The Nutritional Content of Prey Affects the Foraging of a Generalist Arthropod Predator

While foraging theory predicts that predatory responses should be determined by the energy content and size of prey, it is becoming increasingly clear that carnivores regulate their intake of specific nutrients. We tested the hypothesis that prey nutrient composition and predator nutritional history affects foraging intensity, consumption, and prey selection by the wolf spider, Pardosa milvina. By altering the rearing environment for fruit flies, Drosophila melanogaster, we produced high quality flies containing more nitrogen and protein and less lipid than low quality fruit flies. In one experiment, we quantified the proportion of flies taken and consumption across a range of densities of either high or low quality flies and, in a second experiment, we determined the prey capture and consumption of spiders that had been maintained on contrasting diets prior to testing. In both cases, the proportion of prey captured declined with increasing prey density, which characterizes the Type II functional response that is typical of wolf spiders. Spiders with similar nutritional histories killed similar numbers of each prey type but consumed more of the low quality prey. Spiders provided high quality prey in the weeks prior to testing killed more prey than those on the low quality diet but there was no effect of prior diet on consumption. In the third experiment, spiders were maintained on contrasting diets for three weeks and then allowed to select from a mixture of high and low quality prey. Interestingly, feeding history affected prey preferences: spiders that had been on a low quality diet showed no preference but those on the high quality diet selected high quality flies from the mixture. Our results suggest that, even when prey size and species identity are controlled, the nutritional experience of the predator as well as the specific content of the prey shapes predator-prey interactions.

觅食理论（foraging theory）曾预测，捕食者的捕食反应应由猎物的能量含量与体型决定，但越来越多的研究表明，肉食动物会主动调控自身对特定营养物质的摄入量。本研究以狼蛛（Pardosa milvina）为研究对象，验证了“猎物营养组成与捕食者营养历史会影响其觅食强度、取食量以及猎物选择”这一假说。通过调整黑腹果蝇（Drosophila melanogaster）的饲养环境，我们获得了营养品质更高的果蝇——与低品质果蝇相比，这类果蝇含有更多的氮与蛋白质，且脂质含量更低。在第一项实验中，我们针对不同密度的高品质或低品质果蝇，量化了被捕食果蝇的比例与取食量；在第二项实验中，我们测定了实验前饲喂不同日粮的狼蛛的猎物捕获量与取食量。两项实验均显示，被捕食的猎物比例随猎物密度升高而下降，这符合狼蛛典型的II型功能反应（Type II functional response）特征。营养历史相似的狼蛛，对两类猎物的击杀数量相近，但会取食更多的低品质猎物。实验前数周饲喂高品质日粮的狼蛛，其击杀的猎物数量多于饲喂低品质日粮的个体，但此前的日粮类型对其取食量无显著影响。在第三项实验中，我们将狼蛛饲喂不同日粮三周，随后允许其从高品质与低品质果蝇的混合种群中选择猎物。值得注意的是，取食历史会影响猎物偏好：饲喂低品质日粮的狼蛛未表现出明显的猎物偏好，而饲喂高品质日粮的狼蛛则会从混合种群中选择高品质果蝇。本研究结果表明，即便控制了猎物体型与物种属性，捕食者的营养经历以及猎物的特定营养组成，仍会调控捕食者与猎物之间的相互作用。