In the often overlooked habitats of the Chesapeake Bay, North American river otters are revealing critical insights into ecosystem health through a most unexpected means—their messy dietary and hygienic habits. Recent research conducted by Smithsonian scientists at the Smithsonian Environmental Research Center (SERC) in Edgewater, Maryland, has illuminated how these aquatic mammals, notorious for their seemingly untidy behavior, may serve as biological sentinels, providing valuable data on environmental parasites and the broader health of the bay’s aquatic systems. This groundbreaking study, published in Frontiers in Mammal Science on August 14, 2025, delves deep into the intricate relationships between river otters, their prey, and the parasites that intertwine these connections within a subestuary of the Chesapeake.
Otters, despite their charm and significance as apex predators, remain enigmatic in many respects due to their secretive lifestyles. Being primarily nocturnal and semi-aquatic, these creatures evade direct observation, compelling researchers to turn to indirect methods such as scat analysis to unravel their ecological roles. This innovative approach harnesses the reality that otters consistently return to specific terrestrial sites known as latrines, where they consume food, socialize, and deposit feces, which harbor crucial biological data. Through systematic collection and analysis of fecal matter from 18 active latrine sites scattered across both natural and anthropogenic environments within SERC’s domain, researchers employed a combination of microscopy and sophisticated DNA metabarcoding techniques to profile both prey and parasite presence with unparalleled granularity.
The compositional analysis revealed that finfish and crabs overwhelmingly dominate the otter diet, accounting for 93% of the detected prey DNA sequences. Intriguingly, the diet also included amphibians, worms, and sporadic avian species, indicating a diverse trophic interaction network. Notably, the utilization of metabarcoding uncovered consumption of two invasive species—the common carp (Cyprinus carpio) and southern white river crayfish (Procambarus clarkii)—highlighting the otter’s role in potentially regulating invasive populations, which may influence the bay’s ecological balance.
Beyond mere diet composition, what makes this study particularly compelling is its thorough characterization of the rich parasitic community cohabiting with the otters’ prey and, by extension, in their digestive tracts. Parasites from six distinct taxonomic groups were identified, with trematodes (parasitic flatworms or flukes) being the most prevalent. Other detected parasite taxa included microscopic dinoflagellates and other flatworm species known to infect fish epidermal structures such as gills and fins. The critical ecological insight here is that most of these parasites reside primarily in the otters’ prey rather than infecting the otters themselves, suggesting a complex ecological interplay where parasites and predators interact via their shared prey.
The researchers postulate that river otters might confer ecosystem-level benefits by selectively preying upon parasite-infected organisms. Such predation ostensibly reduces parasitic loads in prey populations, effectively weeding out individuals that could otherwise degrade stock health and reproductive viability. This selective pressure imposed by apex predators like otters may play a pivotal role in shaping fishery population dynamics and overall aquatic health. Concurrently, parasites themselves may indirectly facilitate predation by manipulating the behavior or physiology of their hosts, making them more susceptible to capture, thus integrating parasitism into trophic webs in nuanced, often underappreciated ways.
Nonetheless, the study did identify a subset of parasites more directly implicated in infecting the otters, notably certain roundworms (nematodes) and single-celled apicomplexans. The presence of these parasites in otter scat indicates active infection rather than passive passage through the digestive tract, raising questions about the health ramifications for the otters themselves and the potential for disease transmission. Importantly, while the study did not detect known zoonotic parasites—that is, parasite species transmissible from otters to humans—it flagged close relatives of such parasites, including those associated with human gastrointestinal diseases such as cystoisosporiasis. This cautionary finding underscores the migrating role of otters from isolated natural habitats into increasingly urbanized and suburban waterways, where interactions with human populations and domestic animals are becoming more frequent.
With urban sprawl encroaching upon natural habitats, the interplay between wildlife, parasites, and human health crisscrosses into realms of growing significance. Calli Wise, the study’s lead author and a research technician at SERC, emphasizes that as mammals sharing these changing environments, river otters may emerge as vital sentinels for environmental health. Monitoring otter populations and their parasite burdens could hence become a frontline strategy for early detection of emerging environmental risks, encompassing pollutants and disease agents with potential human health impacts.
This study also contributes substantially to the sparse body of knowledge regarding the biology and ecology of North American river otters in the Chesapeake Bay. Historically, otter populations in the region suffered drastic declines due to unchecked fur trade practices and habitat degradation. Their resurgence, bolstered by Maryland’s reintroduction initiatives in the mid-1990s, marks a conservation success, yet the otters’ elusive nature continues to impede comprehensive population assessments. The data garnered from fecal analyses thus fill pivotal gaps, spotlighting diet breadth, parasite-host dynamics, and latrine usage in both natural and human-constructed environments.
The methodologies underpinning this research showcase the power of modern molecular techniques such as DNA metabarcoding in ecological studies. By amplifying and sequencing tiny fragments of DNA from complex environmental samples, scientists can simultaneously identify multiple prey and parasite species, illuminating hidden ecological interactions that are difficult or impossible to observe by conventional means. DNA metabarcoding enhances ecological resolution, enabling accurate dietary reconstructions and parasite inventories that yield insight into trophic webs and environmental health.
Moreover, this research draws attention to how environmental pressures and anthropogenic alterations influence wildlife disease ecology. The appearance of otters in more urban landscapes correlates with increased exposure to pollutants—chemical contaminants, heavy metals, and pathogens—that accumulate in waterways. This multifaceted environmental stress not only impacts otter health but also raises broader questions about pollutant transfer through food webs and the cascading effects on ecosystem services.
Parasitology, often perceived solely through the lens of disease, here emerges as a critical component of ecosystem function. Parasites regulate host populations, mediate interspecific interactions, and can indirectly drive evolutionary pressures. The otters’ consumption of parasite-laden prey illustrates how apex predators integrate into these complex biological networks, where health, ecology, and disease intersect in profound ways.
Collaboration among institutions, including Frostburg State University, Johns Hopkins University, and the University of the Pacific, enriched this research, reflecting the interdisciplinary and multi-institutional nature of contemporary ecological science. These partnerships are vital for leveraging diverse expertise and technological capabilities to explore the nuanced relationships governing wildlife ecology and environmental health.
In conclusion, this revelation that North American river otters’ diets and parasite loads can serve as a window into the health of Chesapeake Bay’s aquatic environments opens exciting avenues for ecological monitoring and conservation strategies. As sentinel species with a metabolic connection to their environment, otters offer a beckoning opportunity to advance our understanding of trophic interactions, disease ecology, and environmental change in a rapidly shifting world. Continued research harnessing molecular tools and ecological insight will be crucial to unravel how these charismatic mammals navigate and reflect the complex, often unseen, biological webs that sustain ecosystem resilience.
Subject of Research: Animals
Article Title: North American river otters consume diverse prey and parasites in a subestuary of the Chesapeake Bay
News Publication Date: 14-Aug-2025
Web References:
Frontiers in Mammal Science Journal
Smithsonian Environmental Research Center
References:
Wise, C., Lohan, K., et al. (2025). North American river otters consume diverse prey and parasites in a subestuary of the Chesapeake Bay. Frontiers in Mammal Science. DOI: 10.3389/fmamm.2025.1620318
Image Credits: Karen McDonald, Smithsonian Environmental Research Center
Keywords: Aquatic ecology, Animal science, Animals, Ecology, Parasitology, Parasites, Trophic interactions, Marine food webs, Animal ecology
