No one knows how prevalent illegal collecting and use of poached specimens is among scientists, not only because of the covert nature of this trade, but also because it can be difficult to define who counts as a scientist and what counts as breaking the law.

Expert-level hobbyists often publish in the scientific literature, for example. But because they are not employed by a university or research institute, they are not subject to strict codes of conduct and may be more prone to cut corners, Mr. Cooper said.

In their paper, Mr. Gabriel and Ms. Sherwood said they were affiliated with the Oxford University Museum of Natural History. But Paul Smith, director of the museum, said that neither are employees: Mr. Gabriel is an honorary associate, and Ms. Sherwood was a volunteer.

“The research was carried out independently and is not connected with the museum in any way,” Dr. Smith said. The paper would have failed to pass the museum’s ethics review, which requires authors to provide proof of legality of their specimens, he added.

The ethics surrounding scientific acquisitions can be hazy. Still, some cases are clearly criminal.

In 2005, for example, a curator of the Charles University Botanical Garden in Prague was convicted in South Africa of collecting over 3,000 protected plants, cuttings and seeds in the Western Cape, according to case records.

In 1999 Earl Thomas Schultz, the former curator of herpetology at the San Diego Zoo, pleaded guilty to fraud and theft after being caught trafficking in imperiled reptiles.

“Sometimes it seemed like I specialized in targeting scientists who felt they were above the wildlife conservation laws,” said Ken McCloud, a retired Fish and Wildlife Service special agent who investigated the latter case.