Reptiles are especially popular. Collectors often have an almost fanatical devotion to their animals and are willing to pay handsomely, especially for rare specimens, said Sandra Altherr, co-founder of Pro Wildlife, a nonprofit conservation group in Munich.
Unethical traders know that snakes, lizards and turtles do not rank as high priorities for law enforcement and customs officials in Western countries.
“Reptiles are coldblooded and not fluffy, and the broad public — including politicians — just isn’t interested in them,” Dr. Altherr said. “Yet there are huge, dangerous loopholes that allow for open trading of the rarest species.”
In addition, many exotic pets originate in developing countries where officials may lack the expertise, motivation or resources to verify that animals about to be shipped out were in fact bred in captivity.
“We don’t have a lot of resources here in the U.S., and developing countries have even less than we do,” said Phet Souphanya, a senior special agent at the F.W.S. “Corruption also goes into the permitting issue — there’s always someone to be bribed.”
Once imported, exotic pets can be legally sold or re-exported. “Those involved in trafficking wildlife know the loopholes inside-out,” said Chris Shepherd, executive director of Monitor, a nonprofit organization that works to reduce illegal and unsustainable wildlife trade.
“They know enforcement agencies’ hands are tied, and they know policy change in favor of conservation does not happen overnight.”
In the United States, the government has to legally prove that animals are not captive-bred — something that is “very, very difficult to do,” said Marie Palladini, an associate professor at California State University, Dominguez Hills.
In the early 1990s, when Dr. Palladini was a field special agent at F.W.S., she helped lead an investigation of pythons smuggled from Papua New Guinea and sold in the United States as captive-bred.
The American importer was eventually prosecuted, but that success required two years of exhaustive work. It also benefited from Papua New Guinea’s willingness to collaborate.
Many countries, however, do not even bother responding to inquiries sent by American agents. And sometimes, officials in exporting countries vouch for suspect shipments. Then American agents have no recourse, Mr. Souphanya said.
“If they’re certifying that their permit process is correct, we can’t tell them, ‘Hey, you guys are wrong,’” he said. “It’s a difficult thing to prove.”
Even when it can’t be proven, there may be other telltale signs that animals were caught in the wild.
Some species sold as captive-bred are notoriously difficult to coax into reproducing. For example, leading zoos around the world over the decades have managed to breed fewer than 50 echidnas — strange, egg-laying mammals that resemble hedgehogs.
Yet in 2016, Indonesian officials permitted PT Alam Nusantara, a Jakarta-based company, to export 45 “captive-bred” echidnas. F.W.S. records show that as early as 2011, the exporter was shipping echidnas labeled captive-born to the United States.
That echidnas appeared on the quota list at all suggests that traders had a hand in setting it, Dr. Nijman said.
“Having been present at those meetings, it felt more like a negotiation between what traders wanted, what regional forestry departments could offer, and what was within acceptable limits for the scientific authority,” he said.
Because of this, he continued, a country’s list of permissible captive-bred animals often appears scattershot and illogical. Reisinger’s tree monitors and spotted tree monitors, for example, suddenly appeared on Indonesia’s list of permissible exports in 2015, only to be removed the following year.
“It doesn’t make sense to invest years and years into breeding a particular species, only to then suddenly no longer export it and change to another species,” Dr. Nijman said.
The more likely explanation? “New entries represent new demand for rare species,” he said. That is, traders received a request, lobbied for the species to be added to the list, found the animals in the wild and exported them — then moved on.
According to Adri Tasma, owner of CV Terraria, a reptile farm near Jakarta, traders rely on Indonesia’s Cites authorities to set sustainable, responsible quotas.
Mr. Tasma specializes in captive-bred green tree pythons, and in 2016 he was allowed to export up to 2,000 of them. But authorities also granted him permission to trade in 56 additional species, including critically endangered Sulawesi forest turtles and rare tricolor monitors.
Mr. Tasma said he did not know why the government gave him permission to export such animals and denied selling them. He added that he isn’t licensed to breed or keep them.
(The Cites trade database indicates that the species were exported from Indonesia to the United States in 2016, but it does not name the companies involved.)