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.

中文翻译：

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直接叠加投资是雌性疣猴的灵活繁殖策略

大多数灵长类动物一次只生产一个后代，但可以根据生态和/或社会条件对连续的后代进行重叠投资（堆叠投资）。“直接叠加投资”发生在女性在前一个婴儿仍在哺乳时怀孕并孕育新后代。我们通过确定女性与第一个婴儿的乳头接触与其后续婴儿的妊娠之间是否存在重叠时间来调查对 Colobus velerosus 的直接叠加投资。然后，我们调查男性群体成员和稳定性对叠加投资的影响，以检验叠加投资是女性避免杀婴的策略的假设。我们从四组 C. velerosus 中的 16 名女性中收集了扫描数据以记录婴儿何时接触乳头，2012 年 5 月至 2013 年 5 月在加纳的 Boabeng-Fiema 猴子保护区。我们研究了三种群体类型，稳定的多雄性、多雌性；不稳定的多雄性，多雌性；和稳定的单雄性，多雌性。我们研究的八位女性直接投资。直接叠加投资的频率在不同的群体类型之间存在显着差异。叠加投资是由于较早的受孕而不是延迟断奶，稳定的多雄性群体中的女性比不稳定的多雄性群体中的女性更早怀孕。稳定的单雄性群体与不稳定或稳定的多雄性群体之间的受孕时间没有差异。各组类型之间最后一次乳头接触的时间没有差异。这些结果不支持杀婴假设的预测，但更大的样本量将有助于更全面地检验假设。不稳定群体中的女性延迟受孕可能是避免杀婴，但是，如果它允许女性有时间在男性中传播亲子关系的确定性。或者，由于频繁的雄性-雄性激动相互作用，不稳定群体中的雌性可能会承受压力或进食时间减少，这两者都会抑制雌性排卵。这项研究有助于越来越多的研究探索婴儿投资策略的灵活性，特别是与男性群体成员身份和稳定性等外部因素相关的灵活性。由于频繁的雄性-雄性激动相互作用，不稳定群体中的雌性可能会受到压力或进食时间减少，这两者都会抑制雌性排卵。这项研究有助于越来越多的研究探索婴儿投资策略的灵活性，特别是与男性群体成员身份和稳定性等外部因素相关的灵活性。由于频繁的雄性-雄性激动相互作用，不稳定群体中的雌性可能会受到压力或进食时间减少，这两者都会抑制雌性排卵。这项研究有助于越来越多的研究探索婴儿投资策略的灵活性，特别是与男性群体成员身份和稳定性等外部因素相关的灵活性。