My favourite ride at Barry Island Fun Fair has always been The Jungle Ride. You sit in a plastic boat in a couple of inches of murky water and wobble through a cardboard wilderness as a series of plastic animals take it in turn to pop out from behind trees and bushes. It's awesome.

Travelling up the Kinabatangan River in Borneo is kind of like The Jungle Ride. The water is just as murky but the boat is a little faster, the wilderness a little wilder and the animals a little furrier. Our stay on the river definitely ranks up there as one of my highlights of our trip thus far.

From base camp near Sepilok, we drove for a couple of hours over gravel roads to a small village on a river where we loaded our packs and supplies onto speed-boats that took us upriver and into the wilderness. On the way up the river we spotted our first wild orangutan, sitting in between two branches of a fig tree about 15 metres up over the water. As we slowed down to stop at the bank beneath him he took a look down at us, then resumed his lunch - slowly chewing his way through branch after branch of the tree's fruit.

'The Man of the Forest' is a solitary, territorial animal, and so spends most of his life on his own. Each time we saw orangutans over the next few days, slowly moving through the trees or sitting watch over the river with their soulful dark eyes, it was hard not to feel that they might be lonely, especially in contrast to the happy, playful packs of macaques roaming the treetops.

Over the next few says we set out on a number of dawn and dusk boat rides up and down-river, one day trek into the jungle and finally one night trek. We saw the most wildlife out on the river (orangutans, long-tailed macaques, proboscis monkeys and gibbons, as well as plentiful hornbills and egrets and even one incredibly menacing estuarine crocodile) but the treks into the jungle were just as exciting.

The rainforest is unlike any other environment I've experienced. As you step out from the safety of camp you can feel some very basic survival instinct flip into high gear as your brain struggles to process the sheer density of new sights and sounds, sorting the dangerous from the mundane: screaming cicadas, whirring mosquitos, the crash of foliage as birds move ahead and all around you the shifting green shadows of lianas, leaves and ferns. Through this aggressive terrain our guide strolled barefoot, only slipping on low rubber shoes at night in deference to the fire ants.

Sarah coped with the stresses of the jungle incredibly well during our stay, especially considering she doesn't like camping, has never been hiking before, and is morbidly afraid of half the things we were actively looking for. Creeping around in the jungle during our night trek with eyes wide and shoulders tensed, her head torch was like a strobe-light as she scanned the branches above her for opportunistic pythons and spiders, but by the time we got back to camp she had the biggest smile of all of us at the things we had seen. (Of course this made it all the more ironic given her subsequent delayed 'reaction' to the experience, but she'll fill you in on that in her next post)

Over the last several hundred years the rainforest has been eroded by various colonial endeavours – tobacco and rubber plantations, hardwood logging – and is now further threatened by the modern blight of palm oil plantations. Oil from the palm's fruit can be used to make cooking oils, soaps, and even bio-fuels, and these plantations now cover the majority of all land in Borneo. Ironically, this actually makes it easier for us to see some wild animals as they are concentrated in a smaller area. Other animals that need more space like elephants fare less well, and the government is struggling to balance the needs of their developing nation (and the benefits of a renewable energy source) with the protection of these endangered species. As we drove home in our minibus, stinking and filthy from the jungle, through the window we saw islands of rainforest in a sea of palm plants, and wondered which way the tide will turn.