Is Olecranon Length Reflective of Adaptations for Speed or Power in Anthropoids?
Authors: Selby Michael S 
Published: November 2023

Olecranon length is thought to reflect locomotor adaptations, in that cursorial mammals have short olecranons and fossorial species have long olecranons relative to overall ulna length, as adaptations for speed and power, respectively. In closely related anthropoids, more arboreal species were found to have relatively longer olecranons, which were argued to be for greater power as a climbing adaptation, than the more terrestrial species. The short olecranon in gibbons has also been suggested as an adaptation for rapid elbow extension, similar to the cursorial condition. However, more recently, the relatively short olecranon in orangutans was argued to be a power adaptation as well. Here, terrestrial quadrupedal, arboreal quadrupedal, and suspensory anthropoid primates are compared to sloths and cursorial carnivores to test the hypothesis that olecranon length in anthropoids reflects adaptations for greater power in arboreal climbing taxa and greater speed in extension in terrestrial or brachiating taxa. Olecranon length (OL) and distal ulna length (UL) were measured from the center of rotation in the trochlear notch. To compare these metrics across a broad size range, a ratio was taken for each metric and a geometric mean of forelimb and hindlimb joint surface areas. An OL/UL ratio was also calculated to compare the lever arm/load arm ratio for the triceps brachii muscle.

The lever arm/load arm ratio (OL/UL) separates the taxa into three distinct groups. Most anthropoids have a ratio that falls between 0.10 and 0.15. Gibbons, spider monkeys, and sloths have a ratio of less than 0.1. Felids, canids, and hyeanids have a ratio greater than 0.15. More terrestrial cercopithecines do not differ significantly for this ratio from arboreal cercopithecines and many ceboids. Canids and felids do not significantly differ from most anthropoids for ulna length, but have significantly longer olecranons compared to all primates. Hominoids, particularly the highly suspensory Asian apes, have significantly shorter olecranons than other anthropoids. Sloths do not significantly differ from hominoids for olecranon length or from Asian apes in OL/UL ratio.

These results suggest that longer olecranons in arboreal anthropoids are not adaptations for power, as the OL/UL ratio did not differ between more terrestrial and arboreal cercopithecines, howler monkeys or cebids (Saimiri and Cebus). However, the most highly suspensory taxa (Asian apes and sloths) have the lowest OL/UL ratio in this sample. Contrary to the prediction of mammals adapted for rapid locomotion having muscles attached closer to the joint, cursorial carnivores have a greater lever arm/load arm ratio than anthropoids. This suggests that cursorial carnivores have relatively longer olecranons to provide for a more powerful lever arm for the triceps muscle, enabling negative work in the forelimb during rapid locomotion. Such force dissipation would be less important in anthropoids that use a compliant gait during arboreal locomotion. The similarities between hominoids and sloths in terms of olecranon length support the hypothesis that olecranon reduction in hominoids is an adaptation for increased forelimb extension.

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