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.
Orangutans are at risk of becoming extinct in the wild, yet scientists are only beginning to get an understanding of the variables that influence their reproduction. Emerging evidence suggests that there are significant differences both between and within populations in variables related to reproduction. Genetic distinctions between orangutans inhabiting the islands of Borneo (Pongo pygmaeus) and Sumatra (Pongo abelii) may explain some of the diversity, yet the role of ecological variation is not fully understood. To address this, researchers from four field sites on Borneo and Sumatra collaborated on an intensive study of ecology, energetics, life history, and endocrine physiology. One major objective was to understand how variation in diet and resulting energy balance influences reproductive function within these populations. The project used a comparative perspective to assess how orangutan reproduction responded under a wide variety of habitats—some with a relatively stable supply of fruits and others with a boom-and-bust pattern of mast fruiting. This has allowed us to assess which aspects of energy availability are the best predictors of reproductive function across populations. Our data also has allowed us to evaluate several hypotheses that may explain reproductive and life history differences between study areas. Finally, through this study we have been able to implement and standardize the collection of orangutan urine samples, the methods for calculating energy intake and expenditure, and the techniques for assaying orangutan hormones across field sites. The data collection phase of this project has now been completed and we have obtained preliminary results. These results show that orangutans in Gunung Palung are in worse energetic condition, as measured by the hormone C-peptide of insulin, than those in other study sites. This is consistent with the long interbirth intervals observed there and provides preliminary support for the Ecological Energetics Hypothesis. We also found the female reproductive hormones were positively and significantly correlated with C-peptide. This suggests that variation in energy balance affects reproductive function. These relationships were consistent within each of the field sites we have samples from, including both masting and non-masting sites, suggesting that ovarian function has a graded response to energy balance rather than just an on/off response to the dramatic shifts of the mast. Additionally, we have found that C-peptide in urine indicates that female orangutans have increased insulin responsiveness during adolescence — a feature that is shared with humans. Furthermore, we have discovered that C-peptide levels in both males and females correspond well to what we know about the energetic needs of the different orangutan life stages. Pregnant females showed increased C-peptide levels compared to cycling and lactating females. Prime flanged males showed much higher C-peptide than past prime flanged males or unflanged males due to their greater energetic demands. These results further demonstrated the utility of this method in providing another non-invasive measure of orangutan physiological status. This project has had significant broader impacts on the development of human resources and on the conservation of wild orangutans. Our methodologies are now being used not only in orangutan field sites, but by many researchers studying primates and humans worldwide. Our analysis of life history data has been the first, to our knowledge, to examine differences in genetic vs. environmental variables in explaining apparent life history differences between closely related species. Our findings about male development contribute to the broader biological question of developmental plasticity and the effect of fetal environment on adult mortality and disease outcomes. This study contributes to a variety of new evidence suggesting that prenatal and early life events, including nutritional status and organizing effects of hormones, can have long-term impacts on individuals. Additionally, we have provided extensive training of American, European and Indonesian students and other scientists. These include over 27 Indonesian students, 45 field assistants and 1 Master’s student. We expect, and have seen, that many of these Indonesian students, and some of the field assistants, will go on to receive further academic training and continue to advance scientific research and understanding in Indonesia. In the United States, we have trained 24 undergraduate students in laboratory methods, 16 graduate students and 2 post-doctoral scholars. Additionally, from US institutions, 5 undergraduate or recent graduates, 2 graduate students and 2 post-doctoral scholars have received extensive field training in orangutan data collection and processing methods. A further 5 graduate or post-graduate students from European institutions have received such training. Finally we have conducted extensive outreach activities through the Gunung Palung Orangutan Conservation Program with communities and schools surrounding our primary study site. These programs are fostering sustainable stewardship of the area's natural resources and building a future where orangutans and other wildlife can thrive alongside the human communities with whom they share the land and these resources.
