Data and code for the study: Influence of Fruit Availability on the Diet and Grouping Patterns of Spider Monkeys (Ateles spp.) in a Relatively Undisturbed and a Degraded Forest in Tropical South America

In mammals, competition for resources is known to be one of the most important determinants of grouping patterns. As a result, some animals have evolved fission-fusion strategies that allow them to balance the benefits and costs of living in large groups. In this study, we evaluated the influence of fruit availability on fruit consumption and grouping patterns in two species of spider monkeys living in ecosystems with different levels of habitat degradation: brown spider monkeys (Ateles hybridus) living in a fragmented tropical forest in the inter-Andean Rio Magdalena valley of Colombia, and white-bellied spider monkeys (Ateles belzebuth) living in a continuous tropical evergreen forest in Amazonian Ecuador. Through focal animal sampling, we recorded diet, subgroup size, and fission/fusion events involving the subgroup that contained the focal animal, and we estimated habitat-wide fruit availability every 2 weeks on phenological transects in the two habitats. We recorded behavioral and ecological data for brown spider monkeys from July 2009 to November 2012 obtaining 2270 hours of focal sampling, and for white-bellied spider monkeys from August 2006 to February 2017 obtaining 7000 hours of focal sampling. We used generalized linear mixed effects models to evaluate the relationship between fruit availability and both fruit consumption and subgroup size. In the relatively undisturbed Ecuadorian rainforest site, both fruit consumption and subgroup size of white-bellied spider monkeys were positively correlated with seasonal fruit availability. By contrast, in the fragmented Colombian tropical forest, we found no significant relationship between fruit availability and either fruit consumption or subgroup size for brown spider monkeys. Although differences between our two study groups may be due to species differences, the two species are very closely related, and spider monkeys in general show similar responses to degraded habitats. Thus, we argue that our results support the hypothesis that grouping patterns in fission-fusion species are influenced by food availability and suggests that habitat degradation might disrupt this relationship.

在哺乳动物中，资源竞争被认为是决定集群模式的最重要驱动因素之一。因此，部分动物演化出裂合-融合（fission-fusion）集群策略，以权衡大型群体生活的收益与代价。本研究评估了果实可获得性对两种栖息于不同生境退化程度生态系统的蜘蛛猴的果实取食行为与集群模式的影响：一是栖息于哥伦比亚安第斯山间马格达莱纳河谷破碎化热带林的棕蜘蛛猴（Ateles hybridus），二是栖息于厄瓜多尔亚马逊地区连续热带常绿林的白腹蜘蛛猴（Ateles belzebuth）。本研究采用焦点动物取样法，记录了目标个体所在亚群的食谱、亚群规模及裂合融合事件，并通过两种生境中的物候样线每两周估算整个生境范围内的果实可获得性。本研究于2009年7月至2012年11月间采集棕蜘蛛猴的行为与生态数据，累计获得2270小时焦点取样时长；于2006年8月至2017年2月间采集白腹蜘蛛猴的相关数据，累计获得7000小时焦点取样时长。本研究采用广义线性混合效应模型（generalized linear mixed effects models），分析果实可获得性与果实取食行为、亚群规模之间的关联。在受干扰程度较低的厄瓜多尔雨林样地中，白腹蜘蛛猴的果实取食率与亚群规模均与季节性果实可获得性呈显著正相关。与之相反，在破碎化的哥伦比亚热带林中，棕蜘蛛猴的果实取食行为与亚群规模均未表现出与果实可获得性的显著关联。尽管两个研究种群间的差异可能源于物种本身的分化，但这两种蜘蛛猴的亲缘关系极近，且蜘蛛猴属物种对退化生境的响应模式整体相似。因此，本研究认为实验结果支持‘裂合融合物种的集群模式受食物可获得性调控’这一假说，并表明生境退化可能会破坏这一调控关系。