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.
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