Orangutans are the most intelligent of primates, next to humans. This wee character is sheltering from the daily bout of torrential rain, and might well make a parasol if in fierce sun. Most of his learning will come from watching his mum, and he will not be weaned until about 4 years old. After that he will start to travel under his own steam, but still in his mother's company until about 10. Orangutans are the first non-human species to use "calculated reciprocity" - weighing up the costs and benefits of gift giving, and keeping track for future payback. When they come into contact with humans they are great mimics, using boats and tools with great panache, and have learnt to play (and to teach others) computer games on touch screens.

Drawing this hand, with all its wonderful creases, brought the orangutan even closer to me... it seems so human-like, and yet of course is very specialised for the grasping and swinging needed for arboreal locomotion. Its four long fingers are curved at rest, creating a readymade hook to grip instantly, and without using its thumb it can lock the tips of its fingers against its palm when holding onto small diameter branches. So walking on the ground, which it seldom does, means walking on its fists, whereas gorillas and chimps are knuckle walkers. The orangutan is very dextrous, and uses both hands and feet to build intricate nests, complete with mattress overlays, pillows and sun or rain shelters; it uses tools for extraction of termites from holes, and for extraction of fruit or nuts from outer coverings. Catch some David Attenborough YouTube clips to see how well they watch us and then use our tools!

Unable to fend for himself, a young orangutan resembles a hairy little rascal, copying everything he sees his mother do, and learning how to live their very specialised arboreal lifestyle. His feet are similar to his hands, with long flexible toes and an opposable big toe (like a thumb) - by the time he is full-grown his armspan will be 2.4m, to enable him to travel and swing between trees despite his large bulk. For now he is practicing physical skills, and very importantly, learning to map the food sources in their territory.

These spectacular trees, growing up to 90m (30 storeys!) high, are called 'emergent' as they tower above the forest canopy - with smooth straight trunks which branch out only up at the 30m level. Down below, this 'tower' of timber is supported by huge buttresses, radiating out from the base and snaking their way for many metres into the surrounding jungle. Big enough to live in, the buttresses provided welcome rest stops on our treks, and somehow offered a personal connection to these mammoth trees. I was intrigued by the intertwining patterns they made.

This painting was great fun! Inspired by traditional biological drawings, and prompted by the long lists of things we identified after each day's trek, I launched into it with the beautiful curled-up 'pill millipede' (centre) looking like some beautiful varnished piece of jewelry... and I was off! Though not painting them all to scale I have been as accurate as I could with the detail of each organism. Borneo is all about "biodiversity" (it is known as one of the world's biodiversity hotspots), a single rainforest tree may be hosting 1000 different insect species. The place just teems with life forms.

A spectacular two day climb took us from steamy jungle through montane forest, which could have been in New Zealand, and into alpine meadows. Finally, after a very welcome lie-down and some delicious noodles at the Laban Rata resthouse, we climbed up the bare granite slopes to the summit - a cool 4095m - in perfect time to watch the sunrise over Borneo. What a sight, seeing the immense shadow of the mountain stretching out over the early morning cloud layer below. From discarding clothes in the heat below, to piling layers on at the icy summit, the whole event is an exercise in contrast - the descent is fairly speedy, but followed by a few days of pain!

One of the first images I knew I was going to paint as soon as I got back to my studio was this one! You are walking in the steamy, buzzing jungle, and suddenly you know someone is watching you. Look up - patterns of dark and light in the profusion of leaves, and there she is, quietly peering at you. She may feed for a while, or just sit and watch - her moves are unhurried, considered - and when she wants to move on, you can follow her progress by watching the tops of the trees bend as she "snag rides" through the canopy. Get too close or annoy her in some way, and you will hear the warning lip-smacking 'kiss' noise. What a privilege to see these magnificent creatures in the wild.

Because we were in the world famous Danum Valley Conservation Area, we were in one of the best places to see orangutan, and we were lucky enough to see one almost every day. These large patient gentle mothers bear only one offspring every 8 or so years, and keep the young one with them (there is a lot to teach it!) for up to 10 years. A quote from Dr Dionysius Sharma, Executive director of WWF Malaysia - "Have you ever gazed into the eyes of an orangutan who knows you mean it no harm? ... Each encounter leaves me humbled beyond words. I have seen such innocence in them. Wisdom superior to many men. Intelligence without ego. Love. Gentleness. And in some, a great sadness".

On our treks through the jungle we several times heard elephants, and often saw their dung (fresh one day, already supporting a forest of delicate white fungi the next day) but despite heading into likely areas, we hadn't been lucky enough to sight any. Then, on my last truck ride out of Danum Valley, there in the moonlight, a small herd quietly strolling across the logging road - what a thrill! No time for my camera, I just watched in awe as they silently disappeared into the gloom of the forest.