North American river otters may be doing more than simply hunting for fish and crabs for their own survival. New research based on the banks of Chesapeake Bay – the largest estuary in the US – has found their droppings teeming with parasites that infect their prey, suggesting the mammals could be helping control disease in the ecosystem by removing sick individuals.
The study, published in Frontiers in Mammal Science, began when a strange scat was spotted on a dock at the Smithsonian Environmental Research Center (SERC) on the western shore of the bay.
“We started this research when a colleague sent me an email about some weird-looking, watery poo that she found at the dock on our campus. In the poo was a fire engine red worm,” says senior author Dr Katrina Lohan, head of SERC’s Coastal Disease Ecology Laboratory. Wildlife cameras confirmed that a North American river otter (Lontra canadensis) – a semiaquatic mammal endemic to North America – was the culprit.
Over the following 11 months, graduate researcher Calli Wise collected scat from 18 otter latrines, averaging 28 samples per site, along a 12km stretch of the shoreline of the Rhode River, a subestuary of Chesapeake Bay.
“Scats usually smelled strongly of fish and were full of scales or crustacean shells,” Wise says. But further analysis was needed to work out exactly what was in the otter poop, so the researchers took the samples to the lab.
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Clues in the scat
In the lab, the team used metabarcoding (a DNA-based technique for identifying multiple species within a mixed sample, such as scat) and microscopy to examine what the otters had eaten and what parasites their meals carried.
“We found evidence of invasive species like common carp and white river crayfish in river otters’ diet,” explains Wise, who says they also discovered evidence of other fish species and crustaceans, including American blue crabs, as well as ducks and amphibians.
The researchers also identified the parasites (though they weren’t able to clearly identify all parasites), then matched these as best they could to the hosts they were most likely to infect.
Many of the parasites found are known to infect teleost fish, a diverse group of ray-finned fishes that are the main prey of river otters. “It is possible that river otters, like other top predators, wouldn’t be able to find enough food to eat without parasites,” Lohan says.
Lohan believes otters may inadvertently be acting as ecosystem engineers, removing diseased individuals from prey populations and potentially influencing their evolution.
“Since so many of the parasites are actually infecting otters’ prey, it could mean that river otters are culling sick individuals from the populations they are preying upon,” says Lohan, explaining that the infected individuals, once eaten, no longer contribute to the gene pool.
“Some of the parasites that infect river otters could potentially also infect humans, who also are mammals,” Lohan adds. “Thus, we could use river otters as ‘disease sentinels,’ and study them to learn about what public health threats occur in certain areas.”
Find out more about the study: North American river otters consume diverse prey and parasites in a subestuary of the Chesapeake Bay
Top image: The team taking samples from 18 active otter latrines. Credit: C. Wise.
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