The evolution of female mating preferences: quantifying the benefits

Understanding the mechanisms that generate and maintain the spectacular phenotypic variation we see in nature is one of the central aims of evolutionary biology. Some of the most eye-catching and intriguing traits are associated with attracting a mate. The enormous and colourful train of the peacock, the extravagant courtship dance of male birds of paradise and the extraordinary stalks of stalk eyed flies- all are a result of female preferences for mating with the most elaborate male. Yet, surprisingly, scientists still do not have a complete understanding of why females should have these preferences; what benefits do females gain from choosing more ornamented males? Current thinking divides the potential benefits into two categories: direct benefits and indirect benefits. Direct benefits are resources females gain directly from the male such as gifts of food, or care for the offspring. In these, the amount or quality of resources a male provides is related to the extravagance of his trait: the more extravagant the male, the better the resource he brings. In contrast, with indirect benefits, the extravagance of a male's trait indicates the quality of the genes he is carrying. Females mating with an extravagant male will therefore produce offspring carrying better genes, and so these offspring will have a greater chance of surviving or reproducing (they will have higher fitness). Theoretical models show that direct benefits can easily explain the evolution of female preferences for extravagant males where they exist. However, direct benefits seem to be absent in many systems, and in these cases indirect benefits are assumed to occur instead. However, the importance of indirect benefits is actually rather controversial. For example, indirect benefits are predicted to be small, making it unlikely that indirect benefits will outweigh the costs to females of expressing a preference. In addition, indirect benefits can be split into good genes benefits and compatible genes benefits. Good genes benefits result from genes a male possesses that will improve the fitness of his offspring regardless of which female he mates with. Compatible genes benefits, on the other hand, result from a particular male-female combination producing offspring with high genetic value for fitness (i.e. particular males and females have genes which are compatible with one another). If females choose males based on compatible genes benefits, different females are not expected to prefer the same males and so female mating preferences will not lead to the evolution of extravagant male traits. Whether females choose based on good or compatible genes benefits depends on two things: the relative magnitude of these benefits and the existence of signals enabling females to choose between males based on these benefits. However, estimates of these two things are rare. In addition, direct and indirect benefits to female mate choice both might occur in the same species, but at the moment they are typically examined in isolation, and so we have little idea of how important they are relative to one another. I propose to provide estimates of these key missing parameters and therefore provide new insights into the potential for direct and indirect benefits to play a role in the evolution of female mating preferences. In the process, this research will generate data that will improve our understanding of the genetic basis to phenotypic traits, a fundamental aim of evolutionary biology that affects our understanding of how genotypes translate to phenotypes and our predictions of how phenotypes will change in response to selection.