Primates include lemurs, monkeys and apes. We're primates too! There is no single feature that is unique to primates as compared to other mammals, but there is a combination of characters that set primates apart, including: opposable thumbs for grasping, flat nails rather than claws in most species, forward facing eyes, and relatively large brains. When you visit the Zoo, you'll find most of the primates at PECO Primate Reserve and The Rare Animal Conservation Center, but there are also several in the Small Mammal House and other locations.