New World Monkeys by Rosenberger Alfred L.;

Author:Rosenberger, Alfred L.;
Language: eng
Format: epub
Publisher: Princeton University Press
Published: 2020-06-23T00:00:00+00:00

The sensorimotor strip in the brain controls tail use

Capuchins are not the only New World monkeys that have evolved a remarkable neurological capacity for interpreting sensation, effecting motion control, and applying dexterity through an appendage. An entire clade accomplished something similar in the form of a prehensile tail. This, too, is imprinted on the external morphology of the brain, in the arrangement of the gyri and sulci of the cerebral cortex found in atelids, in an area called the sensorimotor strip (fig. 7.3). The morphology of the strip is distinctive in these prehensile-tailed monkeys. They present a subdivision that coordinates the tail that is unique in that it is easily seen on the surface of the brain. In other monkeys, even the semiprehensile-tailed Cebus, the homologous region is positioned differently. It is not visible on the lateral surface of the brain because it is only exposed in the deep midline cleft that divides the right and left hemispheres. Enlarged and shifted from the midline to a more lateral position, the atelid tail unit may be packed with more neurons, and it may also provide a more advantageous wiring system that coordinates the tail with the limbs, which are also controlled by the sensorimotor strip. Only one of the atelids, Ateles, has been studied electrophysiologically in the lab in order to document the role of this area. But because the anatomy is uniform among all four, we assume that the same region of the cortex in the others is responsible for producing the same behaviors. This is true even in the decidedly smaller brain of Alouatta.

The atelid prehensile tail is hypersensitive, multiaxial, superbendable, and ends in a strong tip. As in other primates, the tail is furry. But, as previously discussed, on its underside, for the last third of its length, there is a hairless, soft strip of friction skin, embossed with the same kinds of miniscule, patterned dermal ridges that all primates, including humans, have on fingertips, palms, and soles. Because of its specialized, multijointed anatomy, an atelid tail can coil around a branch and twist about its own long axis while carrying the monkey’s full weight. It is strong enough to fling a 20 lb, 9 kg, spider monkey from one perch to another. Most amazingly, the spider monkey’s tail acts like a fifth limb, fully incorporated into locomotor movements during a hand-over-hand, suspensory-style locomotor cycle or during the quadrupedal step cycle that is used in walking and bounding on branches. It is completely functional at all times rather than just being an accessory of occasional value, acting as an emergency brake as it is does in howlers, or in woolly monkeys, where it acts as a stabilizer.

FIG. 7.3. Lateral views of the brains of a prehensile-tailed monkey, left, and a semi-prehensile-tailed monkey, right. Abbreviations for areas representing anatomical regions: T, tail; L, leg; A, arm; F, face. Adapted from Radinsky (1972).

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