Blog

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.

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.

More wildlife stories from around the world

Researchers have unearthed tooth fossils in Ethiopia dating to about 2.65 million years ago of a previously unknown species in the human evolutionary lineage, one that lived in the same time and place as the earliest-known member of the genus Homo to which our own species belongs.

The scientists discovered in the Ledi-Geraru research project area of northeastern Ethiopia’s Afar Region 10 teeth – six molars, two incisors, one premolar and one canine – that they concluded belonged to a new Australopithecus species. The teeth came from two individuals.

Until now, six species of the genus Australopithecus, an important early human ancestor that displayed a mix of ape-like and human-like traits, were known from fossils at various African sites. The researchers said the newly found teeth bore traits indicating they belonged to a seventh species.

READ MORE: China discovers landmark human evolution fossils

A genus is a group of closely related species that share similar characteristics. For example, lions and tigers are from the same genus but represent different species.

The scientists also discovered three other teeth dating to 2.59 million years ago that had traits showing they belonged to the oldest-known species of Homo, one that was first revealed by a jawbone unearthed in the same vicinity in 2013.

Scientists have not yet assigned names to the Australopithecus and Homo species represented by these 13 teeth because of the incomplete nature of the fossil remains. Our species Homo sapiens is the most recent member of the Homo genus, first appearing roughly 300,000 years ago in Africa before later spreading worldwide.

The new dental fossils provide insight into a poorly understood period in human evolution. The close age of the teeth suggests that this newly identified Australopithecus species coexisted in this region with the early Homo species, raising questions about whether they competed for the same resources.

The teeth also indicate that there were four hominins – as species in the human evolutionary lineage are known – that inhabited East Africa at the time. Previous fossils showed that another Australopithecus species and a species of Paranthropus, a hominin possessing a specialized skull adapted for heavy chewing, lived in East Africa during this time. An additional Australopithecus species also inhabited southern Africa, bringing the number of hominins then on the continent to five.

The presence of these contemporaneous hominins illustrates the complicated nature of the human evolutionary process.

“This reinforces the idea that the story of human evolution is not of a single lineage changing slowly through time,” said University of Nevada, Las Vegas paleoanthropologist Brian Villmoare, lead author of the research published on Wednesday in the journal Nature.

“Rather, the pattern of human evolution is similar to that of other organisms, repeatedly branching into multiple species throughout the fossil record, many of whom lived at the same time,” Villmoare added.

The researchers are seeking clues about the nature of any interaction between the Australopithecus and Homo species represented by the 13 teeth.

“We are currently analyzing teeth to see if we can tell if they ate the same thing,” said Arizona State University paleoecologist and project co-director Kaye Reed.

If so, they may have fought over resources, Reed said. Crude stone tools dating to about the same time were previously discovered nearby, Reed said, probably made by the Homo species.

The researchers determined the age of the teeth using a technique that dated feldspar crystals contained in volcanic ash in the sediments where they were discovered based on radioactive decay of the element argon.

The Afar Region, one of Earth’s hottest and lowest places, is an arid expanse of badlands. But at the time of these species, rivers flowed across a vegetated landscape into shallow lakes in a landscape populated by a splendid array of animals. These included giraffes, horses, pigs, elephants, hippos and antelopes as well as predators such as saber-toothed cats and hyenas.

Homo is generally thought to have descended from a species of Australopithecus, though the exact species and the timing have been a matter of debate. Australopithecus eventually died out.

READ MORE: A frigid apocalypse doomed early humans in Europe

Australopithecus includes the famous fossil Lucy, who was a member of the species Australopithecus afarensis who lived approximately 3.18 million years ago. Lucy’s remains were discovered in 1974, also in the Afar Region. The newly discovered teeth had characteristics that showed they did not belong to Lucy’s species, the researchers said.

“This new Australopithecus species is in no way some ‘missing link,’ and we actually don’t think that it was necessarily ancestral to any known species,” Villmoare said.

“Species arose and many went extinct,” Reed said. “Each find is a piece of the puzzle that puts human evolution into a twiggy tree, rather than a linear graphic.”

A single living tree can host about one trillion microbes in its wood, with distinct communities in the inner and outer layers. That is not a rounding error – it forces a rethink of what tree health and forest function really mean.

