“Fitness consequences of female alternative reproductive tactics in house mice (Mus musculus domesticus)”
Manuela Ferrari, Anna K. Lindholm, and Barbara König (Jan 2019)
Female house mice are known to raise their young either alone, or together with one or more other mothers in a communal nest. In the latter case, the mothers all care for the young indiscriminately. Whenever two alternative breeding tactics occurring in the same sex and population in nature are found, the question arises of how they can be evolutionarily stable. Manuela Ferrari and her colleagues at the University of Zurich (Anna Lindholm and Barbara König) studied communal and solitary breeding in a population of free-living house mice during a five year period (2007–2011). They find that females are more successful if they rear their litters solitarily, because pups in communal nests suffer from a lower survival probability. Why does communal breeding occur in the population, and at high rates (70%), if it reduces female fitness?
The researchers show that older – likely also heavier and more experienced – females are less likely to rear their litters communally and instead have more solitary litters. Rearing young in a communal nest might therefore be a tactic that is followed only by females of lower overall condition. Such females might be either unable to rear litters on their own or to monopolize a nesting site and therefore take the least bad option, which is communal breeding instead of not breeding at all. These findings can help us to understand how behaviors that seem to have a cost, such as communal breeding in this example, can still be seen in nature if they are the best an individual can do in a given condition and situation.
Alternative reproductive tactics are defined as discrete differences in morphological, physiological and/or behavioral traits associated with reproduction, which occur within the same sex and population. House mice provide a rare example for alternative reproductive tactics in females, which can either rear their young solitarily, or together with one or several other females in a communal nest. We assessed the fitness consequences of communal and solitary breeding in a wild population to understand how the two tactics can be evolutionarily stable. Females switched between the two tactics (with more than 50% of all females having two or more litters using both tactics), pointing towards communal and solitary breeding being two tactics within a single strategy and not two genetically determined strategies. Communal breeding resulted in reduced pup survival and negatively impacted female reproductive success. Older and likely heavier females more often reared their litters solitarily, indicating that females use a condition dependent strategy. Solitary breeding seems the more successful tactic and only younger and likely less competitive females might opt for communal nursing, even at the cost of increased pup mortality. This study emphasizes the importance of analyzing phenotypic plasticity and its role in cooperation in the context of female ARTs.