Three key factors determine primate social systems.
1.) of resources determine distribution.
2.) The and distribution of food patches determines how a group can be, and whether groups must periodically undergo .
3.) pressures influence whether animals can travel alone, whether must protect . If animals have to travel in groups for protection, an individual's rate of intake will be .

These ecological pressures affect a wide range of though - but primates are unusual in their year-round between and . Females have high levels of , so there might be selection on males for strategies to return females to .

Infanticide has been inferred or observed in 51 species of primates. It has been observed in l, p, co, ce and h.

The early evolution of probably went a bit like this:
1.) Ancestral primate probably had of several young, who were left in the whilst she foraged. Lactation was .
2.) Female life-history selected for and a period. This increased the risk of by males.
3.) Males may have nests at first whilst the the female foraged. This is still seen in .

Infanticide then progressed from males guarding nests whilst females foraged.
1.) declined and infants become less . Females carried young, which increased .
2.) An association between and i may have been an important step in moving between life to constant association.