Pregnancy replacement in mammals – all costs or hidden benefits for females?

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The adaptive value of many behavioural processes may relate to very specific biological or social situations. The Bruce effect reported from different mammalian taxa describes the termination of a pregnancy (very early, in the original definiton of the effect) in reaction to the confrontation to a new, unknown male. It probably has high energetic costs for the female, especially if a broader, less mechanistic definition of the pregnancy termination also in later stages of the pregnancy is applied. The adaptive value has been debated as a counterstrategy to potential future infanticide by the new male (infanticide avoidance hypothesis), but also lab artefacts were considered since in rodent species the effect was reported only from laboratory conditions. We showed in Eccard et al. 2017 that in rodent pairs under experimental, near natural conditions in isolated, breeding pairs half of the test females replaced pregnancies when encountering a new male, which raised questions about the adaptive value and biological function of the process.In the proposed project I aim to experimentally disentangle potential adaptive and mechanistic explanations for pregnancy replacements, focussing rather on the replacement then the termination of pregnancies. Low population density forces females to reproduce with their relatives, so that in the event of an immigration by an unknown male, a pregnancy replacement may allow the female to increase her genetic dissimilary to sire (inbreeding avoidance). Further, the process could be a mechanism of sequential mate choice and should thus depend on the relative quality of original and replacement male (male quality). Hypotheses will be tested in near natural conditions, and infanticidal tendencies of each male will also be monitored. Further, the dynamic interaction of the animals will be tracked on a fine scale, automated tracking devices to reveal proximate triggers of pregnancy replacement, e.g. mate guarding, harassment, avoidance (behavioural, mechanistic exlanations). By the use of rodents as a manageable, experimental system the project will advance our understanding of pregnancy preplacement in mammals, female reproductive strategies and sexual conflict.