McAfee SECURE sites help keep you safe from identity theft, credit card fraud, spyware, spam, viruses and online scams

What role does condition dependence play in sexual selection?

Sexual selection is the dominant theory for the evolution of certain traits in animals. In 1871, Charles Darwin proclaimed that sexual selection was qualitatively different from the process of natural selection and likened this process of sexual trait evolution to artificial selection in domesticated animals.  Observations have shown that in several animal species, males contain conspicuous traits, or ornaments, which appear to be redundant to the functioning and survival, despite the favorability of their female counterparts in selecting individuals with these particular traits. (Kotiaho et al 2002) Darwin offered that these traits are maintained in a species through a process of sexual selection, as opposed to natural selection, as expressing them does not enhance the individual's rate of survival in any way. (Darwin 1871) This suggests that the secondary sex traits expressed by males must be indicative of attributes which will be advantageous to the female if she mates with the male displaying the ornament. These attributes could be a superior immune system for example, or a favourable genetic make-up. There is much evidence however, to show that expression of the ornaments or weapons displayed to attract prospective mates is costly for the bearer and from empirical observations have come models of sexual selection which predict that sexual traits are expressed in proportion to the condition of their bearer. (Andersson 1986; Darwin 1871; Kotiaho et al 2002) The role of condition dependence in sexual selection is discussed herein and we give some consideration to the reliability of experimental evidence leading to the theory of condition dependence in this instance.

Traditionally, two mechanisms are involved in the sexual selection process.  The first mechanism is that of intrasexual selection involving competition between mates of the chosen sex, usually males, for access to females. Competition over mates has resulted in the evolution of weaponry traits such as horns and antlers to increase the success of mates in competitions for females. Large male size has also evolved to provide some individuals with a size advantage over others. (Andersson 1986; Duckworth 2004) The second mechanism of sexual selection involves intersexual selection, or mate choice by the choosy sex (usually females). Mate choice has resulted in the evolution of many secondary sexual traits such as the long and elaborate tail of the peacock, highly specialized courtship displays and calling songs. (Lorch et al 2003; Kotiaho et al 2002)

More experimental investigation into condition dependence is required if we are to better understand the relationship between the expression of sexual traits and condition dependence. (Bakker et al 1999; Candolin 1999, 2000; Holzer et al 2003; Andersson 1986; Møller et al 2003) Most studies which have investigated the condition dependence of certain sexually selected traits and their effect on female mate choice have been performed under experimental conditions and not in nature and the reliability of the results is thus questionable. (Andersson 1986). In addition, the studies cannot divulge information on how males make optimal use of resources whilst exhibiting the sexually selected trait in a natural habitat. Variation in resource availability, temperature, intraspecific competition, as well as risk of predation and parasitism may influence the trade-off outcome between investment in the trait and other life history traits, as mentioned above. (Anderson 1986; Holzer et al 2003)

Scrutiny of the term condition and what constitutes it is hugely important when considering condition dependence in sexual selection as quantification of condition is very difficult. Kotiaho (1999) terms the phenomenon 'abstract' and claims that it is near impossible to 'really know what constitutes a good empirical measure of condition'. (Kotiaho 1999). In terms of sexual selection models, it is agreed by some that condition must be seen as the individual's ability to reflect the overall source of resources which are available for the investment in different traits. Thus a high degree of fitness is attributable to condition. (Cotton et al 2007) The failure to agree on a quantitative measure of condition has remained a limiting factor in the ability to make predictions about sexually selected traits and will continue to do so.

Related Links
To Top