A fifth hand
Why Can a Monkey Hang by Its Tail?
Some monkeys can hang their whole weight from their tail and pick a fruit at the same time. Most cannot.
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Step 1 of 5
Most tails only balance.
A tail swinging out to the side shifts an animal's weight and stops a turn tipping into a fall. Almost every monkey's tail does this and nothing else — it cannot hold on.
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Step 2 of 5
A gripping tail has a bare patch.
Monkeys that hang by their tails have a hairless strip on the underside of the tip, ridged with the same kind of pattern as your fingerprints. Bare, ridged skin grips a branch; fur slides.
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Step 3 of 5
It has its own patch of brain.
The strip of brain that handles touch and movement gives a big area to the tail in these monkeys, the same way it gives a big area to your hands. It is controlled like a limb, not waved like a rope.
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Step 4 of 5
Hanging frees both hands.
The point is not travel, it is reaching. Anchored by the tail, a monkey can lean right out to fruit at the thin end of a branch that would never hold its weight.
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Step 5 of 5
Only the American monkeys have it.
Monkeys in Africa and Asia never evolved it in over thirty million years of living in similar forests. It is a reminder that evolution does not find every good answer — it works from whatever the ancestor happened to have.
A gripping tail is strong enough that spider monkeys sometimes sleep hanging from it, hands and feet completely relaxed.
The short version
A few American monkeys grip with their tails using a bare ridged patch and a limb's worth of brain, which frees both hands to reach.
Try it yourself
Try picking something off a high shelf while holding the doorframe with one hand, then while hooking your foot round a chair leg. An anchor buys reach.