Which characteristic is unique to haplorrhines compared to strepsirrhines?

The presence of postorbital closure is indeed a characteristic that sets haplorrhines apart from strepsirrhines. This feature refers to the bony wall or closure behind the eye sockets that provides greater protection to the eyes and allows for a wider range of vision. Haplorhines, which include tarsiers, monkeys, and apes, generally have this anatomical trait, which is associated with their more advanced visual processing and reliance on vision over olfaction.

In contrast, strepsirrhines, which include lemurs and lorises, lack this postorbital closure. Their eye sockets are more open, which typically correlates with their reliance on a keen sense of smell rather than advanced vision.

Other elements mentioned in the options, such as the unfused mandibular symphysis or the presence of a moist rhinarium, are traits found in strepsirrhines and are linked to characteristics that emphasize olfactory capabilities. The dental tooth comb, prevalent in some strepsirrhines, is also not related to the haplorrhine group. Thus, the distinction provided by postorbital closure is a clear indicator of the evolutionary adaptations that define haplorrhines as a separate group from their stre