Data from: Ecological outsourcing: a pitcher plant benefits from transferring pre-digestion of prey to a bat mutualist

Mutualisms are interspecific interactions where each of the species involved gains net benefits from the other(s). The exchange of resources and/or services between mutualistic partners often involves tasks that species originally accomplished themselves but which have been taken over by or transferred to the more efficient partner during the evolution of the mutualism. Such ‘ecological outsourcing’ can be seen, for example, in several carnivorous plants that have transferred prey capture and digestion to animal partners. However, the outcome of this transfer and its fitness relevance has rarely been quantified.
Using a digestive mutualism between a carnivorous pitcher plant (Nepenthes hemsleyana) and a bat (Kerivoula hardwickii) as a model, we tested the hypothesis that ecological outsourcing is a profitable strategy for the outsourcing partner. To evaluate the value of this mutualism, we conducted a series of field and glasshouse experiments. We measured the benefits of ecological outsourcing by comparing survival, growth, photosynthesis and nutrient content of N. hemsleyana plants fed with bat faeces to those fed with arthropods. To investigate the costs of such outsourcing processes, we repeated the experiment with the closest relative (Nepenthes rafflesiana) that is not adapted to digest bat faeces.
We found that N. hemsleyana plants fed with faeces had increased survival, growth and photosynthesis compared to plants fed with arthropods only. On average, plants covered 95% of their nitrogen demand from faeces under strong nutrient deprivation. Despite N. rafflesiana's higher arthropod capture rate, faeces covered a large part of this species’ nutrient demand as well, suggesting low costs for outsourcing.
Synthesis. Outsourcing prey capture and digestion to the mutualism partner seems to be a beneficial strategy for N. hemsleyana. It may explain the evolutionary trend of several carnivorous plants to lose their carnivorous traits while increasing their attractiveness to mutualistic partners. On a much broader scale, we propose that ecological outsourcing could be one of the major drivers for the evolution and maintenance of mutualisms.

互利共生（Mutualism）是指参与双方（或多方）均能从彼此间获取净收益的种间相互作用。互利共生伙伴间的资源与（或）服务交换，通常涉及将物种原本自主完成的任务，在共生演化过程中移交或转让给效率更高的伙伴，此类现象被称为“生态外包（ecological outsourcing）”。例如，多种食肉植物便将猎物捕获与消化的工作移交至动物伙伴，但这类任务转移的实际效果及其适合度关联，此前鲜有定量研究。

本研究以食肉猪笼草（Nepenthes hemsleyana）与哈氏毛翼蝠（Kerivoula hardwickii）之间的消化型互利共生关系为模型，验证了“生态外包对于外包方而言是一种收益可观的策略”这一假说。为评估该共生关系的价值，我们开展了一系列野外与温室实验。通过对比以蝙蝠粪便为营养源的赫姆氏猪笼草与仅以节肢动物为食的同类的存活率、生长速率、光合能力及养分含量，我们量化了生态外包带来的收益。为探究此类外包过程的潜在成本，我们以不适应消化蝙蝠粪便的近缘物种莱佛士猪笼草（Nepenthes rafflesiana）为材料重复了上述实验。

实验结果显示，与仅以节肢动物为食的个体相比，以蝙蝠粪便为营养源的赫姆氏猪笼草存活率更高、生长更快且光合能力更强。在严重养分匮乏的条件下，该猪笼草平均可通过蝙蝠粪便满足其95%的氮需求。尽管莱佛士猪笼草的节肢动物捕获率更高，但其养分需求的很大一部分也可通过蝙蝠粪便得以满足，这表明生态外包的成本极低。

综合分析：将猎物捕获与消化工作外包给共生伙伴，对赫姆氏猪笼草而言似乎是一项有利的演化策略。这或可解释为何多种食肉植物会逐渐丧失食肉性状，同时增强对互利共生伙伴的吸引力。从更宏观的尺度来看，我们认为生态外包可能是推动互利共生关系演化与维持的核心驱动力之一。