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Putri is an adult female with an infant male son, Pur. Pur looks about two years old, so she should have him with her for the next 3-4 years. We first met Putri in April 2010, and followed her monthly until October. Unlike the first two females we followed, Putri was very uneasy about us. She alternately kiss-squeaked and threw branches at us, or hid in her nest. These early reactions are normal, but because she reacted this way we kept our follows brief (2-3 days).

Putri is especially cunning in sneaking away from us. She was once in a large fruit tree for a long time, but very quiet and mostly invisible in its leafy canopy. From the ground, we kept watch for signs of her in the tree, like a red hairy foot or the rustling of an orangutan moving, and monitored all exit routes from the tree that we could find. The calls of an approaching monkey group distracted us, but only briefly. While turning back to monitor Putri's tree, I spotted a young orangutan in a tree behind us. Someone new, I thought. On closer look it turned out to be Pur. Putri had to be nearby, and careful checking revealed her in the same tree as Pur, better hidden. She must have used the monkey distraction to leave the fruit tree undetected.

Putri habituated to us over repeated encounters and reduced her complaints, as she discovered we simply follow and watch. By August, we were able to follow her for eight consecutive days before she gave us the slip. Especially interesting is that she travels farther than E. Bornean orangutans, Pongo pygmaeus morio, are thought to travel. It is currently thought that morio travel within very small ranges (< 200 ha, 2 km2) because they live in fruit-poor habitat and so rely heavily on vegetation foods like leaves, bark, and pith. These foods are nutritionally poor, so they may also have to minimize their energy use. Their energy-cutting tactics may include traveling less and resting more than orangutans living in better habitat. In the short time that we have followed her, Putri has traveled over an area of 4-5 km2. We stopped following her when we reached the boundaries of our study area but she kept going, so she clearly goes even farther.


Pur is Putri's infant son. We estimate him to be about 2 years old, based on his size, spiderhead hair style, and behavior. He's better coordinated than a one year old, eats some foods on his own, and may move several meters away from his mother to explore, play, or eat.

It is clear that Pur is learning from Putri. He eats the same foods that Putri eats, begging or stealing a portion of the harder-to-get ones. He also copies her in responding to the sight of us. When Putri kiss-squeaks and shakes or throws branches at us, Pur chimes in with his own kiss-squeaks and waving or dropping tiny twigs. At the same time, however, he is seriously curious. Once when Putri met us unexpectedly, she pushed Pur behind her body to hide him while she threatened. Instead of joining in with threats, Pur repeatedly stuck his head out from behind her, to the left, right, and below, to sneak peeks at us. When he's exploring away from his mother, between kiss-squeaks and twig threats, he spends considerable time just staring down at us.

Pur's learning still has a long way to go. He's not good at hiding, for instance. Putri once sneaked out of a large fruit tree while we were distracted by an approaching group of monkeys. We had no idea she had moved until I spotted what I thought was a new orangutan in a different tree. It turned out to be Pur. Putri had changed trees surreptitiously and carefully sat still in the new tree out of our line of sight. Pur, however, kept moving around normally and in full sight of us, and so gave their position away.


Uci is an adult female, the third we found in our Kutai study area early in 2010. She was always unconcerned about our presence so we were able to follow her for several days at a time almost immediately. When we first met Uci, she was accompanied by a young adolescent male that we named Ucok, assuming he was her son. She also attracted a series of adult male suitors so she had probably become sexually receptive in readiness for conceiving her next offspring. In fact, her abdomen was bulging and she spent what seemed like an inordinate amount of time resting during the day, so we thought might already be pregnant.

We were probably right about Uci's readiness to conceive but wrong about Ucok. After seeing them together, our impression was that Ucok didn't really behave like Uci or even look like her. He also left her company and was replaced by another young male, maybe 5-6 years old, that we named Chelsea. Chelsea showed stronger signs of being Uci's son, sharing Uci's nest, periodically nursing, and travelling with her for several months.

Uci was less often accompanied by male suitors by summer, she disappeared at the end of June, and Chelsea appeared alone from August on. A possible explanation is that Uci had indeed been pregnant earlier in the year and had given birth after disappearing. The birth of a new infant would very likely result in Chelsea's separating from her. We have yet to meet her again, but assume we will early in 2011, once trees in our study area come into fruit.


