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Displaying the muscled limbs of a constant climber, a male proboscis monkey scales a tree, preparing to leap across a river in Sabah. Bachelor groups, such as the one he lives in, usually number 10 to 15 monkeys. But as one of the oldest and biggest males in the group, he's about ready to go solo and establish a harem of his own.

In full voice (and full nose) an adult male lets loose with thunderous honks and roars to threaten a potential rival. If the verbal assault fails to intimidate a foe, males will jump violently around, shaking the trees. Rarely do they come to blows.

Proboscis monkey social structure was a relative mystery until pioneering field studies in the past two decades by both Elizabeth Bennett and Carey Yeager showed that the monkeys are highly organized. The basic family unit, or harem group, has one adult male with several females and their offspring. Young males, who are kicked out of harems at an early age, form all-male groups where they bide their time until they reach maturity and form their own harems. Relations among females are usually peaceful, though squabbles do erupt. One evening I saw a female lean down and squawk at a mother and baby who were encroaching on a choice sleeping spot.

After foraging, family groups gather in bands to sleep in trees at river's edge. This proximity of groups is unusual among monkeys, and the reason why they congregate by rivers at night is unclear. A treeless sweep of water may provide visibility for females scoping out potential mates.

Bearing the dark fur and bluish face of infancy, a four-week-old baby rests with its mother in the relative peace of the Lower Kinabatangan Wildlife Sanctuary in Sabah. Though young babies stick pretty close to their mothers, proboscis monkey females share the job of infant care. Mothers may pass their infants off to another female or older sibling from time to time.

Such help would be a must for a mother of twins. Because twins are rare and seldom documented, I was thrilled to spot them from a boat on the river. I went back six months later and saw them again. Their survival seems a hopeful omen.

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