Studying complex social behavior in the laboratory is challenging and requires analyses of dyadic interactions occurring over time in a physically and socially complex environment. In our laboratory we conduct long-term behavioral observations of animals housed in groups in large vivaria that mimic the burrow systems of their ancestral species Mus musculus (pictured).
We have studied groups of males and females. We have found that both male and female mice form highly linear dominance hierarchies where animals occupy unique social ranks. This is illustrated in the pictured sociomatrices. These aggregate observed aggressive behaviors between individuals. Numbers represent the total number of agonistic behaviors that individuals in rows directed towards individuals in columns. This matrix can be converted into a dichotomized 1/0 matrix (on the right). In this example, a 1 represents that the individual in the row is a clear winner against the respective individual in the column. A 0 indicates a clear loser or tied relationship. From these sociomatrices many statistical analyses of social dominance can be applied. We have made many of these methods available in our compete R package
We have shown that mouse hierarchies emerge rapidly and are stable over several weeks. Within each group of 10-12 male mice, typically one clear alpha individual becomes dominant over all overs. There are an additional 2-4 sub-dominant individuals and the remaining individuals are subordinate. However, even subordinate animals determine their relative rank order.
Notably, we have shown that male mice are even able to form linear hierarchies when housed in groups of 30 individuals. In larger groups, we have also observed using social network analysis that animals form distinct sub-communities.
We have also demonstrated that mice are able to attend to their social context and adjust their behavior accordingly. In particular, using pairwise-correlation statistical methods, we show that males monitor the behavior of the alpha male in a hierarchy and avoid engaging in aggressive behavior whilst the alpha male is being actively aggressive. Only the alpha male mouse is monitored and avoided in this fashion. Indeed, the more despotic that each individual alpha male mouse is, the more strongly anti-correlated are the aggressive behaviors of other animals with his behavior.
Below are representative tick plots from six cohorts that show each aggressive behavior by each animal by rank. For readability the x-axis is the number of each aggressive act rather than time. As can be seen, there is inter-hierarchy variability in the degree of activity of animals – but across all groups, those who can be classified as ‘alpha’ or ‘sub-dominant’ engage in the highest levels of activity.
Williamson CM, Franks B, Curley JP. 2016 Mouse social network dynamics and community structure are associated with plasticity-related brain gene expression. Front. Behav. Neurosci. 10, 152. PDF Online Article
Williamson CM, Lee W, Decasien AR, Lanham A, Romeo RD, Curley JP. 2019 Social hierarchy position in female mice is associated with plasma corticosterone levels and hypothalamic gene expression. Scientific Reports. 9:7324. PDF Online Article