It also happens to be 35 degrees and it's like cycling in a steam room, only with the disadvantage of better visibility.

I can see the top of the 200-metre hill and it isn't getting closer.

I can also hear Kiwi Matt, young and wiry and a mountain biking veteran, breathing down my neck and nonsensical pride is not allowing me to let him pass.  Sarah-Louise is meanwhile demurely bringing up the rear, barely a bead of sweat on her un-furrowed brow.

"Nice and slow, no prizes for coming first," our whippet-like Borneo bike guide Dan counselled at the beginning of the slope.  Unsurprisingly, the woman in our group is the only one who's listened, while all three blokes are grinding up the hill, failing desperately to bridge the chasm between delusion and fantasy.

I'm 50 metres from the brow of the hill, my tyres sliding on the dirt road, when Matt sidles up beside me.

"You can do it, big guy," he encourages.

"Bastard," I think, rising jelly-legged from my seat and thrusting down hard one last time onto my pedals.

I beat Matt to the top by the tread on my front tyre, at which point my bike and backside head in different directions, splaying me onto the grit.

"Nice work," smiles Matt, "bit worried about Glen, though."

Five minutes later, I recover my vision and look back at the track we've climbed, curling through groves of rubber trees, part of a 5000-hectare plantation that reaches as far as I can see.  In its midst is Glen, barely moving but still pedalling, his face contorted and the colour of a bruised mango.  Sarah-Louise walks beside him, smiling broadly.

If anybody tells you that climbing Mount Kinabalu is the biggest challenge in Borneo then they haven't done this cycle ride through the provinces of Tuaran and Tamparuli.

OK, apart from this one gruelling ascent it's not really that bad and we soon also get to hare downhill, the motion-created breeze helping reduce torrents of sweat to mere cascades.

Then, in the shade of some trees, Bike Borneo support vehicle driver Christian appears ahead like a fairy godmother, with an esky full of cold drinks and a first aid kit.

At first it's a wordless, fumbling exchange of fluids.  Then Christian tends to our wounds like the team physio.  Glen needs a blood transfusion and an organ donor, I have a showy but inconsequential gash on my leg from my bottom plant and Matt, poor luv, has chafed buttocks.

When we resume, the cycling is a doddle.  We pass riverside villages where sentry dogs barely raise their snouts, cross narrow suspension bridges that bounce and swing and stop for selfies with groups of giggling schoolkids.

Then, nearing our endpoint, the town of Tuaran, the inevitable happens, Glen and Matt careering into each other in a Hamilton/Vettel-like tangle for supremacy.

For a moment, as Matt picks up his chafed behind, Glen lies slumped flat-out beside a cute roadside cow.  Then, with a burst of choice invective, he wobbles to his feet.

Cycling with Bike Borneo hasn't only revealed that male competition isn't pretty but has also unveiled a side to Sabah that tourists rarely see, away from the resorts.  It has introduced us to people from the Dusun and the seafaring Bajau tribes, still struggling to come to terms with the deforestation of their jungle homes for palm oil plantations.

The plight of Borneo's orangutans through loss of that same habitat is well known, the endangered creatures having become a wildlife cause celebre.  Sharing 96.4 per cent of human DNA and with numbers down to 15,000 in the wild, the concern for these so-called "men of the forest" is understandable and, for most visitors to Sabah, a trip to the orangutan rehabilitation centre in Sepilok in the region's east is essential.

Ambling through the centre a few days later, it is impossible not to be drawn to these animals with elastic arms and faces that meld baby-like openness with the gravitas of old age.  The 45-square-kilometre park turned 50 in 2014 and continues to do excellent work, with more than 100 orangutans rescued from logging areas.  Run in conjunction with a British-based charity, the reserve delicately treads the balance between rehabilitation and exposure to the public, with most orangutans eventually re-released into suitable rainforest.

At the end of my visit, I can't resist "adopting" a baby orangutan for my two daughters back home, choosing cheeky "diva" Chikita, who arrived at Sepilok aged only one month old in 2010 and paying £36.50 ($70) to contribute to her care.