Researchers sampled 150 living trees spanning 16 species across the northeastern United States and found that these internal microbe communities are not random. They split cleanly between the inner heartwood and the outer sapwood. There is little overlap with microbes found in leaves, roots, or soil.

Jonathan Gewirtzman, a doctoral candidate at the Yale School of the Environment, led the study with support from collaborators across multiple disciplines at Yale.

Hidden worlds beneath the bark

Most research on trees has focused on what we can see, like leaves, roots, and bark. Yet forests store vast amounts of carbon across living biomass, dead wood, litter, and soils, with global stocks estimated at roughly 861 gigatons of carbon across all pools.

What happens inside wood is not a footnote for the planet. The inner trunk is not uniform. Heartwood is older, drier, and typically more sealed off, while sapwood is younger tissue that moves water and nutrients upward.

DNA reveals microbial residents

The researchers extracted high-quality DNA from woody tissues and profiled bacteria and archaea across both layers to map who lives where.

The team paired the genetic snapshots with measurements indicating microbial activity in the wood, so this was not just a catalog of dormant passengers.

Those choices matter because a microbiome is more than a roster. It is a structured community whose metabolism can change the chemistry of its home. That is exactly what the patterns in wood suggest.

Two neighborhoods in every trunk

The inner wood favored anaerobic life that does not require oxygen, a pattern seen before in studies of heartwood that reported methanogenic communities thriving in low-oxygen conditions.

The outer wood favored oxygen-using microbes suited to the sapwood’s role in water transport and air exposure. This split tracks with basic physiology and chemistry.

These communities include archaea alongside bacteria, and they do work that affects trees and ecosystems. That work includes biogeochemical processes tied to gases and nutrients that, when scaled across forests, can influence larger cycles.

Species signatures that stick

The communities were not just layered – they were species-specific. Profiles inside sugar maple differed consistently from those inside pines, and those differences held across individual trees of the same species.

This pattern is consistent with long-term host filtering and even coevolutionary pressures discussed in plant-microbe research.

“One of the things I found most interesting was how these microbiomes varied across different species,” said Wyatt Arnold, a chemical and environmental engineer and study co-lead.

“For example, sugar maples hosted a very different community than the one within pines, and these differences were consistent and conserved.”

Microbes as tree partners

Calling plants holobionts treats the organism and its associated microbes as one integrated unit. That framing is not semantics, it changes how we think about tree health, stress, disease, and even growth in the field and in cities.

These wood-dwelling partners appear to help cycle nutrients inside trunks and produce gases that may vent to the atmosphere or feed other microbes.

Scaling that up matters because forests act as a major carbon sink each year. Small changes in internal processes can add up across billions of stems worldwide.

Microbes in the carbon equation

Global assessments suggest forests hold on the order of hundreds of gigatons of carbon and continue to remove carbon dioxide from the air annually.

Knowing how much of that storage and flux depends on microbes living in wood is key for realistic models and practical management.

This is not a call to overpromise – it is a step toward tighter estimates of what forest recovery and protection can deliver.

These estimates are informed by a better handle on internal microbial metabolism. They also draw on how it changes with drought, heat, pests, and age structure across regions.

Mapping life across forests

Cataloging these communities beyond the northeastern United States will show how climate, species pools, and land use shape internal wood life.

That includes dry forests, wet tropics, plantations, and urban trees that experience pruning, compaction, and pollution in different ways.

“There is a massive reservoir of unexplored biodiversity, countless microbial species living inside the world’s trees that we’ve never documented,” said Gewirtzman. “We need to catalog and understand these communities before climate change potentially shifts them.”

Microbes matter for tree management

Internal microbiomes can influence decay resistance, wound healing, and interactions with pathogens. They may also shape a tree’s response to drought or flooding by altering internal chemistry and gas transport.

If certain microbe groups boost resistance or growth, they could lead to new diagnostics and treatments alongside breeding and silviculture.

That prospect aligns with a growing literature on shared governance between hosts and microbes in plants. This research stresses that outcomes reflect both partners.

The study is published in the journal Nature.

—–

Like what you read? Subscribe to our newsletter for engaging articles, exclusive content, and the latest updates.

Check us out on EarthSnap, a free app brought to you by Eric Ralls and Earth.com.

—–