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.

捕食作用是影响动物适合度与生活史的关键因素。影响猎物存活的两大核心性状为体型与体色。胸斑赛蝇（Sepsis thoracica）的雄性个体在这两种性状间呈现出S型关联，其界定出一个体型阈值：当体型超过该阈值时，黑色素投入量会急剧下降，进而产生小型黑化（黑色）型或大型琥珀色型两种形态。为解析这种罕见二态性的演化机制，本研究开展了实验室捕食实验，以评估多种节肢动物捕食者（蝽类、蝇类、蜘蛛）对雄性个体体型与体色施加的成虫存活选择强度。实验设置了两种模拟自然栖息地（粪便与草地）的背景，不同形态在该背景下的伪装及/或警戒色效应会存在差异。体型主要受到正向选择（体型越大存活概率越高），该选择强度超过了体色所受的选择强度，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。研究未发现分裂选择，且选择作用并未改变两种性状间的S型关联。综上，对于该蝇类而言，由体型决定的捕食规避与逃逸能力，相较于由体色决定的显眼程度，对无脊椎动物捕食者的防御效果更强；这与脊椎动物捕食者相关研究的结果相悖——在脊椎动物捕食者的场景中，猎物体色发挥关键作用，且体型受到负向选择。由于节肢动物捕食者对昆虫种群具有显著调控作用，无脊椎动物捕食者施加的正向定向选择，可能是驱动胸斑赛蝇（S. thoracica）乃至昆虫整体体型演化的重要动力。