How Do Infanticidal Male Bank Voles (Myodes glareolus) Find the Nest with Pups?

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

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.

Rodent mothers increase vigilance behaviour when facing infanticide risk

Infanticide, the killing of unrelated young, is widespread and frequently driven by sexual conflict. Especially in mammals with exclusive maternal care, infanticide by males is common and females suffer fitness costs. Recognizing infanticide risk and adjusting offspring protection accordingly should therefore be adaptive in female mammals. Using a small mammal (Myodes glareolus) in outdoor enclosures, we investigated whether lactating mothers adjust offspring protection, and potential mate search behaviour, in response to different infanticide risk levels. We presented the scent of the litter's sire or of a stranger male near the female's nest, and observed female nest presence and movement by radiotracking. While both scents simulated a mating opportunity, they represented lower (sire) and higher (stranger) infanticide risk. Compared to the sire treatment, females in the stranger treatment left their nest more often, showed increased activity and stayed closer to the nest, suggesting offspring protection from outside the nest through elevated alertness and vigilance. Females with larger litters spent more time investigating scents and used more space in the sire but not in the stranger treatment. Thus, current investment size affected odour inspection and resource acquisition under higher risk. Adjusting nest protection and resource acquisition to infanticide risk could allow mothers to elicit appropriate (fitness-saving) counterstrategies, and thus, may be widespread.