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Most primates produce one offspring at a time but can overlap investment in consecutive offspring (stacked investment) depending on ecological and/or social conditions. "Direct stacked investment" occurs when a female conceives and gestates a new offspring while a previous infant is still suckling. We investigate direct stacked investment in Colobus vellerosus by determining if there is a period of overlap between nipple contact with a female's first infant and gestation of her subsequent infant. We then investigate the effect of male group-membership and stability on stacked investment to test the hypothesis that stacking investment is a female strategy to avoid infanticide. We collected scan data to record when an infant was in nipple contact from 16 females in four groups of C. vellerosus, from May 2012 to May 2013 at Boabeng-Fiema Monkey Sanctuary in Ghana. We studied three group types, stable multimale, multifemale; unstable multimale, multifemale; and stable unimale, multifemale. Eight of our study females directly stacked investment. The frequency of direct stacked investment varied significantly among the group types. Stacked investment was due to earlier conception rather than delayed weaning, with females in stable multimale groups conceiving a subsequent infant earlier than those in unstable multimale groups. The timing of conception did not differ between stable unimale groups and either unstable or stable multimale groups. There was no difference in time to last nipple contact among group types. These results do not support the predictions of the infanticide avoidance hypothesis, but a larger sample size would help to test the hypothesis more comprehensively. Delayed conception by females in unstable groups may be infanticide avoidance, however, if it allows time for females to spread paternity certainty among males. Alternatively, females in unstable groups may be under stress or experience reduced feeding time due to frequent male–male agonistic interactions, both of which can suppress female ovulation. This study contributes to a growing body of research that explores flexibility in infant investment strategies, particularly in relation to external factors such as male group membership and stability. [ABSTRACT FROM AUTHOR]