BITE FORCES USED BY JAPANESE MACAQUES (MACACA-FUSCATA YAKUI) ON YAKUSHIMA ISLAND, JAPAN TO OPEN APHID-INDUCED GALLS ON DISTYLIUM-RACEMOSUM (HAMAMELIDACEAE)

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. Teg pits had a mean indentational area of 1.44mm(2) (S.D. 0.28mm(2)), while 15 elongated scratches had a mean width of 1.26mm (S.D. 0.27 mm). The gall resembled a lightwood in its mechanical properties and had a microhardness of 80.4 MPa. Assuming that at least two marks were formed in any given bite, indentational analysis gave a mean estimate of bite forces of 232 N (maximum 291N) to produce pits and 255N (maximum 487N) to produce scratches. These forces are consistent with, but at the high end of, limits predicted by anatomical analysis.