The existence of a relationship between the social ties an individual has to other family members and its further role within the family was tested in feral house mice (Mus musculus domesticus), according to the ‘Social cohesions hypothesis’. It is predicted by the hypothesis that individuals not forming strong social ties are the first who emigrate.

House mice were studied using a population cage system that allowed continuous observation of individually marked animals. Data on time staying with other animals (social ties), aggressive interactions, body weight, reproduction, and emigration were collected daily. The results may be summarized as follows:

  • 1
    Male emigrants were less integrated in cohorts of male littermates compared with their brothers of the same age. These male cohorts appeared to protect single males from attacks by the dominant male. No difference could be observed in social ties to other family members.
  • 2
    After weaning, there was no difference in social ties of male and female offspring. However, after sexual maturation social ties of males decreased significantly while those of females remained almost constant.
  • 3
    Female emigrants showed the same intensity of social ties as their resident sisters.
  • 4
    No difference could be found between social ties of females becoming pregnant and their nonreproductive sisters of the same age. Reproduction or reproductive suppression could not be explained by having more or less contact with other reproductive females.
  • 5
    Dominant males spent least of all time with other family members.

The social cohesions hypothesis has to be rejected in analysing proximate causes of emigration. In house mice, male emigration was caused by aggression of the dominant male in competition for the top rank within the group. This was enhanced by a lower integration in the group of same-aged brothers but is not related to a lack of integration into the family.