SEXUAL CONFLICT AND SEXUAL SELECTION: LOST IN THE CHASE

Risking infanticide or losing precious time? Delay of reproduction in a short-lived mammal

Nonparental infanticide, or killing of conspecific young infants, is an extreme form of sexual conflict that is widespread throughout the animal kingdom. One of the female counterstrategies to reduce the damage of infanticide is the "Bruce effect," ie the termination of a pregnancy sired by the former breeding male after the invasion of a new breeding male. Using bank voles (Myodes glareolus) as experimental subjects we phenotyped males for infanticidal types (attacking unrelated pups or not), and we exchanged the breeding male in the early second trimester of a potential pregnancy, allowing the female to terminate the pregnancy and breed with the new male (pregnancy replacement). We found that the proportion of late litter was significantly higher if the second male encountered was infanticidal, and males' probability to reproduce was affected by both infanticidal tendency and the sequence in which it was presented to the female. We found no connection between infanticidal tendencies and male quality, and females' choice was not affected by male quality or female parity. Consequently, the infanticidal status of the males and the sequence in which the males are presented may trigger females to exchange a pregnancy and reproduce with an infanticidal male. Thus, the functional Bruce effect may be considered as a form of sequential mate choice.

How is female mate choice affected by male competition?

The plethora of studies devoted to the topics of male competition and female mate choice belie the fact that their interaction remains poorly understood. Indeed, on the question of whether competition should help or hinder the choice process, opinions scattered throughout the sexual selection literature seem unnecessarily polarised. We argue, in the light of recent theoretical and empirical advances, that the effect of competition on mate choice depends on whether it results in the choosy sex attaining high breeding value for total fitness, considering both direct and indirect fitness benefits. Specifically, trade-offs may occur between different fitness benefits if some are correlated with male competitive ability whilst others are not. Moreover, the costs and benefits of mating with competitive males may vary in time and/or space. These considerations highlight the importance of injecting a life-history perspective into sexual selection studies. Within this context, we turn to the sexual selection literature to try to offer insights into the circumstances when competition might be expected to have positive or negative implications for pre-copulatory female choice. In this regard, we elaborate on three stages where competition might impact upon the choice process: (i) during mate detection, (ii) mate evaluation, and (iii) in dictating actual mating outcomes. We conclude by offering researchers several potentially rewarding avenues for future research.