Our findings indicate that female bank voles gain delayed non-genetic benefits from polyandry in the form of increased recruitment, probably via infanticide avoidance. As in other rodent species (see Ebensperger & Blumstein 2007
for review), copulation itself and/or sexual contact with a certain female may inhibit infanticidal behaviour in bank vole males. Because both the number of mates and matings were the same in the two treatments, genetic benefits as an explanation for our results can be excluded.
Despite the prominence of polyandry and its benefits in empirical and theoretical studies during the last decades, the generality of hypotheses for different taxa is still unclear. Wolff & Macdonald (2004)
argued that the origin and benefits of polyandry in mammals might be different from other taxa, especially because infanticidal behaviour is extremely common. Thus, and because genetic benefits as the evolutionary origin of polyandry are questioned (Yasui 1998
), our evidence for a non-genetic benefit may provide a general explanation for the evolution of this female reproductive behaviour in bank voles and other infanticidal mammal species.
Once established, however, polyandry leads invariably to the possibility of genetic benefits (Jennions & Petrie 2000
), contributing to its maintenance. In fact, female bank voles may also gain long-term genetic benefits from polyandry, in the form of offspring with increased reproductive potential (Klemme et al. 2008
). Additionally, other direct benefits are possible. For example, multiple mating in bank voles also increases the probability of pregnancy initiation owing to increased stimulation. However, because this stimulus can also be achieved by repeated matings with the same male, this benefit does not necessarily explain polyandry (Klemme et al. 2007
Are there alternative explanations for our results? Maternal effects could also possibly explain decreased offspring numbers in monandrous populations (Simmons 2005
). If females of an infanticidal species encounter strange males during pregnancy or lactation, they may perceive the potential for offspring loss and consequently decrease the resources they provide for the offspring. However, given the relatively large investment of females in this study until the first contact with strange males (at least two weeks of pregnancy), an increased effort in nest protection, which is effective in this species (Ylönen & Horne 2002
), would seem a better cost-benefit strategy. Further, female infanticide, which is common in bank voles (Ylönen et al. 1997
), as well as other mortality factors, may also have affected recruitment. However, if so, we would expect the magnitude of these effects to be similar among both treatments. Actually, female infanticide in addition to other mortality factors may explain the generally low recruitment rate in this study, despite the significant difference between the treatments (). Moreover, because some studies suggest that social interactions with the pregnant female are important for infanticide to be fully inhibited (e.g. Elwood 1985
), and because males and females were housed separately until release to the field, some male infanticide may have occurred even in polyandrous populations. However, if so, the effect of infanticide avoidance via polyandry should be even stronger under natural conditions.
In conclusion, this study provides, to our knowledge, the first field evidence that polyandry is an efficient strategy for increasing offspring survival, probably via the avoidance of male infanticide. However, maternal effects cannot be entirely excluded. More studies are needed to test the general validity of the hypothesis and may help explain the ultimate origin of polyandry in bank voles and other infanticidal species.