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Female orangutans evolve subtle response to forced sex

Female orangutans are forced to copulate against their will more frequently than has been observed in any other mammal. Scientists have generally believed that this is because females spurn mating with inferior “unflanged” males. Rejected males have no chance to mate unless they use coercion–or so it was thought. But new studies, using the first...

Female orangutans are forced to copulate against their will more frequently than has been observed in any other mammal. Scientists have generally believed that this is because females spurn mating with inferior “unflanged” males. Rejected males have no chance to mate unless they use coercion–or so it was thought.

But new studies, using the first hormonal data from wild orangutans, collected noninvasively from the urine of females, suggests that orangutan sex may be a lot more subtle than meets the eye.

Although coerced to mate by most males they encounter, the females may have evolved advantages in their mating interactions to influence who gets to father their offspring and to protect the resultant babies from being killed by the males who didn’t.

“Rather than being helpless victims of forced sex, female orangutans employ subtle counterstrategies,” says Cheryl Knott, a Boston University anthropologist and National Geographic emerging explorer, who led the research.


Photo by Tim Laman

In the orangutan world males with flanges–or cheek pads–are also the dominant males. They defend territories and emit loud “long calls” to attract receptive females. The cheeky ornaments are perhaps attractive to females because they show that the orangutan has ‘made it’ to flanged male status, which perhaps indicates better genetic quality, and thus make those that have them good candidates to sire healthy offspring.


Photo of flanged male orangutan by Tim Laman

NGS-Grant-logo.jpg“Using the first hormonal data from wild orangutans, we show that around ovulation females preferentially encounter and mate with prime males whose impressive size and ornamentation are probable indicators of genetic quality,” Knott and others write in their research paper, which was published by the biological research journal Proceedings of the Royal Society B.


But when not ovulating, females mate willingly with unadorned males and those past their prime, the scientists discovered.

Knott and her team came to this conclusion after observing hundreds of encounters between male and female wild orangutans in Gunung Palung National Park, West Kalimantan, Indonesia. The 220,000-acre (90,000-hectare) sanctuary contains a resident population of 2,500 wild orangutans.


Photo of Cheryl Knott in the field by Tim Laman

Orangutan mating is often lengthy and can include elements of both coercion and cooperation, the researchers noted. Nonetheless, by devising a method to rate sexual behavior, the scientists were able to determine when the females were primarily resistant to the males and when they were primarily receptive.

Almost a thousand urine samples were collected on filter paper from 10 of the females involved in the encounters, enabling the researchers to determine their reproductive status.

By combining all the data, the researchers found that ovulating females mated almost exclusively with prime males, perhaps in part because they engineered encounters with prime males by responding to the long calls made by those males.


Photo by Tim Laman

Unflanged males do not make long calls, so rather than “sit and wait” for mates as the prime males do, they must search for potential partners. When they find them, the data show, they often have their way, but typically and unbeknown to them when the females are not fertile and have little or no chance of becoming impregnated.

“Females mated most frequently with unflanged males overall, but they did so exclusively when conception risk was low,” the scientists concluded. “A single peri-ovulatory [period of fertility] mating with a past-prime male was highly resisted, while non-periovulatory matings met less resistance, and pregnant matings were not resisted at all,” they observed

Strategy of paternity confusion

Lowered mate selectivity outside of the peri-ovulatory period is consistent with another form of risk avoidance, the researchers said–”the anti-infanticide strategy of paternity confusion.”

“This strategy, wherein females mate with potentially infanticidal males in order to increase their perception of paternity probability, is common in…primates as well as some species of carnivores and rodents,” the researchers noted.


Photo by Tim Laman

Although infanticide has not been observed in wild orangutans, the scientists say that willingness to create confusion about paternity by mating during pregnancy, and avoidance of long calls from strange males, all indicate female strategies to reduce infanticide.

orangutan facts.jpgSo while to the observer female orangutans are often indiscriminately forced to mate by any males that encounter them, what this research suggests is that the females ultimately may have more control over who gets to pass his genes on to future generations. 

Said Knott, “Because orangutan don’t have sexual swellings [a signal of fertility to potential mates in other female primates], we couldn’t tell just by looking at them when they were ovulating. Now, with this new hormonal data, we see that females can use this lack of a visual signal to their advantage in their mating interactions.”

The research paper Female reproductive strategies in orangutans, evidence for female choice and counterstrategies to infanticide in a species with frequent sexual coercion, was published by by Cheryl Denise Knott, Department of Anthropology, Boston University, Melissa Emery Thompson, Department of Anthropology, University of New Mexico, Albuquerque, Rebecca M. Stumpf, Department of Anthropology, University of Illinois, Urbana-Champaign, and Matthew H. McIntyre, Department of Anthropology, University of Central Florida, Orlando.

The research was sponsored in part by the National Geographic Society’s Committee for Research and Exploration.

The photos on this page are courtesy of Tim Laman. You might like to see more of his pictures of orangutans on the National Geographic Magazine Web site Orangutans in the Wild.

Watch this National Geographic video about Kalimantan’s orangutans:

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