However, in 1981, English anthropologist Alan Walker found that the microwearing patterns on the molars were inconsistent with a diet high in hard foods, and were effectively indistinguishable from the pattern seen in the molars of fruit-eating (frugivorous) mandrills, chimps, and orangutans. The microwearing on "P. boisei" molars is different than that on "P. robustus" molars, and indicates that "P. boisei", unlike "P. robustus", very rarely ever ate hard foods. Carbon isotope analyses report a diet of predominantly C4 plants, such as low quality and abrasive grasses and sedges. Thick enamel is consistent with grinding abrasive foods. The microwear patterns in "P. robustus" have been thoroughly examined, and suggest that the heavy build of the skull was only relevant when eating less desirable fallback foods. A similar scheme may have been in use by "P. boisei". Such a strategy is similar to that used by modern gorillas, which can sustain themselves entirely on lower quality fallback foods year-round, as opposed to lighter built chimps (and presumably gracile australopithecines) which require steady access to high quality foods.