Social Hierarchies and Social Networks
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).
Each observed behavior between individuals can be aggregated into a sociomatrix as seen on the far left. Numbers represent the total number of agonistic behaviors that individuals in rows directed towards individuals in columns. This matrix can be converted into a dichotomozied 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 behavior can be applied. We have made many of these methods available in our compete R package. We have studied groups of both males and females. One significant finding (discussed more in this paper) is that groups of male mice form extremely linear dominance hierarchies where animals occupy unique social ranks. This is illustrated in the dichotomized sociomatrix by the significant number of 1's that appear above the diagonal (indicating a hierarchically organized sociomatrix). This finding forms the basis of a number of projects within our lab.
We are focused on utilizing novel statistical methods to further characterize social dynamics spatially and temporally. For example, we are able to track the formation and maintenance of social hierarchies using a temporal pairwise contest model - the Glicko rating system (left).
Another advantage of applying temporal measures is to evaluate the consistency and stability of social ranks over time. We find that in general the mouse social hierarchies established in our vivaria are incredibly linear and stable.
For an interactive look at studying the temporal dynamics of hierarchies see this description of our recent study in Animal Behaviour.
Another class of methods we apply to social data are social network analyses. For instance, we can construct social networks of groups of mice for different types of behaviors (right).
This enables us to compare how different behaviors are associated with one another at an individual, relationship and group level. We can also use social network analysis to determine the level of influence or power that individuals have over other individuals within a social network.
We also investigate neurobiological differences between animals of varying social hierarchy and network position. For instance, we have previously reported associations between amygdala and hypothalamic CRF mRNA expression and hippocampal GR and BDNF mRNA expression and social dominance. We also investigate how relationships between neuroendocrinological markers and social network position may vary according to social context.
We are use social network methods to investigate the spatial organization of mice. In Williamson et al, 2016, Frontiers in Behavioral Neuroscience, we describe how 30 male mice organize themsevles into different network sub-communities. We also report in this paper how dominance rank is negatively associated with the relative mRNA expression of DNMT1.
Another research theme in the lab is how mice are able to attend to their social context and adjust their behavior accordingly. In this paper, 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. Some more details are available here.
In Williamson et al 2017 Hormones & Behavior we show that subdominant male mice are able to rapidly respond to the emergence of power vacuums.
When an alpha male is removed from a hierarchy, subdominant males rapidly (within 3 minutes) recognize that there exists a social opportunity and they aggressively exert their own dominance over all other animals in the group. These males socially ascend to become the new alpha
males and are able to stay at the top of the hierarchy. This demonstrates great social competence on behalf of these males to be able to so quickly respond to a change in the social context of the group.
We have also found that this change in social context leads to increased GnRH mRNA expression in the mPOA of mice where the alpha has been removed.
Future work will identify which brain regions show increased activation in response to such changes in social context.