39 - Collagen Fiber Orientation in the Midshaft Humerus of Virunga Mountain Gorillas (Gorilla Beringei Beringei)

Poster Board Number: 39
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Abstract Body : Collagen fiber orientation (CFO) observed in bone microstructure has long been studied to reveal form-function relationships, based on the hypothesis that collagen fibers orient themselves preferentially within bone to accommodate different loads. Studies have demonstrated that collagen fibers aligned longitudinally are better able to resist tensional loads, and transversely aligned fibers are more resistant to compressional loads.

This study addresses whether there is a consistent patterning of CFO at the midshaft humerus that reflects locomotor development in wild mountain gorillas from Volcanoes National Park (VNP), Rwanda. Behavioral studies indicate that while the predominant locomotor behavior in adult mountain gorillas is terrestrial knuckle-walking quadrupedalism, infants show more variable locomotor behaviors and increased arboreality. Around two years of age, infants undergo a behavioral shift to a more “adultlike” form of knuckle-walking quadrupedalism, with a significant decrease in locomotor behaviors such as climbing.

Using a naturally accumulated skeletal collection of mountain gorillas from VNP, our first aim is to determine whether infants accommodate this shift in locomotor behavior by a changing CFO pattern in older individuals (i.e., > 3 years). Specifically, we expect less variation in CFO among older individuals, given their less variable locomotor repertoire. Our second goal is to determine the spatial distribution of CFO across regions (i.e., concentric rings and radial sectors) within cross-sections.

100-micron sections of midshaft humeri from gorillas (n = 10) of known age were prepared for circularly polarized light (CPL) imaging, following established protocols. In CPL, birefringent collagen fibers oriented transverse to the optical axis refract light and appear bright, while light refracting from those oriented longitudinally (i.e., parallel) to the optical axis appear dark. CPL images were converted to grayscale in Fiji and binned using an 8-bit color look-up table. Percent area represented by LUT bins was analyzed to test for age differences among whole cross-sections, and across concentric rings and radial sectors within cross-sections, using Kruskal-Wallis and Friedman’s ANOVA.

No significant age differences were found in CFO; however, a predominance of longitudinal CFO was found across all ages. Significant regional variation was found between radial sectors and between concentric rings within cross-sections for the two age groups, but post-hoc test results were not significant. Infants show a slight decrease in mean bin value (1.33), indicating more longitudinal CFO, versus older individuals (1.47).

These results expand on a prior study looking at CFO of the midshaft femur. At both skeletal sites, infants showed increased variability in CFO compared to older individuals, as well as a predominance of longitudinal CFO for all individuals. The midshaft femur shows more significant regional patterning between concentric rings within cross-sections of infants, as revealed by post-hoc tests (p < 0.05), than the humerus. This may indicate a transition to adult-like knuckle-walking quadrupedalism around three years of age, due to differences in load distribution across skeletal elements. This study provides preliminary evidence for regional variation of CFO and demonstrates the potential for the internal structure of bone to advance an understanding of the adaptive response of bone to locomotor behavior.