Abstract:
Stable isotope analysis of primate tissues in tropical forest contexts is an increasingly popular
means of obtaining information about niche distinctions among sympatric species, including
preferences in feeding height, forest canopy density, plant parts, and trophism. However, issues
of equifinality mean that feeding height, canopy density, as well as the plant parts and plant
species consumed, may produce similar or confounding effects. With a few exceptions,
researchers have so far relied largely on general principles and/or limited plant data from the
study area as references for deducing the predominant drivers of primate isotope variation.
Here, we explore variation in the stable carbon (δ13C), nitrogen (δ15N), and oxygen (δ18O)
isotope ratios of 288 plant samples identified as important to the three primate species from the
Polonnaruwa Nature Sanctuary, Sri Lanka, relative to plant part, season, and canopy height. Our
results show that plant part and height have the greatest effect on the δ13C and
δ18O measurements of plants of immediate relevance to the primates, Macaca sinica,
Semnopithecus priam thersites, and Trachypithecus vetulus, living in this monsoonal tropical
forest. We find no influence of plant part, height or season on the δ15N of measured plants.
While the plant part effect is particularly pronounced in δ13C between fruits and leaves,
differential feeding height, and plant taxonomy influence plant δ13C and δ18O differences in
addition to plant organ. Given that species composition in different regions and forest types will
differ, the results urge caution in extrapolating general isotopic trends without substantial local
baselines studies.