The US Fish and Wildlife Service has listed the southern white rhinoceros as threatened under the Endangered Species Act.

The measure will put under the act’s protection the last remaining unprotected species of rhinoceros, a move that the organization says could help protect the other four highly endangered species of rhinos – black, Sumatran, Indian, and Javan – from poaching.

The southern white rhino is not as close to extinction as its four cousins, with about 20,000 white rhinos in protected areas and private game reserves in four countries: South Africa, Namibia, Zimbabwe, and Kenya. (The southern white rhino is actually a subspecies of white rhino; its northern counterpart is believed extinct in the wild.)

In contrast, the Javan rhino, the most threatened of the five rhino species, is now believed to number just 35 individuals, all living in Ujung Kulon National Park in Java, Indonesia. Vietnam’s last Javan rhino was declared extinct in 2010.

But the white rhino’s omission from the act had meant that traffickers, if caught during transactions, could pass off any rhino horn as a white rhino's, says Craig Hoover, head of the wildlife trade and conservation branch of the US Fish and Wildlife Service. Mr. Hoover notes that the white rhino’s horns are physically indistinguishable from those cut from the other rhinos.

“This is really to address what was a loophole in how we can regulate the movement of rhino horns in the US,” he says.

According to a statement from the office, the new classification "prohibits the sale or offer for sale in interstate commerce of this species and its parts and products, consistent with all other rhinos."

The announcement comes after South Africa, home to about 80 percent of the world’s rhinos, reported a severe spate of white rhino killings within its borders over the last year and a half. Last year, that country alone recorded 668 white rhinos poached, as well as 446 white rhinos slaughtered in the first six months of this year, the US embassy said in a statement. That compares to 448 dead in 2011 and 333 dead in 2009.

Before 2008, the highest poaching toll since about 1990 was largely just in the single digits, says Hoover.

The dramatic uptick in killings stems from a catastrophic collision of two factors: booming wealth in Asia, especially in recently prosperous Vietnam, and the branding of crushed rhino horn folk remedies, says Hoover.

“Historically, the horn has been used as a fever reducer in Asia,” says Hoover. “But that’s not new. What is new is that rhino horn has suddenly became the cool thing, particularly in Vietnam, for things like a hangover cure or a cancer cure – none of which is grounded in any research or scientific fact.”

At this point, a kilogram of rhino horn commands about $60,000 to $65,000, making rhino horn worth more by weight than gold, according to US Fish and Wildlife Service. Rhino horn is made of keratin, the protein in human fingernails, and, if not for the puffed-up demand, would not be valuable.

“Now you see why people are getting involved in illegal rhino horn trade,” says Hoover.

Indeed, the surging price has spawned a coterie of international criminal syndicates that rival drug and arms cartels in the sophistication of their methods. The US, while not a major player in a market that largely spans from Africa to Asia, has nevertheless emerged as a worrisome linchpin in illegal rhino horn trading.

That’s because Americans are the largest importers of rhino hunting trophies in the world, says Teresa Telecky, Director of Wildlife for Humane Society International, noting that hunters brought some 688 trophies into the US between 2002 and 2012.

“Basically, there are a lot of horns in this country,” says Dr. Telecky.

In the US, hunting trophies from both black and southern white rhinos can be imported with proper documentation from either South Africa or Swaziland, but they cannot be sold. Those trophies consist of the full rhino head, and both black and white rhinos have two horns.

The US, then, has become a hotspot for traders looking to buy up horns that will win big cash rewards in Asia. To date, a massive, undercover criminal investigation known as Operation Crash, led by the US Fish and Wildlife Service, has resulted in 14 arrests and six convictions for illegal rhino horn trading. Those convictions included two Chinese businessmen caught buying horns in the US for illegal sale in China, one of whom attempted to package the rhino in a postal box marked as bearing sweets and handicrafts.

In its March 2013 report, the Convention of International Trade in Endangered Species (CITES) also identified a rise in the theft of rhino horns in the US.

As the threat to rhinos becomes increasingly dire, attempts at combating the problem have also become more creative, with most aimed at curbing the ballooning price of a horn. Some conservationists have advocated legalizing the rhino trade, in hopes that doing so will convert rhino poaching into rhino farming, since rhino horns that are shaved off can regenerate, with no need to kill the animal for its horn.

That solution, though, is saddled with major questions about sustainability, as is its unclear if regenerating horns could sustain an industry that is expected to draw more participants if legalized. One Chinese pharmaceutical company is currently experimenting on how to harvest rhino horns efficiently and without injury to the animal.

In another surprising turn, private rhino owners in South Africa, who consider the animals essential to tourism, have begun poisoning the horns. The controversial practice, which does not harm the rhinos but is aimed at making those who ingest the horn powder extremely ill, seeks to warn away would-be consumers in Asia, says Telecky.

“People in South Africa are really just so desperate to protect the rhinos,” she says. “They’re at their wits’ end about what to do.”

Telecky’s organization has taken a less extreme approach, attempting to rebrand how Vietnamese businessmen perceive the horns. Vietnamese businessmen are among the product’s biggest buyers, purchasing horns as status symbols, she says.

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White rhinos – when allowed to live – are enthralling animals. In white rhino social groups, adult males cordon off for themselves a plot of land about one square mile and rim it with dung piles to form a symbolic fence. Dominant males then patrol that land to ensure that breeding females do not leave the plot and will lunge – horn first – after any male that expresses interest in its preferred mate.

And these are big beasts: White rhinos are the second largest land mammals in the world after elephants, placing them in a group of big-boned animals that historically have not fared well on the planet. Vanished Brobdingnagian animals include the Elasmotherium, a four-ton, furry rhino that went extinct during the Pleistocene.