How river otters’ terrible food hygiene benefits ecosystem health monitoring

North American river otters have terrible hygiene when it comes to their food. They eat, play and defecate in the same place. But their unhealthy habits make them ideal for detecting future health threats in the environment, according to scientists.

In a new study published in Frontiers in Mammal Science, Smithsonian scientists analyzed the otters’ diets and “latrine” habitats in the Chesapeake Bay for the first time. They discovered river otters often eat food riddled with parasites—and that may not be a bad thing for the larger ecosystem.

“River otters are impressive apex predators that play a vital role in ecosystems,” said Calli Wise, lead author of the study and a research technician at the Smithsonian Environmental Research Center (SERC). “The parasites consumed by river otters may also teach us about the health of the environment.”

River otters are among the most elusive animals in the Chesapeake. They’re nocturnal, semi-aquatic and generally shy around people, so live sightings are rare. Once abundant across North America, their numbers dwindled due to the fur trade and habitat degradation.

A Maryland reintroduction program in the mid-1990s helped their populations rebound across the state. But even as they bounce back, scientists still don’t have precise estimates as to their population numbers in the Bay region. And many other aspects of their behavior and diets remain obscure.

“It is shocking how little information there is about their biology and ecology,” said Katrina Lohan, co-author and head of SERC’s Coastal Disease Ecology Lab.

Since live otters are difficult to observe, biologists rely on what they leave behind. Specifically, their feces. Otters leave the water periodically to congregate at latrines—sites on land where they eat, socialize and leave fresh droppings as scent marks for other otters. By studying the feces (or “scat”) from otter latrines, scientists can get a sense of what the otters are eating.

The latest study looked at scat from 18 active latrines on the SERC campus in Edgewater, Maryland. Most were natural sites, such as beaches or riverbanks, but a few latrines appeared on man-made structures like docks or boardwalks. The biologists took the scat samples back to the lab, where they surveyed the samples under the microscope and ran DNA analyses using a technique called metabarcoding.

Finfish and crabs formed the staples of otter diets—accounting for 93% of all prey items in the DNA analysis. The otters also ate amphibians, worms and the occasional bird. The researchers even found evidence that otters ate two invasive fish: the common carp and the southern white river crayfish.

But the DNA analyses also uncovered a host of parasites from six different taxonomic classes teeming in the otter scat. The vast majority were trematodes—parasitic flatworms also known as “flukes.” Other parasites included microscopic dinoflagellates and other flatworms known to infect the gills, skin or fins of fishes.

Most of the parasites likely infected the otters’ prey, not the otters themselves—and the otters probably weren’t any worse off for eating them. In fact, Lohan suggested, otters may be helping the overall prey populations by eating parasite-infected animals, since this weeds out sicker fish and crabs. Meanwhile, parasites may be helping the otters catch prey that would otherwise elude them.

“While parasites have negative impacts on individuals, they are extremely important in food webs,” Lohan said. “It is possible that river otters, like other top predators, wouldn’t be able to find enough food to eat without parasites.”

However, a few parasites in the study, such as roundworms and single-celled apicomplexans, are known to infect mammals. The scientists suspect these parasites directly infected the otters themselves, rather than their prey.

This study did not detect any parasites in river otters that can infect humans. But some of the parasites were closely related to ones that can cause human disease, including the gastrointestinal disease cystoisosporiasis. As river otters are appearing more often in urban and suburban areas, the likelihood of them encountering something that could affect human health is also rising.

“As river otters move into more urban waterways, they will be increasingly exposed to pollutants and parasites of concern to humans,” Wise said. “As mammals, river otters may be disease sentinels that we can study to learn more about environmental risks to humans.”