Supplementary Material for: Does Oxygen Stable Isotope Composition in Primates Vary as a Function of Vertical Stratification or Folivorous Behaviour?

Stable isotopes of oxygen often vary within a community of primates. For example, folivorous monkeys that forage in the upper reaches of the forest canopy tend to evince high δ¹⁸O values, whereas those that prefer the understory tend to have lower δ¹⁸O values. Given that leaves also have high δ¹⁸O values, particularly higher in the canopy, there is uncertainty as to which behavioural variable – vertical stratification or folivory – is the primary determinant of variation in δ¹⁸O values. Here, we explore further δ¹⁸O values from the Taï Forest monkeys (<i>n</i> = 7 species; <i>n</i> = 33 individuals) by examining the interaction between diet and vertical stratification, thereby allowing us to differentiate the effects of each covariate. We found that δ¹⁸O values varied as a function of mean canopy height, but not folivory, resolving uncertainty about the primary cause of δ¹⁸O variation. This outcome revolves largely, but not entirely, on the behaviours of <i>Procolobus verus</i>, a highly folivorous but understory forager. Relatively elevated values in <i>Cercopithecus diana</i>, a frugivorous but middle-to-high canopy forager, raises the possibility that plant reproductive tissues (e.g., fruits, flowers) may be increasingly sensitive to evaporative fractionation at higher forest canopy levels. Overall, our results further affirm the value of using δ¹⁸O values to estimate the vertical behaviour of primate species in a fossil assemblage.