The evolution of infidelity in socially monogamous passerines: neglected components of direct and indirect selection

A critical evaluation of Arnqvist and Kirkpatrick's (2005) study suggests that their approach is likely to provide great insight in the future when it is possible to quantify all the different forms of selection acting on both male and female behavior. In addition to demonstrating the potential power of the approach they used to understand questions in evolutionary biology, the study by Arnqvist and Kirkpatrick (2005) has stimulated renewed interest in a number of areas surrounding the central question of female polyandry in socially monogamous birds. As discussed above, there are a number of important forms of selection that Arnqvist and Kirkpatrick (2005) either did not include in their model or which are currently very difficult to quantify. As in other taxa (Bretman et al. 2004), it is quite possible that all females in a population are copulating with multiple males to use a genetically loaded raffle (Ball and Parker 2003) to find the most genetically compatible partner and increase their direct and indirect fitness (Jennions 1997). Such a scenario would constitute a strong positive selective force on polyandrous behavior by females and yet would be very difficult to detect because there is no forensic trace of EPCs that do not result in EPP, and investigations of postcopulatory female choice and infertility in wild birds are extremely challenging. Nonetheless, these neglected possibilities could significant alter our perspective of the true function of EPP to males and females, and they are deserving of further empirical work.