Disproportionately long, spindly limbs inspired the spider monkey's common name. Their deftly prehensile tails, which may be up to 305 cm (35 inches) long, have very flexible, hairless tips and skin grooves similar to fingerprints. This adaptation to the spider monkey's strictly arboreal lifestyle serves as a fifth hand. Adults reach an average body length of 50 cm (20 inches) and a weight of 6.4 kilograms (14 pounds).