Crashing through undergrowth, splashing through creeks, Cheryl Knott races to keep up with the 100-pound ape adroitly clambering through the lush canopy overhead. She\’s following the wild orangutan, whom she calls Beth, through the Indonesian rain forest, documenting the female\’s daily search for fruit to feed herself and the newborn infant who clings to her reddish fur.

The scene typifies Knott\’s many research expeditions to Gunung Palung National Park in an Indonesian province on the island of Borneo, where the associate professor of anthropology has been studying orangutans since 1992. In addition to observing and documenting the endangered species\’ behavior, Knott and her field team of Western and Indonesian researchers gather samples of the orangutans\’ food, which she\’ll later analyze for calorie and nutrient content, and of their urine, which she\’ll test to measure the animals\’ hormone levels. Her not-so-glamorous role as an \”orangutan pee collector\” earned Knott a place in Popular Science\’s 2005 list of the worst jobs in science. \”Have I been pissed on? Yes,\” she told the magazine.

Knott began studying orangutans as a graduate student at Harvard in the early 1990s. \”I was interested in a general sense in reproduction because evolution operates through reproductive success,\” she says. While her initial interest was in human reproduction, she explains, \”I started to realize that we actually knew a lot more about humans than we did about our closest relatives, the great apes.\” With plenty of other primatologists focused on chimpanzees and gorillas, humans\’ nearest cousins, Knott found a niche studying our next closest kin, orangutans.

The long-limbed, big-bellied apes proved to be the ideal research subjects, as their reproductive behavior is unique in many ways. Female orangutans, for example, give birth only once every six to nine years, the longest birth interval of any mammal. In addition, there appear to be two different types of adult male orangutans—one type with large bodies and wide cheek flanges, and another type that, while also sexually mature, remains smaller and without the impressive flanges. The existence of two male morphologies is fairly common in insects and fish, but it\’s practically unheard of in mammals.

After nearly two decades of tracking elusive orangutans through the forest, and positioning plastic sheets below their nightly nesting spots to catch falling urine, Knott is well on her way to solving both of these puzzles. In each case, the answers lie in the availability of food in the forest habitat to which orangutans have adapted.

\”We tend to think of the rain forest as this cornucopia of food and fruit all the time,\” says Knott, \”but actually—even though the temperature doesn\’t change like it does in the temperate zones—there are big fluctuations in fruit. And these Southeast Asian rain forests fluctuate much more than those in Africa or South America.\” Every few years, the Bornean rain forest bursts into a \”mast fruiting\”—up to 80 percent of its trees produce fruit simultaneously—that provides a feast of exotic rain-forest fruits. The masts are followed by years of low fruit production, leaving orangutans with little else to eat than leaves and bark. As an adaptation to this boom-and-bust cycle, orangutans are very good at storing fat, which they live on during lean times. Knott\’s analysis of orangutan hormones shows their unique reproductive characteristics also reflect the unstable food environment.

Her research has revealed a direct link, for example, between orangutans\’ long birth intervals and the spotty availability of their food. Her lab analyses show female orangutans\’ reproductive hormones increase when their nutritional status improves, so they\’re most likely to become pregnant during a mast fruiting or other fruit peak. The females—single mothers who get no help from their mates in providing nutrition for their babies—then carry and nurse their young for about six years, using up vast amounts of energy, resulting in suppressed hormone levels. Once a juvenile orangutan is weaned, the mother may need to wait for the next mast fruiting, which could be two or three years away, to reach hormone levels at which she could become pregnant again.

\”So the pattern of the forest affects their hormone levels and whether they can get pregnant,\” says Knott. \”I think that makes a lot of sense, but we weren\’t really aware of it before I began my research.\”

Inconsistent food availability may also be the key to solving the mystery of dimorphism in male orangutans. While humans and other mammals develop from adolescents to adults within an expected time frame, male orangutans reach their fully developed adult stage—characterized by wide cheek flanges, large throat patches, and the production of long bellowing calls—at unpredictable times. Some develop these traits at age 10, others at 15 or 20, and some may never develop them at all, retaining an adolescent-like morphology throughout their lifetime. Scientists have long observed this phenomenon but have been at a loss to explain it.

Knott\’s recent studies reveal that sustaining the prime state requires massive energy resources, so males don\’t stay big and flanged for long. Jari Manis, a male in peak condition when Knott encountered him in 1997, was a shadow of his former self—shriveled cheeks, wasted muscles, sunken eyes—when she saw him again nine months later. Jari Manis means \”ring finger,\” and if she hadn\’t spotted the damaged finger for which he was named, Knott says she may not have even recognized the once-magnificent male.
Cheryl Knott’s research camp lies within Gunung Palung National Park (marked by the ‘x’ on the map), in the Indonesian portion of the ­island of Borneo (in yellow). Wild orangutans are found only on Borneo and the neighboring island of Sumatra.

\”In the zoo, where you don\’t have problems with food, you can be a big male for 20 years or so,\” she says. \”In the wild, they only seem to be able to maintain that size for a fairly brief period.\” Knott has shown that ovulating females prefer to mate with prime males. Thus, because females are fertile so rarely, timing of the prime state is critical. \”So it doesn\’t make sense to have a fixed developmental period,\” Knott says. \”There\’s no point in being a big male if you can\’t use it to your advantage reproductively.\”

Why some males never develop the prime morphology is an unanswered question, but Knott is finding clues in the hormone levels of male orangutans: Having high testosterone levels early in life appears to put males on a faster trajectory toward full prime development.

Fascinating as these findings are in their own right, orangutan behavior particularly interests Knott—who is not only a biologist but a biological anthropologist—for what it explains about human behavior. As she teaches students in her Anthropology 335 course, \”The Ape Within,\” comparing human traits with those of other primates helps us identify characteristics that are unique to humans and understand how those traits evolved.

The reproductive cycles of most mammals, for example, are synchronized with the seasons, timing conception so that offspring will be weaned when food is most abundant. In humans, however, female fertility corresponds to current, not future, food availability: Women\’s reproductive hormones rise when they gain weight and drop when they lose weight. \”We thought maybe humans were unique in that,\” says Knott, but her research shows that humans share this trait with other long-lived primates.

By contrast, watching Beth and other lone female orangutans raise their offspring one at a time underscores the uniqueness—and importance—of human cooperation. With multiple family members helping to feed their children, human females can have overlapping dependent offspring—a newborn, a two-year-old, and a five-year-old, for instance. \”So we have short birth intervals, but we can still have a long period of development,\” says Knott, \”and how we do that is through a social structure that allows it.\”

What else might orangutan behavior reveal about our own? Knott hopes the species will survive long enough for her and future researchers to find out. Wild orangutans currently live only in the forests of Borneo and nearby Sumatra, forests that are rapidly being destroyed by illegal logging. Like most of today\’s primatologists, Knott devotes considerable time to conservation projects aimed at saving the forests in which she conducts her orangutan fieldwork because—as her own research has proved—the lives of these animals are inextricably linked to the rhythms of their fragile forest home. If the trees are lost, so are the orangutans and the lessons they have yet to teach us.