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Later Cayo Santiago Cohorts Had Smaller Rhesus Macaque Vertebrae

Rhesus macaque on Cayo Santiago, Puerto Rico
Research · Anatomy & morphology

A study of free-ranging rhesus macaques (Macaca mulatta) from Cayo Santiago, Puerto Rico, found that vertebrae became smaller across animals born between 1951 and 2002, a span representing roughly 10–12 generations in the population. The analysis used three-dimensional landmark data from six vertebrae in 210 known individuals — 132 females and 78 males.

The researchers assessed overall vertebral size, vertebral body height and shape while accounting for age at death. Overall size and vertebral body height declined with year of birth across the vertebral column. Comparisons between an early cohort born from 1950–1965 and a late cohort born from 1985–2002 showed significant size differences in both sexes.

Vertebral reductions were stronger in females

The reductions were stronger in females than in males. Male vertebrae were generally about 10–15% larger than female vertebrae. Sexual dimorphism increased only modestly in overall vertebral size — by about 1% between the early and late cohorts — but increased by about 8% in vertebral body height. The decline in vertebral body height was most pronounced in females, leading the authors to interpret the pattern as females becoming shorter over time while males largely maintained overall body length. In contrast, the study found no consistent evidence that vertebral shape changed over time.

The authors place the findings alongside earlier work showing temporal reductions in body mass, skull size and limb-bone dimensions in the Cayo Santiago population. They interpret the vertebral results as part of a broader reduction in skeletal size rather than a change restricted to one body region. Because the monkeys receive provisioned food, the authors consider simple resource shortage an unlikely explanation and instead discuss increasing population density and climatic stress as possible contributors. The population rose to nearly 2,000 animals during the period represented by the sample, far above an estimated optimum of about 300. Females may be especially sensitive because pregnancy and lactation are energetically costly, and Cayo Santiago females begin reproducing at about four years of age while full skeletal maturation is reached only around seven. The study did not directly test these mechanisms, so they remain hypotheses for the stronger female change.

Sources and image credit

  1. Villamil, C. I., Negrón, J. J., & Middleton, E. R. (2026). Temporal Changes in Vertebral Morphology of the Free-Ranging Rhesus Macaques From Cayo Santiago, Puerto Rico. American Journal of Biological Anthropology, 189(4), e70249. https://doi.org/10.1002/ajpa.70249Primary open-access research article first published 13 April 2026. Three-dimensional landmark data from six vertebrae in 132 female and 78 male rhesus macaques born from 1951 to 2002 showed decreases in overall vertebral size and vertebral body height over time. Females decreased more than males, increasing sexual size dimorphism slightly, while there was no consistent evidence of temporal change in vertebral shape.

Image: BDNf cynocephale — rhesus macaque on Cayo Santiago, Puerto Rico, photographed 1 April 2014; Wikimedia Commons / Flickr; representative animal from the study population, not an identified individual from the 2026 sample · CC BY 2.0 · Source

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