There's an uneasy alliance between humans and long-tailed macaques, discovers Jonathan Killick.
Our guide leaps out of the boat onto a plastic lawn chair embedded in the sand, lifts his hands to his mouth and begins to screech and howl.
"I'm calling the monkey king," he tells us.
Near Vietnam's Cat Ba Island and Halong Bay, Monkey Island is home to clear blue waters, soft white sand, tropical fruits, and a troop of long-tailed macaques.
We journey there on a boat made from a mishmash of planks that has an equally homemade-looking outboard engine slapped on the back steered by the driver's foot.
Just after the guide's call, I see them. About 15 monkeys descend from the trees onto the sand and peer at us from a distance waiting to see what we've brought for them.
It's yesterday's scraps. Watermelon rinds, cucumbers, and carrots have all been thrown into a big sack and handed out to those of us daring enough to try feeding the monkeys.
They seem friendly as they gratefully take a melon rind from your hand. Then a small claw swipes out at you and the monkey's face curls into a snarl that bares a mouth full of teeth.
This is a crucial moment that defines the social exchange between man and monkey. I stand my ground and the beast and I lock eyes for a moment before both standing down.
It's a primal procedure of fight or flight. If you show fear for even a moment they will take the opportunity and attack.
Down the beach another tourist makes the mistake of running. A monkey tears across the sand chasing him down and in a panic he falls over. Fortunately other nearby tourists chase the monkey off and save him from getting nipped.
A 10-year-old boy in our group is not so lucky and leaves with a bite mark on his leg.
The guides give the monkeys cans of Fanta and beer and they just love it -- it's a sight that will trouble many westerners. The drunker they get, the more unpredictable the macaques become.
A dominant male is particularly keen on a locally produced brew, 333, fighting off all others who want a sip. He skulls down two and a half beers and it only whets his thirst.
When you encounter these monkeys they let you get close and the temptation is to get out your phone and take a snap.
The monkeys are accustomed to people handing them things so as soon as a smartphone is presented to them, they take it. There must be hundreds of smartphones up in the jungle, our guide guesses.
From time to time the guide sneaks up behind us and grabs the back of our leg to watch us jump. To be honest, you have to watch your back because some of the male monkeys are bold.
The small ones are pretty cute. They timidly approach you then reach out and snatch the bag of chips you've offered them and scramble up a tree to wildly devour it.
The cafe staff on Monkey Island know exactly how the monkeys work. One moment they'll be feeding the monkeys Oreos and the next moment the exchange will go sour and they'll be yelling and chasing a monkey down the beach.
There's an uneasy alliance between humans and monkeys there. The monkeys rely on the food the locals and tourists provide, and locals rely on the tourists that the monkeys attract.
But their interactions always end with fight or flight.
It's the law of the jungle.
Getting there: Cathay Pacific offers connections to Hanoi via Hong Kong.