanogaster*, allometry did not explain the majority of evolutionary divergence of sexual shape dimorphism [45], while in *Podarcis bocagei* and *P. carbonelli,* Kaliontzopoulou *et al*. [82] showed that allometry was present and even differed in males and females. These inconcordant results suggest that there is a multifaceted interaction between sexual size dimorphism, sexual shape dimorphism and allometry. Examining size alone shows only a piece of the mechanisms contributing to allometry, thus attention needs to also focus on

As seen in the examples in this chapter, much of the evidence on processes underlying sexual shape dimorphism is incongruent. One area needing attention is that of the correlation between sexual shape dimorphism and fecundity advantage, as shape may impact egg carrying capacity as size does. More work is needed to assess genetics and sexual shape dimorphism, and studies continue to argue that sexual selection causes sexual shape dimorphism due to male-male combat and mate choice, while others argue for natural selection via environmental factors and interspecific competition. No doubt that all of these factors play a role in influencing the evolution of sexual shape dimorphism, but what are the patterns? Do vertebrates tend to follow one trend while invertebrates follow another? In

allometry and it's relationship with sexual shape dimorphism.

Chelsea M. Berns *Department of Ecology, Evolution and Organismal Biology, Iowa State University, Ames, Iowa, USA* 
