Orangutans are at risk of becoming extinct in the wild, yet scientists are only beginning to understand the variables that influence their reproduction. Existing evidence suggests significant reproductive variation both within and between populations. Genetic distinctions between orangutans inhabiting the islands of Borneo (Pongo pygmaeus) and Sumatra (P. abelii) may explain much of the diversity, yet ecological variation may also play a significant role. To address this, researchers from four field sites on Borneo and Sumatra will collaborate on an intensive study of ecology, energetics, life history, and hormonal physiology in orangutans. One major objective is to understand how variation in diet and resulting weight gain or weight loss influences reproductive functioning. This will be evaluated through measurement of hormones in urine and make possible the documentation of changes in reproductive status, such as when an individual begins cycling after giving birth. These reproductive changes will be linked to changes in food availability and energy status. Urine samples will be used to validate innovative new techniques for the non-invasive assessment of physical condition which can then be applied to other species. The results of this project will have a potentially important impact on understanding the evolution of life history traits (such as the age at first reproduction and the length of the interval between births) in animals. Models of life history variation predict slower, longer life histories, with longer intervals between births, in species living in stable, low mortality environments. A contrasting model, from the field of reproductive ecology, indicates that higher reproductive rates, and faster life history variables, are found within populations of great ape species (including humans) occupying higher quality habitat. Thus, these two models make different predictions for the comparison of Sumatran orangutans, who live in an environment of more food availability, compared to Bornean orangutans, where food fluctuates more. Investigating the relative importance of life history evolution versus shared physiological responses between these two species will serve as a model for comparative studies of other species. The study focuses on collaboration across field sites to maximize data and to allow for replication of findings across study areas utilizing identical methodologies while improving the infrastructure and facilities at each site; this process will facilitate and provide opportunities for the training of students in behavioral observation and laboratory techniques. Orangutans share substantial genetic and physiological similarities with humans, yet have inter-birth intervals two to three times as long, thus the study of orangutan reproduction has important applications to understanding the evolution of human life history and reproductive adaptation. Finally, the study of reproduction in endangered species has important conservation applications in quantifying the influence of habitat degradation on reproductive rates. By providing knowledge essential for conservation management, deterring illegal logging and through involvement in conservation efforts in surrounding communities, this project will bring increased attention to the last remaining populations of orangutans, and the synergy of scientific and conservation goals may be one of the best strategies at hand for saving these species and their habitats.
