Cooperation and Exploitation in Communally Nursing House Mice (Mus musculus domesticus)

Cooperative behaviours are ubiquitous in nature and puzzle scientists ever since Darwin formulated his theory for evolution through natural and sexual selection. Why should an individual help another one, if selection favours those maximising their own fitness? Indirect fitness benefits as first described by Hamilton, and alternatively, direct fitness benefits for cooperating individuals (for example through reciprocity) helped to explain the evolution of cooperation over the last 60 years. Understanding the evolution and maintenance of cooperative behaviours in nature nevertheless requires identifying the potential for conflict and the mechanisms in place to prevent exploitation. This thesis focuses on communal nursing in house mice (Mus musculus domesticus). Female mice show two alternative reproductive tactics; rearing their Young either solitarily, or pooling their litters in one nest and caring for all pups indiscriminately. Communal breeding can be observed in many different species, which is remarkable given the high costs of parental care. The potential for exploitation seems high, whenever females differ in the amount of care they provide to the joint litter, or the number of young they have in the nest. I combined experiments in the laboratory with data from a free living population to asses whether there is potential for conflict and to quantify the fitness consequences of the two alternative reproductive tactics in house mice. Communally nursing females invested according to the total number of pups in the nest, but not their own litter size, making them highly vulnerable to exploitation (Chapter 1). Accordingly, females with relatively fewer offspring in the nest overinvested. In the laboratory, females tried to avoid situations of high conflict by reducing their propensity to cooperate when females differed in litter size (Chapter 2). Furthermore, I found no evidence that females enforced their partner’s continued investment (Chapter 3). Overall, it seems females had only limited options to reduce the risk for exploitation after the formation of the communal nest, emphasising the importance of deciding beforehand whether, or with whom, to communally nurse. Data from a wild population revealed that communally nursing females suffered an increased pup mortality, resulting in a lower reproductive success for females rearing a larger proportion of their litters communally (Chapter 4). Older, and probably heavier females, were more likely to rear their litters solitarily, indicating that it represented a condition dependent alternative reproductive tactic. Younger females may have been unable to rear litters solitarily, therefore opting for communal nursing as a "best-of-a-bad-job", even at the cost of losing some of their offspring. The research presented in this thesis demonstrates the strong potential for conflict among communally breeding females and reveals that an apparent cooperative behaviour does not necessarily always result in benefits for all individuals involved. Plastic and condition dependent alternative reproductive tactics might nevertheless maintain such behaviours in the population, highlighting the importance for future research about the evolution of plasticity and its effect on cooperation