| Abstract: | Extreme reduction of the hallux is unique to the orang-utan (Pongo pygmaeus) among Primates. Hallucal diminution has advanced so far that 60% of orang-utans lack both distal phalanges and nails. Absence of these structures occurs significantly more often in females than in males. Hypotheses on possible genetic control of the condition have been tested and indicate that either single gene inheritance or polygenic inheritance with variable expressivity is involved. Reduction of pollex and of hallux in Pongo have advanced with selection for a specialized four-digit grasp in hands and feet. Diminution has progressed farther in the great toe than in the thumb due to selection for fine manipulation in the latter digit. |