Chelsea is a young male that we estimated to be 5-6 years old. We first encountered him alone and as far as we could tell, he was a female—thus the name. More careful binocular inspection showed that Chelsea is in fact male, but the name stayed. He was also on the fringes of Uci's sphere, perhaps hanging back because of her male suitors. Over the time that we observed him, he joined Uci, stayed with her for several months, and behaved like her son. His age and Uci's sexual attractiveness both suggested he might separate from her soon. Signs of disagreement were already there. When Uci rested for a long time, Chelsea often communicated that he wanted to get going by approaching her and whining continuously. When she ignored him (she mostly did), he eventually gave up, moved a short distance away, and ate or played on his own—but periodically slumped down on a branch as if unbearably bored, bored, bored.

He also showed he was developing a mind of his own when he and Uci were trapped in a giant fig tree for three days. There appeared to be only one way in, swaying branches of a nearby fruit tree close enough to grab one of the fig's outermost branches. The fig's branches were too brittle to sway, however, and the outermost branch broke as Uci and Chelsea entered, making the gap to the fruit tree too wide for them to cross. For three days they stayed in the fig (it had lots of fruit), periodically trying to leave but always turning back at the large gap. Finally Uci took the risk, safely crossed the gap into the fruit tree, made a bridge of her own body, and let Chelsea climb out along her shoulders. Then what did Chelsea decide to do but sway the fruit tree and climb back into the fig. And of course he couldn't get back out. Lucky for him Uci eventually took mercy and helped Chelsea get out by swaying a fruit tree branch close enough for him to grab.

Chelsea was still with Uci at the end of June 2010, when we last followed her. When we met him again, late in August, he was alone. He may have left Uci by choice, given the signs he'd been showing of having different ideas about what to do than Uci had. This is consistent with his estimated age: as a juvenile, he would be ready to become semi-independent of his mother. Alternatively, he may also have been forced away by Uci, if she had indeed been pregnant and gave birth in July. When we found him alone in August, he appeared to be nervous about being followed and gave us the slip within a day.


We met Darwin at the end of January, 2011. We figure Darwin is a near-adolescent male, although it took some looking and thinking to figure it out. At first look, he seemed more like a female than a male: abdomen very round, facial features less robust, and eating the number one priority. The explanation may be that he is becoming independent, perhaps having recently left his mother's care and now approaching adolescence. He was traveling alone when we found him and throughout the time that we followed him, but he still had a juvenile hair style and body. Male orangutans experience a growth spurt at adolescence, so Darwin's eating obsession might indicate his body's new urge to grow big, fast. He probably will, since he never stopped eating the whole time we followed him and probably killed more than his share of trees and vines that week.

Darwin has proven to be a very calm orangutan. It took virtually no time to habituate him to our presence and observation: we were able to follow and observe him for eight days the first time we found him. He made a couple of feeble attempts to vanish, but gave up quickly and got back to eating. He finally did vanish, but apparently not because he was trying to get away from us, after coming to the ground to eat ginger stems. It is very difficult to follow these orangutans when they are on the ground in ginger-rich areas, because the patch of ginger stems creates a sea of vegetation cover about 1 m high—just high enough to hide an orangutan. Once Darwin was feeding on gingers and traveling on the ground, he, like others, disappeared within minutes.


Otoy is a "cheekpadder", a mature adult male with flanges, who appears to reside in the center of our research area. When we followed him in April and May 2010 he was often near Uci and Chelsea, probably interested in Uci as a mate. She showed no interest in Otoy and resisted mating with him, however, so either she didn't like him (females do have preferences) or she was already pregnant and no longer interested in male suitors.

Since then, we have often found and followed Otoy alone. He has been relatively tolerant of our observing him as long as we keep our distance and don't follow him for long. He also once found us, appearing near our post, and stayed for three days. Over those three days he repeatedly circled the camp, in the trees and on the ground, and observed us. The exception to his tolerance followed Otoy's meeting a younger (unflanged) adult male. Otoy chased the younger male him and gave a long call in his direction. The younger male fled, hid at the top of a tree, and finally disappeared. Otoy finally gave up the chase and resumed foraging but, probably still worked up, threatened our observers while he was traveling on the ground.

Otoy often travels and forages on the ground, making it harder for us to follow him from a safe distance and easier to lose him. Indeed, he seems to use the strategy of dropping to the ground when he loses patience with being followed, and puts it into action where he can immediately disappear into tangled brush or bamboo groves.