Bite forces used by Japanese macaques (Macaca fuscata yakui) on Yakushima Island, Japan to open aphid-induced galls on Distylium racemosum (Hamamelidaceae)

Abstract

Japanese macaques on Yakushima Island have been seen attempting to open thick, woody plant galls in order to eat aphids contained within them. We analysed a sample of galls and gall fragments with toothmarks and found that 22% were still intact indicating a failure to open them. These marks were examined and measured. Ten pits had a mean indentational area of 1.44 mm2 (S.D. 0.28 mm2), while 15 elongated scratches had a mean width of 1.26mm (S.D. 0.27 mm). The gall resembled a light wood in its mechanical properties and had a microhardness of 80.4MPa. Assuming that at least two marks were formed in any given bite, indentational analysis gave a mean estimate of bite forces of 232N (maximum 291 N) to produce pits and 255 N (maximum 487N) to produce scratches. These forces are consistent with, but at the high end of, limits predicted by anatomical analysis.