The term prosimian refers to a group of primates (suborder Strepsirhini) that includes lemurs, lorises, and bushbabies (also referred to as galagos). Prosimians exhibit physical characteristics that are considered ancestral or "primitive". Many have a specialized dental structure, called a tooth comb, for feeding and grooming. Prosimians have reduced visual acuity, compared to monkeys and apes, and rely more heavily on olfaction (sense of smell). Many have a reflective layer in the back of the eye (tapetum lucidum), which aids night vision. Prosimians have a grooming claw on the second toe of each foot and special features to the ankle bone for grasping vertical branches. The reproductive systems of prosimians also differ from other primates.
There are more than 100 species and subspecies of lemurs, each with distinctive characteristics and behavioral patterns. Lemurs are endemic (or native only to) the island of Madagascar.
The extant prosimians typically weigh under 14 pounds, but as recently as 500 years ago there were lemurs in Madagascar as large as gorillas. All of the large lemurs went extinct after the arrival of humans, due to human hunting and human-caused environmental changes. Today, the survival of prosimian populations continues to be threatened by human activities, such as the pet trade, bushmeat trade, and habitat destruction. Lemurs are the most endangered vertebrates on Earth!