How a social lifestyle helped drive a river otter species to near extinction

After wild rubber prices collapsed in the early 20th century, rubber merchants in the Amazon turned to the wildlife trade to keep their businesses afloat. They targeted many species, including two river otters: the giant otter and the neotropical otter. Only one of these species, though, the giant otter, was driven to near extinction. And new research on the patterns of the hunting trade has revealed how the giant otter’s biology, including its monogamous tendencies and boisterous social lifestyle, may have undermined its survival.

At least 23 million Amazonian animals, including the otters, were hunted for their hides from 1904 to 1969. They were killed mostly for their fur, which was desirable in U.S. and European markets. A 1967 law outlawed commercial hunting, but demand for otter fur didn’t really decline until 1975, when Brazil adhered to the Convention on the International Trade in Endangered Species, known as CITES. By that time, however, the neotropical otter had declined in numbers and the giant otter had been driven to near extinction, disappearing entirely from parts of its historical range.

Since then, the neotropical otter has recovered its numbers, and the giant otter population, while still endangered, appears to be increasing in Peru, Colombia and Brazil.

That includes the upper Rio Negro of northwest Brazil, home to the indigenous Baniwa people. The Baniwa were curious whether their fishing habits would somehow affect the return of the giant otters, a question that eventually came to the attention of ecologist Natalia Pimenta, of the National Institute for Amazonian Research in Manaus, Brazil, and her colleagues. But before the scientists could answer the Baniwa’s question, they needed to know more about the otters’ past.

During the years that the otters were hunted commercially, Pimenta says, it was common for companies to register their fur sales. These historical documents — many of which are in the Amazonas Museum at the Federal University of Amazona, in Manaus, thanks to Pimenta’s coauthor André Antunes of the National Institute for Amazonian Research — let the research team reconstruct how many otter hides were sold and how prices changed over time.

The other key source of data was the Baniwa people themselves. Baniwa hunted river otters in the Rio Içana, part of the upper Rio Negro, and sold the skins to white traders. Pimenta and her colleagues interviewed 11 Baniwa men who were old enough to have participated in otter hunts. “I was very well received in the communities where I went,” Pimenta says. “Although few hunters are alive, I encountered many indigenous [people] who had witnessed their relatives on otter [hunts] and who were able to reproduce in detail how the activity was carried out in the region.”

One Baniwa recalled, “I must have been less than 10, but I remember the Içana full of white traders’ boats, in search of skins of jaguar, margay, river otter and giant otter. The trade lasted nearly 10 years, until the animals disappeared from around here. No one saw giant otters again.” This would have been somewhere around 1960.

The historical records confirm that giant otter hides had become rarer in trade by then, with neotropical otters taking their place. But why did the giant otters nearly disappear before their neotropical cousins? For one, they were the larger otter species, which made a single pelt more valuable than the smaller neotropical otter’s. However, the two otters have vastly different lifestyles that make the giant species far more vulnerable to human hunting.

Giant otters live in social groups of up to 20 especially noisy animals. That makes them easy to find and efficient to hunt. In those groups, there’s usually just one monogamous pair of breeding animals. Kill one of that pair — and the large male would be a good target — and you could break up the whole group, the team notes March 30 in PLOS ONE.

Neotropical otters, in contrast, are smaller, solitary, quiet creatures that usually come out in the daytime but, when faced with pressures from humans, can turn nocturnal. That makes them harder to hunt, as well as less profitable. The female otters can mate with multiple males in a territory, so even if hunters take out a few big male otters, females still have options. As a result, the neotropical otters were not only never subjected to the same intensity of hunting as their larger cousins, but there were also better equipped to handle their losses.

The researchers’ detective work paints a historical picture of the hunting practices and the economy of the time. But the work also has practical implications for modern otter populations. “From the species’ response to hunting pressure, we can suggest management and conservation strategies directed to each species,” Pimenta says.