When preferred foods are scarce, one strategy employed by primates is to switch to an alternative food item of lower quality or preferability, i.e., a fallback food. In the montane rainforest of Nyungwe National Park in southwestern Rwanda, Rwenzori black-and-white colobus (Colobus angolensis ruwenzorii) (hereafter Rwenzori colobus) form a supergroup comprising hundreds of individuals. Over 13 mo we investigated how this supergroup uses resources in periods of resource abundance vs. periods of resource scarcity. Based on 5603 feeding records we first identified preferred foods and then identified fallback foods as those food categories or species whose consumption increases when preferred foods, or preferred food categories, are less available. When the availability of 19 preferred food items was low, fruticose lichens (Usnea sp.) contributed >50% of the monthly diet for the Rwenzori colobus. Moreover, consumption of lichens was significantly negatively related to the availability of preferred foods. Fruticose lichens can therefore be considered a fallback food for Rwenzori colobus that sustains the supergroup during periods of reduced food availability. This result, in combination with previous findings that mature foliage in Nyungwe is of high quality and does not elicit feeding competition, points to the importance of resources in facilitating supergroup formation. However, several other montane forests in Eastern Africa also harbor fruticose lichens and yet support only small groups of Angolan colobus, suggesting that additional factors such as sufficient forest size and limited fragmentation and hunting pressure by humans are required for supergroups to form.

中文翻译：

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Rwenzori Black-and-White Colobus (Colobus angolensis ruwenzorii) 在卢旺达的饮食和后备食物的使用：对超群形成的影响

当首选食物稀缺时，灵长类动物采用的一种策略是改用质量或偏好较低的替代食物，即备用食物。在卢旺达西南部纽恩圭国家公园的山地雨林中，鲁文佐里黑白疣猴（Colobus angolensis ruwenzorii）（以下简称鲁文佐里疣猴）形成了一个由数百只个体组成的超群。在超过 13 个月的时间里，我们调查了这个超级群体在资源丰富时期与资源稀缺时期如何使用资源。根据 5603 份喂养记录，我们首先确定了首选食物，然后将后备食物确定为在首选食物或首选食物类别较少时消费量增加的食物类别或物种。当 19 种首选食物的供应量较低时，果状地衣（松萝属）做出了贡献 > Rwenzori 疣猴每月饮食的 50%。此外，地衣的消费与首选食物的可用性显着负相关。因此，果状地衣可以被视为鲁文佐里疣猴的备用食物，在食物供应减少期间维持超级群。这一结果与之前的发现相结合，即 Nyungwe 的成熟树叶质量高，不会引起进食竞争，表明资源在促进超群形成方面的重要性。然而，东非的其他几片山地森林也拥有果状地衣，但仅支持安哥拉疣猴的小群体，这表明超级群体的形成还需要额外的因素，例如充足的森林面积、有限的破碎和人类的狩猎压力。