The postorbital bar is a bone which runs around the eyesocket of strepsirrhine primates. This is in contrast to the higher primates, haplorrhine, which have evolved fully enclosed sockets to protect their eyes.
One theory for this evolutionary difference is the relative importance of vision to both orders. As haplorrhines (tarsiers and simians) tend to be diurnal, and rely heavily on visual input, strepsirrhines are mainly nocturnal and have a decreased reliance on visual input.
- Campbell, Bernard G., Loy, James D. (2000). Humankind Emerging (8th edition). Allyn & Bacon. p. 85.
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