While orangutans are undoubtedly Sabah's big wildlife draw, other equally appealing creatures are under threat from loss of habitat.  Among them is nature's oddest-looking monkey, the proboscis, which I see for the first time in a sanctuary at nearby Labuk Bay.  With their long, drawn-out noses, soulful eyes and wisps of fine orange hair around their faces, these monkeys summon up a skinny Gerard Depardieu playing a Renaissance courtier.

Equally droll is the sight of the world's smallest bear taking a nap, like a dark-furred Winnie the Pooh after too much honey, against a tree at Sepilok's sun bear conservation centre.  These little guys are simply too cute for their own good, captured and kept as pets in tiny cages across south-east Asia as well as being victims of deforestation and poaching for body parts, such as paws, used in Chinese medicine.

Dexterous climbers that feed on the forest canopy, they are known as sun bears for the horse-shoe shaped markings on their chests and grow 120-150cm long and 60-78cm high.  The species is found only in south-east Asia  and has declined by 30 per cent in the past 30  years, according to the International Union for the Conservation of Nature.   So the work of the sun bear centre, founded by Malaysian biologist Siew Te Wong and opened to the public in 2014, is crucial to their survival.

We have conservationists to thank, too, for helping preserve one of the world's most abundant marine environments, around Sipadan Island, off Sabah's south coast, where I venture next.  Sipadan was first declared a nature reserve in 1917, and, in 2009, it became a protected marine park.  Visiting divers are now obliged to purchase permits and to stay on neighbouring islands, like Mabul, with a maximum number of 120 allowed each day.

More than 3000 species of fish have been recorded on the reefs and drop-offs in the Celebes Sea surrounding Sipadan, and the three dives I do here are among the most spectacular I've done.   The backdrop of dive sites like barracuda point and white tip alley is alluring enough in itself: deep blue water washing over a mass of healthy, swaying soft corals interspersed with giant bommies and stands of hard, branching staghorn.   Feeding or resting on the reef is everything from diminutive, brightly-hued nudibranch molluscs and clouds of minnows to grandpa-like green turtles watching the world go by.

Yet it's for aggregations of bigger fish that I'll always remember the reefs around Sipadan.  Having become almost lost inside a huge funnel of jackfish whirling around me on one dive, I am met on the next one by a silver wall of chevron barracuda so tall and wide I cannot see its edges.

Being confronted by a thousand toothy grins seems a fitting end to my quest for adventure in Sabah, a journey that has seen me indulge in cartoon-esque rivalry among its northern jungles and commune inland with "men of the forest", sweet little bears and ginger monkeys with comical oversized hooters.

TRIP NOTES

MORE INFORMATION 

sabahtourism.com

GETTING THERE

Air Asia flies twice daily to Kuala Lumpur from Sydney and Melbourne, with onward connections to Kota Kinabalu. On-board hot seats (in quiet zones and emergency exits) are available for purchase. See airasia.com.

STAYING THERE

Shangri-la Rasa Ria on Pantai Dalit Beach, Tuaran, Sabah, has more than four hundred spacious rooms, six restaurants and its  own nature reserve, including baby orangutans. shangri-la.com/kotakinabalu/rasariaresort

Four Points by Sheraton, Sandakan, is a good base for visiting the nearby Sepilok and Labuk Bay wildlife sanctuaries.  Rooms from Malaysian ringgit 195 ($69). starwoodhotels.com/fourpoints

The luxurious Mabul Water Bungalows, on the island neighbouring Sipadan, has four-day, three-night packages costing RM 4308 ($1526), including three dives daily.  Waterbungalows.com

TOURING THERE

Orangutan rehabilitation centre, Sepilok Road, 23km from Sandakan, open daily 9-12, 2-4pm, feeding times at 10am and 3pm. Adults RM 30/children 15.  sabahtourism.com

Labuk Bay Proboscis Monkey Sanctuary, Jalan Labuk, Sandakan.  Open daily 9.30-5pm, RM 60 adults, 30 children. proboscis.cc

Bornean Sunbear Conservation Centre, next door to the orangutans, Sepilok, Open daily 9-3.30, admission RM30 adults/15 children.  www.bsbcc.org.my

The writer was a guest of Tourism Malaysia and Sabah Tourism