Adaptations of animals to commensal habitats: population dynamics of house mice (Mus musculus domesticus) on farms

1. Commensal mammals live in habitats that appear to provide both benefits and costs compared with natural and semi-natural (non-commensal) habitats. These commensal habitats offer potentially rich food resources but are also characterized by instability in time and space. We expected to demonstrate that animals in these habitats have high reproductive rates to counter high mortality rates and show flexibility in spatial organization. House mice (Mus musculus domesticus) are unusual because they are able to persist entirely in both commensal and non-commensal habitats, and so can provide a test of the distinctiveness of commensal populations.

2. We studied populations of commensal house mice on two neighbouring farms in North Yorkshire, UK, for 2 years by capture–mark–recapture using the robust design. A total of 568 house mice were captured, with a total of 1053 recaptures. Population size varied from nine to 93 individuals, estimated with closed population models. Apparent survival was surprisingly low (0·54 per month) and was best modelled as constant across age, sex and time.

3. In situ reproductive recruitment occurred throughout the study and was numerically more important than immigration. Immigration was important during only two intervals and was probably from untrappable areas within the study site. Breeding throughout the year allowed the population to persist despite low survival rates. The results suggested that births and deaths had more influence on the overall population dynamics than movement.

4. The population was divided into subgroups to represent the territorial, demic structure known to be present in commensal house mouse populations. Dispersal between subgroups within the population was limited, representing only 6·6% of recaptures. The low rates of dispersal suggested that house mice responded to their environment as consisting of aggregated patches of suitable habitat.

5. In comparison, non-commensal house mice have lower mortality, seasonal breeding, and individuals move more frequently and further than commensal house mice.

6. These differences illustrate the responses of house mice to the specific opportunities and demands of commensal habitats and demonstrate the importance of a flexible life-history strategy for animals exploiting these habitats.