Kojima Wataru, Sugiura Shinji, Makihara Hiroshi, Ishikawa Yukio, Takanashi Takuma
1 Graduate School of Agricultural and Life Sciences, The University of Tokyo, Bunkyo-ku, Tokyo 113-8657, Japan.
Zoolog Sci. 2014 Mar;31(3):109-15. doi: 10.2108/zsj.31.109.
Male sexually-selected traits often impose an increased risk of predation on their bearers, causing male-biased predation. We investigated whether males of the sap-feeding Japanese rhinoceros beetle Trypoxylus dichotomus were more susceptible to predation than females by comparing the morphology of beetles caught in bait traps with the remains of beetles found on the ground. The males of this species are larger than the females and have a horn on the head. We found that predation pressure was greater for males than for females, and that larger individuals of both sexes were more vulnerable to predation. We identified two predators, the raccoon dog Nyctereutes procyonoides and jungle crow Corvus macrorhynchos, by monitoring sap-site trees with infrared video cameras. Raccoon dogs visited sap-site trees at night, while crows came after daybreak. The highest frequency of visits by both predators was observed in the first half of August, which matches the peak season of T. dichotomus. Raccoon dogs often left bite marks on the remains of prey, whereas crows did not. Bite marks were found on most of the remains collected at two distant localities, which suggested that predation by raccoon dogs is common. Size- and sex-dependent differences in the conspicuousness and active period of T. dichotomus probably explain these biased predation patterns. Our results suggest that having a large horn/body is costly in terms of the increased risk of predation. Predation cost may act as a stabilizing selection pressure against the further exaggeration of male sexual traits.
雄性性选择特征往往会增加其携带者被捕食的风险，导致雄性偏向的捕食现象。我们通过比较诱饵陷阱中捕获的甲虫形态与地面上发现的甲虫残骸，来研究吸食树液的日本犀金龟独角仙的雄性是否比雌性更容易被捕食。该物种的雄性比雌性体型更大，且头部有角。我们发现雄性的捕食压力大于雌性，而且两性中体型较大的个体更容易被捕食。通过用红外摄像机监测树液采集点的树木，我们确定了两种捕食者，即貉Nyctereutes procyonoides和大嘴乌鸦Corvus macrorhynchos。貉在夜间光顾树液采集点的树木，而乌鸦在黎明后到来。两种捕食者的最高访问频率出现在8月上半月，这与独角仙的高峰期相匹配。貉经常在猎物残骸上留下咬痕，而乌鸦则不会。在两个遥远地点收集的大部分残骸上都发现了咬痕，这表明貉的捕食很常见。独角仙在显眼程度和活动时期上的大小和性别差异可能解释了这些有偏向的捕食模式。我们的结果表明，拥有大角/大体型在捕食风险增加方面是有代价的。捕食代价可能作为一种稳定选择压力，防止雄性性特征进一步夸张。
