Maryland researchers uncover how North American river otters shape local ecosystems through diet, parasite spread, and habitat use.

Investigating Life Along the Chesapeake Bay

North American river otters have long inhabited the Chesapeake Bay, yet little is known about how their environment influences their daily lives. To explore this, researchers from the Smithsonian Environmental Research Center (SERC) in Maryland studied where these otters feed, socialize, and defecate, as well as what they eat. Their findings are published in Frontiers in Mammal Science.

“River otters in the Chesapeake Bay eat a wide range of animals, including those that live in the water and on land. Parasites, too, are crucial parts of their diet,” said senior author Dr Katrina Lohan, a parasite ecologist and head of the Coastal Disease Ecology Laboratory at SERC. “We also found that river otters use manmade structures for latrines or choose areas with specific characteristics.”

From Dockside Discovery to Scientific Study

The project began when a colleague sent Dr Lohan an email about “weird-looking, watery poo” found on a campus dock. Inside the scat was a fire engine red worm—likely a parasite. Wildlife cameras later captured a river otter defecating on the dock, prompting further investigation.

Graduate researcher and first author Calli Wise collected samples from 18 active otter latrines over 11 months, averaging 28 scats per site. “Scats usually smelled strongly of fish and were full of scales or crustacean shells,” Wise noted. Samples were collected with gloves and sterile tools to prevent DNA contamination.

Latrines as Ecological Hubs

Latrines serve multiple purposes for otters: they are used for scent-marking, eating, playing, socializing, and resting. In the Chesapeake Bay, most were located near shorelines with woody debris, low overhanging vegetation, and easy water access. Some were situated on man-made features such as docks, boardwalks, and shoreline staircases.

Diverse Diet, Including Invasive Species

Back in the lab, researchers identified otter prey using two methods—metabarcoding (a DNA-based approach) and microscopic analysis.

“We collected most scats in the summer, when river otter diet is likely diverse,” Wise said. “We found evidence of invasive species like common carp and white river crayfish in river otters’ diet.”
Otters were found to consume primarily fish and crustaceans such as American blue crabs, but also occasionally ducks and amphibians.

Parasites as Ecological Indicators

Metabarcoding revealed that many parasites present in otter scat originated from their prey, particularly teleost fish—a diverse group of ray-finned fishes. Some parasites were capable of infecting otters directly.

“It is possible that river otters, like other top predators, wouldn’t be able to find enough food to eat without parasites,” Lohan explained. The presence of parasites suggests otters may help regulate prey populations by targeting sick individuals, potentially influencing evolutionary processes.

Some parasites found could also infect humans, highlighting the potential of using river otters as “disease sentinels.” As Lohan concluded: “Thus, we could use river otters as ‘disease sentinels,’ and study them to learn about what public health threats occur in certain areas.”

Study Limitations

The researchers noted challenges in identifying all parasites due to incomplete genetic databases and the inability to link specific scats to individual otters.

Key Findings

• River otters in Chesapeake Bay have a varied diet, including fish, crustaceans, amphibians, ducks, and invasive species.
• Latrines are multifunctional hubs used for defecation, social interaction, and feeding.
• Many parasites in otter scat originate from prey, with some capable of infecting otters or humans.
• Otters may act as ecosystem engineers by removing infected prey from the gene pool.
• Potential use of otters as “disease sentinels” for public health monitoring.

Original Publication
Journal: Frontiers in Mammal Science
DOI: 10.3389/fmamm.2025.1620318
Method of Research: Experimental study
Subject of Research: Animals
Article Title: North American river otters consume diverse prey and parasites in a subestuary of the Chesapeake Bay
Article Publication Date: 14-Aug-2025
COI Statement: The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Original Source: https://frontiersin.org/news/2025/08/14/otter-poo-reveals-animals-shape-ecosystems

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