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 () 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.