March 12, 2014

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Two major predators of Japanese rhinoceros beetle. Top: raccoon dog and the remains of a beetle preyed on by this animal. The red arrowhead indicates a bite mark. Bottom: jungle crow and the remains of a beetle preyed on by this animal.© Wataru Kojima

The remains of dead Japanese rhinoceros beetles (Trypoxylus dichotomus) are often found near to broadleaf trees that leak sap such as the sawtooth oak and Quercus serrata, looking as if something has just eaten the abdominal portion. This has been suspected to be the result of predation by crows, but had not been confirmed by research.

A research group including then-third-year doctoral student Wataru Kojima at the University of Tokyo Graduate School of Agricultural and Life Sciences used automatic filming of sap-site trees to show that these beetle carcasses were the result of predation by raccoon dogs and jungle crows. The researchers also examined the feeding marks left by each predator, discovering that the raccoon dog left what appears to be a small tooth hole in the beetle carcass. On this basis, an analysis of beetle carcasses collected from around the Kanto region suggests that some 60 to 80 percent of predation in this area is by raccoon dogs. In addition it was found that both jungle crows and raccoon dogs preyed on males more than females, and on males with larger horns than on males with smaller horns.

While having a larger horn is advantageous when competing with other males, it is probable that this also has a high predation cost as such males are also more easily found by natural enemies, resulting in high predation pressure. This research was carried out in collaboration with the Forestry and Forest Products Research Institute.

Paper Information

W. Kojima, S. Sugiura, H. Makihara, Y. Ishikawa, and T. Takanashi, “Rhinoceros Beetles Suffer Male-Biased Predation by Mammalian and Avian Predators,”doi: 10.2108/zsj.31.109 (未定)