Humeral cross-sectional shape in suspensory primates and sloths

Area of Science:

• Paleoanthropology
• Primate Anatomy
• Biomechanics

Background:

• Cross-sectional geometry of long bones in African apes correlates with locomotor behaviors.
• More circular humeral cross-sections are observed in arboreal and suspensory chimpanzees compared to terrestrial gorillas.
• This suggests circularity adapts to multidirectional loading during acrobatic locomotion.

Purpose of the Study:

• To investigate if a more circular humerus reflects increased forelimb suspension use in other primates and nonprimate mammals.
• To compare humeral cross-sectional geometry (Imax/Imin ratios) across anthropoid primates and suspensory sloths with varying degrees of forelimb suspension.
• To correlate morphological data with behavioral data on locomotion.

Main Methods:

• Obtained midshaft humeral cross-sections from anthropoid primates and two genera of suspensory sloths.
• Calculated the ratio of maximum to minimum second moments of area (Imax/Imin) for each cross-section.
• Compared these ratios within and between groups, correlating findings with known locomotor behaviors.

Main Results:

• Suspensory sloths exhibit circular humeri.
• More suspensory hominoids and atelines have more circular humeri than quadrupedal taxa and Cebus monkeys, respectively.
• Considerable overlap exists between taxa, and finer comparisons reveal variations not aligned with predictions.

Conclusions:

• Humerus Imax/Imin ratios are informative for distinguishing generalized locomotor modes (suspensory vs. quadrupedal).
• Additional structural data are necessary for more precise assessments of locomotion.
• The study highlights the complexity of linking bone morphology to specific locomotor behaviors.