Sexual Selection and Life History Allocation


A comprehensive explanation for sexual trait diversity depends on integrating life history theory and sexual selection to compare costs and benefits of sexual traits. A universal cost for all sexual traits involves the resources required to create and maintain them, which are consequently unavailable to other life history characters. This resource trade‐off typically causes covariance between an organism's resource budget and its level of sexual trait expression, which is known as condition‐dependence. Condition‐dependence has several implications. It may be particularly important for ornaments signalling genetic quality, by helping to maintain the genetic variation that favours mate choice. It may also reduce extinction risk in sexually selected populations. Further life history studies of sexually selected traits are required, mindful of the inherent difficulties in quantifying resource acquisition and allocation, not least of which is that these processes are not independent of one another.

Key Concepts:

  • Trait evolution is restricted by trade‐offs, where the benefit of increased investment in a trait is opposed by the cost of investment.

  • The acquisition and allocation of resources is central to life history theory, but these processes are extremely difficult to quantify.

  • Phenotypic correlations are not necessarily good metrics for the strength or importance of a trade‐off between traits.

  • Condition‐dependence in mating systems featuring choice for indirect genetic benefits can help to maintain genetic variation for ornamental traits.

  • Sex‐role reversed systems have life history features that may clarify some of the outstanding questions in sexual selection.

Keywords: sexual selection; life history; condition‐dependence; acquisition; allocation; fitness; trade‐offs; genetic quality

Figure 1.

The Y‐model: (a) The total energy, A, acquired by the individual, and the fraction, B, that it allocates to the life history trait R. The remaining fraction is allocated to the life history trait S. (b) Population wide variation in A is large and variation in B is small, leading to a positive phenotypic correlation between R and S (see hatched area). (c) The reverse case. Figure reproduced with permission from Van Noordwijk and De Jong .

Figure 2.

An allocation trade‐off exists between weapons (thoracic horns) and testes in the beetle Onthophagus nigriventis. (a) Large males produce two horns on the thorax. (b) Cauterised males grew larger and invested disproportionately in testes compared to horned males. (Inset) Residual testes weight for cauterised and control males after controlling for body weight. (c, d) Analysis of 25 congeneric species shows that allometric slope steepness for log testes weight against log body weight declines as both body size (c) and horn allometry steepness (d) increases. These results indicate the decreasing marginal benefits of investment in testes with increasing horn and body size, presumably because larger males can better monopolise their mates and avoid sperm competition altogether. Figure reproduced with permission from Simmons and Emlen .

Figure 3.

Male Rhamphomyia dance flies prefer females with larger, more mature eggs. The effect of egg length on abdominal area in females of (a) R. sociabilis, a species in which females cannot inflate their abdomens, reveals a strong positive relationship. In R. longicauda (b), abdominal area is a poorer indicator of egg length. Females of this species inflate their abdomens by swallowing air before entering the mating swarm to exaggerate their apparent size. Figure reproduced with permission from Funk and Tallamy .



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Houslay, Thomas M, and Bussière, Luc F(Jan 2012) Sexual Selection and Life History Allocation. In: eLS. John Wiley & Sons Ltd, Chichester. [doi: 10.1002/9780470015902.a0023667]