Indonesian cruise is a luxe look at animals both touching and terrifying but all in trouble

In the jungles of Borneo, the proboscis monkeys are long-nosed, pot-bellied and sad-faced. They look like ageing clowns (until you notice the erections: apparently proboscis harems are considerable affairs). Of an evening, they sit in the meranti trees, legs splayed, hands on knees, chewing with an air of resignation through the never-ending leaf salad that surrounds them. The males sport a white ruff that makes them look as if they're in pyjamas - a ginger onesie perhaps.

The proboscis offers a humbling reminder of the basic instincts we share, the selfish gene and the biological imperative, stripped of all artifice. Perhaps they are too easy to anthropomorphise as they frown, chew their lips and scratch their behinds. But if we recognise kinship, or feel any sense of responsibility, we should be concerned about the fate of our cute, lovable, randy cousins. Because they may not be around much longer. The proboscis is one of nine species of monkeys and apes whose extinction may well occur in our lifetime, on our watch.

Which is why I am aboard a swanky boat in the Indonesian archipelago. My fellow passengers are supporters of the Durrell Wildlife Conservation Trust. Founded by the late British naturalist and writer Gerald Durrell, the trust is committed to protecting endangered species.

Saving such animals is normally quite hard work. All over the world, dedicated field workers are living in shacks of plywood and tar paper, struggling daily through steamy jungles, battling leeches and loggers, hoping to protect shrinking habitats and prevent lawless poaching, which are the chief threats to many species' survival. Our part in the crusade is rather less arduous: we sunbathe, we snorkel, we eat too much, we make a serious dent in the nautical wine cellar. Occasionally we listen to a few agreeable lectures, but our cruise ship does the legwork, ferrying us in style from tropical islands to jungle rivers to visit three of the creatures the trust is hoping to sustain - the Bornean orangutan, the Komodo dragon and the Bali starling. Part of our fares for this sybaritic expedition is going towards sustaining the trust.

The ship was a few days into its schedule when I joined at Kumai, a dusty river town on the southern shore in Kalimantan. I waited on a rickety dock while mosquitoes feasted on my ankles until, sometime before midnight, a cluster of distant lights appeared in the darkness. As they drew nearer, they formed themselves into a small cruise ship, the Orion.

we have an appointment with the orangutans. Leaving the Orion at anchor, we head up the Sekonyer river in klotoks, traditional double-decker wooden boats. Smoke from slash-and-burn agriculture hangs over the river like early mist. The clearance of the great forests for palm-oil plantations is the continuing scandal of Kalimantan, with half the orangutan habitat lost over the past 20 years as a result.