Review of “reproductive parameters of female orangutans”
Wyatt HilyardM. ScogginANTH 410March 30, 2013 Review of “Reproductive Parameters of Female Orangutans” In October 2012, Birute Mary Gadikas and Alison Ashbury published a paper entitled“Reproductive Parameters of Female Orangutans (Pongo pygmaeus wurmbii) 1971-2011, a 40-Year Study at Tanjung Puting National Park, Central Kalimantan, Indonesia.” The article detailsand summarize observations and data collected concerning female orangutan reproduction,collected over a 40 year period. For the study, they were concerned with four main reproductiveparameters: age at first reproduction, interbirth interval (IBI), sex ratio at birth, and infantmortality. The authors applied their findings to several ecology hypotheses, concluding that theecological energetics hypothesis (increased diet quality leads to a faster rate of reproduction) isbest supported by the study. As with many primate studies, this project can be put to use in several different ways. Theinformation can be used to better understand orangutans and provide superiorconservation/rehabilitation projects in the future. It can also be used for human evolution studies;due to their close evolutionary and genetic ties to humans, “documenting orangutan life history isuseful for identifying derived features of human life history.” The methodology of the project seems to be pretty sound. The work was carried out overthe 40-year period by either Galdikas herself or assistants who had been trained by her, whichminimizes drastic differences in methodology and data collection. I imagine there is still somedegree of difference in personal data collection style, but it is kept at a minimum. The largest
problem I found, which they point out as well, is that they did not start with the first generationof mothers from birth. Their ages and age at first reproduction had to be estimated based onphysical and behavioral clues. It is not something that can be fixed at this point, just a margin oferror that has to be considered. As part of the study, the authors also compare and contrast their site, Camp Leaky inTanjung Puting, Indonesia, with a site in Sabah, Malaysia, Sepilok Orangutan Sanctuary. Notonly are the two projects run differently, but they are dealing with two different species andenvironments. The orangutans at Camp Leaky are healthier, have higher reproductive rates, andsignificantly lower infant mortality rates than those at Sepilok. There are three main factors forthis: different species, different environments, and different provisioning. Camp Leakeysorangutans are Pongo pygmaeus wurmbii, whereas those at Sepilok are Pongo pygmaeus morio.It is a slight difference in subspecies due to geographical location, but it is worth noting that theyare thus genetically different. Galdikass team provisions the orangutans once a day on what they call a provisioningplatform, which is designed to increase sociality among the individuals and decrease dependencyon the researchers. The project leader at Sepilok believes feeding platforms actually decrease thehealth of mothers due to increased social stress and increased exposure to parasites. This ties into another facet of the study, sex ratio at birth. The table on page 67 shows the proportions ofmales at birth: 0.63 at Camp Leakey, 0.07 at Sepilok (rehabilitants), 0.51 in a zoo population.This very large disparity is explained by a few factors. The increased stress at feeding platformscauses the mothers to have a disproportionately high number of daughters to sons. Also, due tothe nature of orangutan life in Galdikass site, “it is advantageous for mothers at Camp Leakey todifferentially produce male offspring who will emigrate from their natal area rather than females
who will remain close by and increase feeding competition around the feeding platform.” Aboutprovisioning, they conclude that the methods of feeding “may play a role in shaping birth sexratios, and social connectivity may play a casual role in infant mortality rates.” I really liked the tables on pages 64 and 65, detailing the age at first reproduction of themothers and pertinent data of their offspring. They are well organized and documented, as are allthe tables. All the raw data of the study is very nicely laid out in the whole article; they makenote of cases and data points that were excluded and explain why. They also include the reminderthat the study is not necessarily over: “It is our hope that in the future, as more data becomeavailable, it will be possible to conduct three-way analysis, including third-generation mothers astheir own category.” In summation, the study seems to have been well planned and executed, as has the article.I dont see any major flaws in methodology or findings. They conclude that the ecological lifehistory model is not supported by their study, but it may explain reproductive variation on anevolutionary scale. The ecological energetics model is much more suited to their study, showingno problems when applied to both variation between populations within a species, and multiplegenerations within a population. I look forward to the findings of this study being applied tobenefit rehabilitation of orangutans, as well as being introduced as new information for humanevolutionary studies.