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(1) In man, thickening of the skin on the hands and feet caused by friction and pressure.
(2) Most catarrhine monkeys have ischial callosities, or hairless areas of heavily fat-padded skin in the inferior dorsal hip region. In large anthropoid apes, calluses on the dorsal side of the middle phalanges of the fingers are used for support while walking.
(3) Cushionlike thickenings of the skin and fatty tissue on the lower surface of the feet in all tylopods (camels and extinct forms) and over the carpi, elbows, knees, and breast in all camelids.
(4) Nuptial callosities are rough masses on the forelegs of male frogs.