The role of male quality in sequential mate choice: pregnancy replacement in small mammals?

Abstract

Females mainly increase their reproductive success by improving the quality of their mates and need to be discriminative in their mate choices. Here, we investigate whether female mammals can trade up sire quality in sequential mate choice during already progressed pregnancies. A male-induced pregnancy termination (functional 'Bruce effect') could thus have an adaptive function in mate choice as a functional part of a pregnancy replacement. We used bank voles (Myodes glareolus) as a model system and exchanged the breeding male in the early second trimester of a potential pregnancy. Male quality was determined using urine marking values. Females were offered a sequence of either high- then low-quality male (HL) or a low- then high-quality male (LH). The majority of females bred with high-quality males independent of their position in the sequence, which may indicate a pregnancy replacement in LH but not in HL. The body size of the second male, which could have been related to the coercion of females by males into remating, did not explain late pregnancies. Thus, pregnancy replacement, often discussed as a counterstrategy to infanticide, may constitute adaptive mate choice in female mammals, and female choice may induce pregnancy replacement in mammals.

Keywords: male quality; pregnancy replacement; pregnancy termination; sequential mate choice; sexual selection.

Figure 1.

Birthday, total number and distribution of bank vole litters across experimental days 18–30 with sequential mate choice. Females were introduced to the first male at day 0. The first male was removed after 5–8 days (mean 7 ± 1 day) and replaced by the second male. Litters were assigned to males based on birth date, more than18 days after the introduction of the respective male (bank vole pregnancy lasting 20 ± 2 days [73], dataset 2), and by genetic paternity assignment during the critical period (days 25–29, dataset 1). Births are colour coded by assignment and dataset.

Figure 2.

The number of bank vole litters sired by the first and second males in a sequential mating treatment, where each male stayed with the female for one week. Males differed in quality (HL = high–low quality; HH = high–high quality; LH = low–high quality), and delivery of litters sired by the second male probably included a pregnancy replacement, indicating mate choice. Births are colour coded by assignment and dataset.