In one of Central America’s most polluted rivers, an endangered species of crocodile is thriving despite living in waters that have become a cesspool for Costa Rica’s capital, experts say.

Every day, garbage and sewage from homes and factories in San Jose flow into the Tarcoles River, spewing tires and plastic into the surrounding mangroves.

Still, around 2,000 American crocodiles have adapted to life in the toxic river, a testament to the country’s decades-long struggle against waste management.

“It’s a super contaminated area, but that hasn’t affected the crocodile population,” said Ivan Sandoval, a biologist at the National University of Costa Rica.

“The Tarcoles River is the most polluted river in Costa Rica and one of the most polluted in Central America. Heavy metals, nitrites, nitrates and a large amount of human excrement can be found,” added the crocodile expert.

According to the International Union for Conservation of Nature (IUCN), after decades of hunting and habitat loss, there are only about 5,000 crocodile species remaining, found in 18 countries.

The organization lists the crocodylus acutus as “Vulnerable,” but says its numbers have been increasing in recent years. The population of Costa Rica is “healthy and robust”.

In fact, the large reptiles that bask in the sun and occasionally feed on fish that come up the channel from the sea seem unfazed by about 150 species of bacteria Sandoval says have been spotted in the river.

He describes the carnivores as “living fossils” with the ability to survive very harsh conditions.

“They haven’t had to change anything in millions of years, they’re perfectly engineered.”

Sandoval said Costa Rica’s crocodile population has been “recovering” since the 1980s and warns of the threat from tourist activity.

The river’s crocodiles are a major draw for foreign visitors, who take boat tours to see the creatures up close.

Some feed the animals, which is forbidden, and Sandoval worries they are getting too used to being around humans.

Juan Carlos Buitrago, 48, the captain of one of the tour boats, says he and other locals regularly pull hundreds of tires and plastic waste out of the water.

He enjoys the river’s fauna, with macaws flying overhead at sunset, but wishes his countrymen would stop soiling his “office”.

“We can’t hide the pollution,” he tells AFP.

Costa Rica has impressive environmental records, with a third of its territory marked for protection, 98 percent renewable energy and 53 percent forest cover, according to the United Nations Environment Agency.

However, the law is not always strictly applied, as in the case of the Tarcoles river.

Attorney and environmentalist Walter Brenes, 34, said all of Costa Rica’s rules and regulations “do not solve the problem.”

He said the country needs “real public policies fully focused on protecting wildlife.”