Social Structure; The Life of a Female

Primates use aggression to reinforce the existing hierarchy, but they also use more subtle tactics.  Here is how it works - a tasty food source is discovered by a lower ranking animal. A more dominant animal will approach the lower-ranking animal - and the lower-ranking animal will walk away.  This behavior is called 'displacement' and it is the dominant animal's non-violent way of letting another know that they are in charge.  Sometimes a high ranking animal will displace another for no more reason than to sit where he/she is sitting.  These little everyday reminders reinforce what appears to be a stable hierarchy in the group, where each drill knows its place.

Drills have a rigid social hierarchy.  Babies grow up under the protection of their mothers, sisters, and brothers.  In an altercation, a female will protect her offspring.  Matrilines (a family of related females) support eachother against unrelated matrilines.  A matriline has a distinct ranking within the larger social group.  (i.e. one matriline will rank over another matriline, which is stronger than everyone but the first, and so on...).  There is also a rank among the females within a matriline (i.e. first, second, third, etc.) - a mother will usually support a younger daughter over an older one (which makes sense, as the younger is usually smaller and needs to be protected more).

The female ranking system is thought to remain stable over time, but some things can impact this hierarchical order - such as which female the dominant male likes best!  When a female is in estrus, she exhibits a genital swelling that the male is attracted to - a high ranking female often directs aggression at a low-ranking female that has become 'attractive' to the male.  Dominant males will often break up a fight between females by supporting one female over another, depending on his preferences.  In altercations between females, it is important which female is stronger than the other, but always, the outcome of an altercation depends on who else in the group supports you!