I completed my Master of Arts at the University of Calgary under the supervision of Dr. James D. Paterson and my Doctor of Philosophy at the University of Calgary working with Dr. Pascale Sicotte. Research for my M.A. and Ph.D. was completed at the Boabeng-Fiema Monkey Sanctuary in Ghana where I studied social organization in ursine (or white-thighed) colobus (Colobus vellerosus). I contrasted ecological factors (i.e., range size, food availability, scramble competition for food) with social factors (i.e., infanticide risk, male sexual coercion, male advertisement, female mate choice) to explain variation in group size and group composition. I found that the rate of infanticide by immigrant males was high and that resident males often defended infants. While food competition constrained group size at the upper end of the continuum, infanticide risk played a more important role in determining social organization, limiting group size earlier and having more of an effect on group composition. Infanticide risk increased with female group size because immigrant males were attracted to larger female groups, thus females attempted to keep group size small by resisting female immigration, evicting maturing nulliparous females, and dispersing when groups became at risk for male takeover. Male quality was found to be important to decreasing the risk of infanticide and females attempted to immigrate into groups where the male(s) won more encounters against the male(s) in the females’ own group. Within groups, male agonistic display output indicated fighting ability; however, males did not vary in their testosterone or parasite levels except when they dropped in rank and were involved in male-male fights. Females did not appear to use male agonistic display to inform their mate choices and seemed to mate promiscuously, perhaps as a counter-strategy to the high rates of infanticide.