Mating strategy predicts the occurrence of the Bruce effect in the vlei rat Otomys irroratus

Abstract

[The Bruce effect is a phenomenon in which pregnant females terminate their pregnancies when exposed to strange males, and is hypothesised to be a female counter-strategy to infanticide risk. Since uncertain paternity decreases the incidence of infanticide, we tested the prediction that the Bruce effect would occur in mating systems where paternity is more certain (polygyny) rather than in mating systems where paternity is less certain (promiscuity). Using populations of the vlei rat Otomys irroratus that were either polygynous or promiscuous in nature, we studied the Bruce effect in captivity. Pregnancy termination almost always occurred in polygynous females but never in promiscuous females ( N = 10 per population). We expected that the signal–receptor (male–female) system of pregnancy termination is present in polygynous but not promiscuous populations and predicted an absence of the Bruce effect in encounters between individuals of different mating systems. This occurred in promiscuous females. However, 50% of polygynous females terminated their pregnancies, indicating that males of both mating strategies might carry signals to disrupt pregnancy but only polygynous females respond to these signals. The Bruce effect may be an adaptive response to infanticide risk in polygynous populations, although studies of free-living populations would be important to confirm our findings., The Bruce effect is a phenomenon in which pregnant females terminate their pregnancies when exposed to strange males, and is hypothesised to be a female counter-strategy to infanticide risk. Since uncertain paternity decreases the incidence of infanticide, we tested the prediction that the Bruce effect would occur in mating systems where paternity is more certain (polygyny) rather than in mating systems where paternity is less certain (promiscuity). Using populations of the vlei rat Otomys irroratus that were either polygynous or promiscuous in nature, we studied the Bruce effect in captivity. Pregnancy termination almost always occurred in polygynous females but never in promiscuous females ( N = 10 per population). We expected that the signal–receptor (male–female) system of pregnancy termination is present in polygynous but not promiscuous populations and predicted an absence of the Bruce effect in encounters between individuals of different mating systems. This occurred in promiscuous females. However, 50% of polygynous females terminated their pregnancies, indicating that males of both mating strategies might carry signals to disrupt pregnancy but only polygynous females respond to these signals. The Bruce effect may be an adaptive response to infanticide risk in polygynous populations, although studies of free-living populations would be important to confirm our findings.]