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Monkeys flourish in the tropical and subtropical regions of South and Central America, Asia and Africa. Monkeys also are found in regions outside of the tropics in both Africa and Asia (but never in the Americas). Although monkeys everywhere have similar external appearances, a closer look reveals unique and distinctive differences between the American and Old world monkeys.
NOSTRILS
New World monkeys
Flat nosed (or platyrrhine)
Nostrils are far apart and open to the side
Old World monkeys
Downfacing nose (or catarrhine)
Nostrils are closer together and open downward or forward
(Apes and humans share this nasal feature as well.)
PREMOLARS and MOLARS
New world monkeys
Three rather than two premolars
Molars are relatively large
The last molar is comparatively small or sometimes absent
Old World monkeys
Two premolars
Premolar in the mandible is SECTORIAL. That is, it's specialized for sharpening the upper canine
Molars have sharply connected cusps
EAR REGION
New world monkeys
Tympanic membrane connected to external ear by a bony ring
Old World monkeys
Tympanic membrane connected to external ear by a bony tube (is visible on the outside of the skull)
TAILS
New world monkeys
Some species (howlers, spiders) have prehensile tails
Old world monkeys
All have tails but lack prehensility feature
Sitting pads (ischial callosities) around the tail region. These thickly calloused skin areas supports the animals while they sit in trees (or on ground) to feed, rest, or sleep
HANDS
New world monkeys
Thumb orientation lies in line with other digits. Opposes the next digit in a scissorslike grip
(Spider monkeys have lost their thumbs)
Fingernails = some species have fingernails on big toe
Old world monkeys
Thumbs are rotated and more opposable, more like ours (Hanuman langurs, baboons)
(Thumbs reduced or absent among colobine monkeys of Africa)
Fingernails and toenails are present on all digits
INFANT CARE
New world monkeys
Male involvement common
Old world monkeys
Male involvement rare or absent in most species (gibbons and siamangs are the exception to this non-male involvement)
OTHER DIFFERENCES
New world monkeys
Few species have one male multifemale groups like many of the Old World monkey species
Have scent glands rely more on scent to mark territories than Old World species
Old world monkeys
Prominent sexual skin around the anus and vagina in females which swells during estrous cycle
HABITATS
New world monkeys
Confined to arboreal habitats
Rely heavily on fruit and less on foliage in comparison to Old World monkeys
Old world monkeys
Tolerate a wider range of habitats, from rainforest through savanna fringe or open savanna to high mtn ranges and semiarid regions even urban environments (remember the hanuman langurs).
Many spend some or most of day on the ground
Some species (leaf-eaters) have specialized digestive tracks for processing low-value food