Strontium isotopes track sex-biased dispersal in Kibale

Stable isotopes have been useful for reconstructing diets and habitats of fossil species, but aspects of socioecology, such as group composition, have remained elusive. In a recent Royal Society Open Science publication, Marian Hamilton and Sherry Nelson explore the use of strontium isotopes to detect male and female dispersal patterns using KCP’s primate skeletal collections. Strontium isotope ratios vary across different bedrocks. Spatial differences are incorporated into plants growing above those rocks, and then into the animals that feed on those plants. Consequently, strontium ratios in skeletal tissues that develop at different times in the life cycle have the potential to trace an animal’s movements from childhood to adulthood. Kibale National Park proved to be an ideal location for this study because bedrocks, and therefore strontium, varied throughout possible migration routes within and between the primate communities of Kanyawara, Ngogo, Sebitole, and Kanyanchu. KCP’s skeletal collection offered a rare opportunity to develop this method because, unlike typical museum collections, all animals were from the same location, with opportunities to also isotopically sample the habitat itself using transects of plant and water samples. Furthermore, the collections include primates that vary in their group compositions, with species in which males stay in their natal group, and others in which females stay and males migrate. The collection also provided opportunities to explore effects of ranging behavior and life history variables on isotope ratios. Primate species at Kibale vary in day and home range sizes as well as length of the juvenile period, with chimpanzees having more extended development compared to monkeys. Hamilton and Nelson found that by incorporating these ecological and environmental variables into their methods, strontium isotopes effectively reflected group composition. These new methods can now be applied to fossil records, including exploring questions regarding early human ancestors in which group compositions and their ramifications, such as mating systems and social bonds, are unresolved.