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Do All Primates Have Five Fingers on Each Hand?

Unlike most primates, spider monkeys don’t have thumbs. Their four long fingers essentially form a hook, an evolutionary adaptation resulting from their arboreal lifestyle, making it easier for them to traverse tree branches quickly and efficiently. A thumb might snag on branches as the creatures move gracefully through the trees with a hand-over-hand motion, a type of locomotion called brachiation.

Look, Ma, three hands:

  • The spider monkey’s ancestors had an opposable thumb, but over time it morphologically disappeared due to lack of use.

  • The spider monkey's long, specialized tail is another morphological adaptation. A prehensile tail allows them to move through the canopy with ease.

  • There’s a distinct pattern of lines on a patch of skin at the tip of their tails, almost like a fingerprint. This patch of skin, basically a friction pad, helps the tail grip surfaces -- almost like a third hand.

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More Info: Smithsonian magazine

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