Temporal Changes in Vertebral Morphology of the Free-Ranging Rhesus Macaques From Cayo Santiago, Puerto Rico

The objective of this study is to assess temporal change in vertebral size and shape in macaques born on Cayo Santiago from 1951 to 2002 in order to clarify patterns of temporal change identified in the cranium, appendicular skeleton, and body mass.

We assessed overall size, vertebral body height, and overall shape using a published dataset of 3D coordinate data representing six vertebrae from 132 female and 78 male Rhesus macaques from Cayo Santiago. We performed multiple linear regression with year of birth and age at death as independent variables, and assessed differences between early (1950-1965) and late (1985-2002) cohorts using standard statistical methods.

We found decreases in overall size and vertebral body height over time across vertebrae. Females decreased more than males, resulting in greater sexual dimorphism over time. We found no evidence of vertebral shape change. Incidentally, we found that thoracolumbar vertebral body height is not sexually dimorphic in this group and that males largely maintain vertebral body height over time.

Our results are consistent with an overall reduction of the skeleton in this population rather than localized reductions in specific body regions or in soft tissue mass. Females displayed more extreme changes than males, and this pattern may suggest that females are under stronger selection for environmental reasons, possibly due to the physiological demands of pregnancy. Female-male differences may also reflect early female age at first birth, which occurs prior to full skeletal maturation and may directly affect female growth.