The lorisiform wrist joint and the evolution of “brachiating” adaptations in the hominoidea

TLDR

Lorisine primates possess distinctive wrist joint traits—such as a reduced ulna tip, a proximal ulnar head, and a displaced pisiform—that are absent in other quadrupedal primates but characteristic of extant hominoids and have been interpreted as adaptations to arm‑swinging locomotion. The study aims to determine whether hominoid‑like wrist features in lorisines evolved to enhance wrist mobility for cautious arboreal locomotion. The authors find that these features may have evolved in both lorisines and large hominoids to enhance wrist mobility for cautious arboreal locomotion involving little or no leaping.

Abstract

Abstract In lorisines ( Loris, Nycticebus, Perodicticus, Arctocebus ), the tip of the ulna is reduced to the dimensions of a styloid process, a new and more proximal ulnar head is developed, and the pisiform is displaced distally away from its primitive contact with the ulna. In some Nycticebus , intra‐articular tissues separate the ulna from the triquetrum. These traits are not seen in other quadrupedal primates, but they are characteristic of extant hominoids. Among hominoids, these features have been interpreted as adaptations to arm‐swinging locomotion. Since hominoid‐like features of the wrist joint are found in lorisines, but not in New World monkeys that practice arm‐swinging locomotion, these features may have been evolved in both lorisines and large hominoids to enhance wrist mobility for cautious arboreal locomotion involving little or no leaping. Most of the other morphological traits characteristic of modern hominoids can be explained as adaptations to cautious quadrupedalism as well as to brachiation, and may have developed for different reasons in different lineages descended from an unspecialized cautious quadruped resembling Alouatta .