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Orangutan diet: lessons from and for the wild
Hardus, M.E.
Publication date
2012
Link to publication
Citation for published version (APA):
Hardus, M. E. (2012). Orangutan diet: lessons from and for the wild.
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8
SUMMARY
As a result of the continuing loss of their forest habitat, orangutans are forced to live in fragmented and disturbed forests and mostly occur in relatively small populations. Consequently, the Sumatran orangutan (Pongo abelii) is listed as critically endangered, and the Bornean orangutan (Pongo pygmaeus) is listed as endangered by the IUCN (Red List 2011). In fact, without direct and committed intervention, such as strict law enforcement on plantation development, (illegal) logging and hunting, their habitat may be gone during the next few decades. Although a number of large pristine forests have been designated as protected areas on both Sumatra and Borneo, logging and illegal clearing for plantation development still occurs inside many of these areas. Moreover, large numbers of orangutans live outside the protected areas, where conservation regulations are even less recognized, to the point of being effectively absent. Several researchers have investigated how Bornean orangutans survive in degraded landscapes, and they have found that these populations appear to cope better than their Sumatran counterparts, as indicated by there being less reduction in density in response to logging. Behavioral data are, however, virtually absent for Sumatran orangutans, but are critical for helping the survival of the species.
One of the main research priorities of this project is thus to determine the impact of logging on the remaining pristine forests of Sumatra and on the lives of the orangutans (chapter 2). In Ketambe logging has significantly modified forest structure and orangutan food resources, specifically important fallback and liana-derived foods. Individual orangutans behaved differently when in logged as opposed to primary forest; with their having moved more and rested less in the former. With the exception of figs, which were virtually absent in the logged area, the proportion of other broad dietary categories (i.e. fruit, leaves and bark) in both forest categories remained overall similar. Life after logging seems energetically more expensive for orangutans. Based on the results of this study, we provide recommendations for conservation research and guidelines for reduced-impact logging.
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As logging significantly affects orangutan food resources, one of the main challenges for individuals in these areas is in adequate food acquisition. To comprehensively assess the effect of logging on orangutan diet, it is necessary to understand the composition of an individual’s diet at the plant species level, diet overlap (overlap in time spent feeding on the same species and overlap in species consumed) between individuals and how diets vary seasonally (chapter 2). After controlling for food availability and home-range differences between individuals with similar energetic requirements (i.e. adult parous females), the majority of individual orangutans consumed a diet of distinct species throughout the year. Their diets were thus significantly different from each other, with the exception of figs, which were the only food species that caused an increased diet overlap between individuals when available in the forest, mostly because orangutans may come together in large fig trees during periods that these trees provide large crops.
This study has uncovered that individuals of a population exhibit different diet compositions, which indicates that they do not strictly consume what is available in the forest, and that each individual actively (i.e non-randomly) finds a specific way to satisfy its energetic needs. This is illustrated by the rare cases of hunting and consumption of slow loris meat (Nycticebus coucang), observed only in one particular female with infant in the population (chapter 4). These captures seemed to be opportunistic but orangutans may have used olfactory cues at close range to detect the prey. Slow loris captures occurred only during periods of low fruit availability, suggesting that meat may represent a filler fallback food for orangutans. Orangutans ate meat more than twice as slowly as chimpanzees (Pan
troglodytes), suggesting that group living may function as a meat intake accelerator in
hominoids. Using orangutan data as a model, time spent chewing meat per day would not have required an excessive amount of time for our social ancestors (australopithecines and hominids), as long as meat represented no more than a quarter of their diet.
To optimize the capacity of maintaining viable populations in the wild, reintroduction of animals back into the wild has become increasingly important for conservation. Orangutans are, always released into unfamiliar forests in which knowledge about what to eat will be crucial to assure their short- and long-term survival. As such it is important to understand the orangutan’s natural predispositions towards novel foods compared to those they are familiar with, in other words, what is their acceptance and consumption rates of novel foods (chapter 5). In an experimental setting, orangutans (Sumatran, Bornean, and their interspecific hybrids) chose novel food more often, but
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remained more cautious during feeding than towards familiar foods. Although both orangutan species consumed novel food in similar amounts, before consumption, Sumatran orangutans chose novel food significantly less promptly than Bornean orangutans. Reduced acceptance of novel food in Sumatran orangutans might partly explain why Bornean orangutans cope better with logging and the subsequent effects on food availability.
To increase the chances of success in reintroduction projects, is it thus necessary to learn how to stimulate an increase to the degree of novel food acceptance and consumption (chapter 6). Repeated exposures in an experimental setting did not lead to an increase of acceptance of novel foods, but did increase the amount of food consumed by orangutans. Repeated exposures also decreased explorative behavior. The presence of other orangutans resulted in increased acceptance of the novel food items as well as an increase in the amount of the novel food items consumed compared to when they were alone. Repeated exposures and sociality may therefore benefit ex-captive orangutans in augmenting and diversifying their diet and, once practiced before being released, may aid individuals’ adaptation to a new forest habitat. This will likely improve their chances of survival. Therefore, it is recommended that orangutan reintroduction projects increase the emphasis on repeated exposure and social feeding on novel food items during the rehabilitation stage of the reintroduction process.
