The Atelidae are the largest New World primates. In Alouatta and Lagothrix, females are much smaller than males, while in Ateles and Brachyteles, males and females are more similar to one another in body size. All of the Atelidae possess prehensile tails, which are bare on the distal underside. The tails are very sensitive, and are used for grasping much like an extra hand. All of the atelids also have 36 teeth, with a dental formula of (I2/2 C1/1 P3/3 M3/3). The relative size
and shapes of their teeth, as well as their jaws and chewing muscles, vary with their respective feeding adaptations.
Atelidae range in color from pale buff or gray (Brachyteles) to dark black (some Alouatta and Ateles). Some species of Alouatta are sexually dichromatic in body color. Alouatta, Lagothrix, and Brachyteles arachnoides have completely black faces. Oreonax has a lighter muzzle, many species of Ateles have distinctly paler eye patches, and Brachyteles hypoxanthus has pink and white patches of skin in variable patterns on the face. The latter also exhibit variation in pigment on the scrotum.
In both Ateles and Brachyteles females, the clitoris is pendulous and elongated. Male Brachyteles also have relatively large testes. Both of these genera also have long hooked fingers, and long limbs and tails relative to their bodies. Lagothrix and Alouatta have more compact bodies and limbs, and relatively shorter tails. Lagothrix travels by suspensory locomotion much less than Ateles or Brachyteles, but is faster and more agile than Alouatta. Alouatta possesses an enlarged hyoid bone, which contributes to the projection of long distance roars. Alouatta also has an elongated hindgut associated with the slow rate of food passage.
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