Data from: Viability selection by invertebrate predators in the polyphenic scavenger fly Sepsis thoracica

Predation is a major factor influencing the fitness and life history of animals. Two key traits affecting prey survival are body size and coloration. Sepsis thoracica males display a sigmoid relationship between these two traits, defining a size threshold above which investment in melanin drastically drops, producing small melanic (black) or large amber morphs. In trying to understand the evolution of this rare dimorphism, we performed laboratory predation experiments to estimate the intensity of adult viability selection exerted by various arthropod predators (bugs, flies, spiders) on male body size and coloration. Selection was performed against two different backgrounds mimicking the natural habitat (dung and grass) in which the camouflage and/or warning effect of the morphs should vary. Body size was mainly under positive selection (larger survived better), which overpowered selection on coloration and varied somewhat among predator species but not backgrounds. No disruptive selection was found, nor did selection change the sigmoid relationship between the two traits. We conclude that, for this fly, predator evasion and escaping skills determined by body size are more effective against invertebrate predators than its conspicuousness determined by coloration, contrasting what has been found for vertebrate predators, where prey coloration is important and negative selection on size dominates. Because arthropod predators have strong effects on insect populations, the positive directional selection imposed by invertebrate predators is likely an important force driving the evolution of body size in S. thoracica and insects in general.

捕食（predation）是影响动物适合度（fitness）与生活史（life history）的核心生态因子。影响猎物存活的两个关键性状为体型（body size）与体色（coloration）。塞氏粪蝇（Sepsis thoracica）雄虫的这两类性状间呈现S型关系（sigmoid relationship）：存在一个体型阈值，超过该阈值后黑色素（melanin）合成投资会急剧下降，进而产生两种形态——体型较小的黑化（melanic）型（黑色）与体型较大的琥珀色形态（amber morphs）。为探究这种罕见二态性的演化机制，我们开展了实验室捕食实验（predation experiments），以评估多种节肢动物捕食者（arthropod predators，包括蝽类、蝇类、蜘蛛）对雄虫体型与体色施加的成虫存活选择（adult viability selection）强度。实验设置了两种模拟其自然生境（natural habitat）的背景——粪堆（dung）与草地（grass），以考察不同形态的保护色（camouflage）及/或警戒色（warning effect）效应在此环境下的差异。体型主要受到正选择（positive selection）作用：体型越大存活概率越高，该选择强度超过了对体色的选择，且在不同捕食者物种间存在小幅差异，但在不同背景间无显著差异。未检测到歧化选择（disruptive selection），选择作用也未改变两类性状间的S型关系。我们据此得出结论：对于该蝇类而言，由体型决定的逃逸与规避捕食能力，相较于由体色决定的醒目程度，在对抗无脊椎动物捕食者（invertebrate predators）时更为有效；这与脊椎动物捕食者（vertebrate predators）驱动的选择模式形成鲜明对比——后者中猎物体色发挥关键作用，且体型受到负选择（negative selection）主导。由于节肢动物捕食者对昆虫种群具有强烈调控作用，无脊椎动物捕食者施加的正向定向选择，很可能是驱动塞氏粪蝇（S. thoracica）乃至昆虫类群体型演化的重要动力。