SATTAHIP NAVAL BASE, Thailand—Here at Thailand’s largest navy base, an enemy lurks within: packs of marauding monkeys that have taken over parts of the facility in their search for chips, nuts and maybe a bar of chocolate or two.

And the sailors are fighting back. Their weapon of choice? Monkey vasectomies.

Squeezed out of the nearby jungles by new housing developments and tourist resorts, the long-tailed macaques have chewed through telephone cables, knocking the entire base off the grid. They break into offices and rip up documents. They drag rocks onto roads to slow down cars and trucks so they can jump aboard and rummage for food. They’ve torn down signs—including one urging visitors not to feed them.

“They’re smart, but they’re a problem. I am sick of them,” said Commander Suranart Jiemjit, whose superiors ordered him three years ago to head off the monkey menace.

But now, as the monkey population swells to 10,000, Cmdr. Suranart is implementing the new vasectomy strategy to defend the navy from attack without hurting their furry neighbors.

Long-tailed macaque monkeys at the Royal Thai Navy base at Sattahip. Photo: James Hookway/The Wall Street Journal

Thailand’s navy has tried sterilizing the macaques before. Three years ago Cmdr. Suranart brought in a team of veterinarians to trap males and then castrate them. It didn’t work. The vets found that castrating the males overturned the monkeys’ social hierarchy. Dominant males who were castrated were sometimes ostracized from the troop and females would simply find a fertile male to mate with. The population continued to grow.


Culling, or hunting, the population wasn’t an option, either. Most Thais are Buddhists and frown upon the practice.

Now the navy is pursuing a different tactic. Vasectomies, while trickier to perform, make the monkeys easier to control and doesn’t upset the tribe’s social order.

Praphat Sathitwat, who works on the problem for the local civilian government of the Thai district of Sattahip, says the new approach seems to be working.

“I see fewer babies than before, so this suggests that the population is growing slower than before and will stabilize,” Mr. Praphat said. Around 280 monkeys have had vasectomies so far, and the navy plans to recruit a veterinarian to handle the procedures full time.


It is a neat solution to a problem prevalent across Asia. As economies grow, wild animal populations increasingly find themselves competing with humans for habitats. Snakes slither through subdivisions in Bangkok, while tigers in India slink closer to people and wild boars run loose in parts of Hong Kong.

In Thailand, monkeys have become a nuisance in various towns and Sattahip is a good example of the trend. The area around the naval base is a popular weekend getaway and is expected to become an even more attractive tourist destination, as the Thai government develops a nearby airfield into another international gateway. It is also an ideal location for monkeys, with thickly forested hillsides sloping down toward aqua-blue waters of the bay, where the navy’s frigates, corvettes and a helicopter wait primed for action.

Vasectomies, however, only work if you can catch the monkeys in the first place. And that is becoming something of a problem.

Sailors used to set cages with the door open and lure the macaques inside with fruit and other treats. If they were lucky, the sailors were sometimes able to slam the door shut and cart them off for surgery.


But the monkeys are getting wise to that. They won’t enter the traps if they sense humans are nearby. Others try to get the food out without setting foot inside the trap.

“They’re learning,” said Mr. Praphat, who also complained about how the monkeys are stealing his golf balls during his weekly 18-hole games near the base. “They think they are eggs,” he said.

A monkey grabbing food at the Sattahip naval base. Photo: James Hookway/The Wall Street Journal

Sailors sometimes now resort to firing pebbles from slingshots to keep the monkeys at bay. At a recent ceremony to commemorate the navy’s anniversary day, a petty officer stood guard with a wooden catapult as white-suited officers laid out offerings of jasmine, roast duck, and brandy before a monument to the navy’s founder, the 19th century Prince of Chumphon, Admiral Abhakara Kiartiwongse.

Another sailor attempted to distract the macaques by laying out a plate of spicy Thai curry.


Worse, there is another threat lying just offshore. For years, the navy dumped the few monkeys they could catch on a small island called Monkey Island, a short boat ride from the Sattahip base. They reproduced quickly, eating up the few resources available there and now wait for tourist boats to drift by so they can beg for snacks.

And, they can swim.

Write to James Hookway at james.hookway@wsj.com