title: Conflict in the communal nest: investigating female competition in house mice
Creators: name: Bottell, Lisa
Publisher: name: University of Liverpool Repository
Year: 2014

description: Female-female competition has been relatively overlooked in favour of male-male competition for mates, but it can also have important reproductive consequences. There are an increasing number of studies describing conditions where females compete to obtain breeding rank, gain access to or control resources or actively defend young. Communally breeding females are thought to be relatively egalitarian, sharing the cost of parental care with other females. Hence little attention has been paid to the potential for competition in such breeding systems, despite evidence of aggression and reproductive suppression between females. This thesis therefore explores the extent of competitive behaviours between female wild house mice (Mus musculus domesticus), a species with communal care of young, and investigates the physiological effects of competition and its consequences for breeding success and reproductive output. I examined the effects of age and other characteristics that may predict the degree of female competition. I identified that body mass, relative age of social partners, urinary testosterone concentration and reproductive experience were all useful predictors for the amount of competitive behaviour observed between female pairs. Following competitive female interaction I found that urinary testosterone and protein output increased, but there was no significant change in body mass and no significant effect on oestrus cycle length. Older females (&gt; 12 months) with competitive experience had larger adrenal glands compared to females previously housed with their sisters, suggesting a possible stress response to competitive interaction. There was also evidence that competitively housed females had enlarged clitoral glands, which may play a role in signalling social status. As female house mice were found to compete and assume social ranks, I investigated the impact of female social status on male mate choice and mating behaviour. There was no evidence of significant male preference for more or less competitive females prior to or after competitive interaction in a choice test with restricted access to females or when presented with female odours. To investigate breeding behaviour I introduced female pairs to a male in semi-naturalistic enclosures, filming continuously over a four day period to examine mating attempts and female behaviour. Interestingly males mounted less competitive females either exclusively or preferentially during the test, with a small number of competitive females interrupting mating behaviour between their social partners and the male. Therefore males may prefer female partners that are less likely to act aggressively towards their advances. The effect of female competition on reproductive success was examined by comparing breeding success of subjects under solitary and communal breeding conditions. Despite the prediction that reproductive success increases for secondary litters in house mice, reproductive output was significantly reduced for more and less competitive females in the communal nest compared to previous output in a solitary nest. This finding illustrates the negative impact of competition on reproductive success. Females that gave birth first in communal nests also had significantly fewer pups present on post natal day one compared to females that gave birth second. Interestingly female offspring of more competitive females in this experiment went on to produce larger litters on average than females born to less competitive females. Litters were also likely to be male biased if females had been reared in a competitive environment rather than a solitary nest, suggesting that competitive ability and rearing environments can both influence reproductive success for offspring. These results, together with evidence in the literature, suggest that competition does occur between communally breeding females, and that reproductive success can be affected as a result. However competition between communal females may be less intense than between females in cooperative systems, where reproductive skew is biased towards one or two individual females in a group. Using a comparative analysis I found that cooperatively breeding species had increased reproductive output compared to other polytocous species, which is likely to be influenced by the presence of non-breeding helpers in the nest site. Cooperative species also had decreased inter-litter intervals compared to non-cooperative species, as well as a reduction in lactation length and protein content of milk. Communal species were found to have increased offspring growth and reduced sexual size dimorphism, suggesting that competition between females may have resulted in selection for increased female body mass. Together these results illustrate the significance of female competition in wild house mice, with consequences for mating behaviour and reproductive success, as well as the evolutionary implications of female-female competition in mammalian species residing in communal breeding systems.