The research focused on five elements of well-known toxicological concern: cadmium (Cd), lead (Pb), chromium (Cr), mercury (Hg), and arsenic (As). Between 2018 and 2021, the team collected 50 fecal samples from four wildlife species—Amur tiger (Panthera tigris altaica), sika deer, wild boar, and other principal prey species—alongside 52 environmental samples of soil, vegetation, and water drawn from the same terrain. In total, at least one of the target elements was detected in 90 of the 102 samples, a detection rate of 88.2 percent. That figure alone is striking: the Tumen River Basin is not an industrial sacrifice zone but a mosaic of forest, farmland, and protected areas within the Northeast China Tiger and Leopard National Park, where anthropogenic disturbance is relatively limited. Yet metals were present nearly everywhere the team looked.

The analytical approach combined field collection with laboratory quantification techniques drawn from established environmental chemistry practice. Metal concentrations in fecal material were measured using atomic absorption and atomic fluorescence spectrometry, methods whose sample preparation requirements and detection limits are well documented in the literature. Because environmental contaminant data routinely contain values below detection limits, the authors handled censored data with established statistical conventions, substituting values below the limit of detection following standard practices for nondetect estimation. Cross-species comparisons, correlation analyses, and exploratory regression models were then used to ask whether fecal metal concentrations tracked environmental gradients—distance to cropland, distance to water bodies, and forest canopy height—and whether they rose with position in the food chain.

The species-level results were anything but uniform. Among the four wildlife species examined, mean fecal concentrations of arsenic, cadmium, and mercury were highest in Amur tiger feces, with mercury averaging 0.149 ± 0.187 mg·kg⁻¹. This pattern is biologically plausible. Tigers sit at the apex of the food web, and mercury in particular is known to bioaccumulate in carnivores; a companion study on Siberian tigers in the Russian Far East has documented measurable mercury burdens in the same subspecies, likely linked to aquatic prey pathways. The finding that the largest predator carries the highest fecal loads of these three elements will sharpen concerns about cumulative contaminant exposure in a big cat population that is only recently recovering from near-extinction in China.

But the picture inverted for the other metals. Mean concentrations of chromium were highest in sika deer feces, averaging 23.433 ± 3.754 mg·kg⁻¹—a remarkably high value—while lead peaked in wild boar. Neither pattern follows a simple biomagnification narrative. Chromium’s dominance in a grazing herbivore likely reflects plant-soil transfer and possibly incidental soil ingestion, a pathway increasingly recognized as a major exposure route for wild herbivores; recent work on African savannah mammals and Svalbard reindeer has emphasized that soil swallowed during foraging can dwarf dietary uptake as a contamination route. Wild boar, as omnivorous rooters that consume soil, tubers, and agricultural crops, are almost engineered for environmental exposure, which may explain their lead burden. Ingestion of soil is a known determinant of contaminant exposure in free-ranging pigs and livestock.

Perhaps the most consequential finding is what the data did not show. Cross-species comparisons revealed no consistent increase in fecal metal concentrations with trophic position. Classical ecotoxicology would predict that non-essential heavy metals biomagnify up food chains, as documented in reviews of trophic transfer and bioaccumulation of hazardous metals. Here, the herbivores outstripped the apex predator for some elements while the tiger led for others. The authors interpret this species- and element-specific pattern as evidence that exposure pathways in this landscape are governed less by food-chain amplification than by the specific feeding ecology, habitat use, and foraging behavior of each species—what an animal eats, where it eats, and how much soil it inadvertently ingests matter more than where it sits in the trophic hierarchy.

To quantify this, the team developed a novel metric: the fecal-to-diet concentration ratio, or FDCR. Applied across the Amur tiger’s three principal prey species—sika deer, wild boar, and roe deer-like ungulates—the FDCR compared fecal metal concentrations with expected dietary concentrations based on vegetation. The ratio exceeded 1 for every species–metal combination, meaning feces were consistently enriched relative to the diet, and the spread was enormous: the highest value was 111.176 for chromium in sika deer, while the lowest was 1.506 for mercury in wild boar. FDCRs far above unity suggest either substantial excretion of absorbed metals through the gut, contribution from non-dietary pathways such as soil ingestion and drinking water, or both. Either way, the metric provides a practical, non-lethal index of how contaminant intake translates into excreted burden—something that previously required sacrificing animals or fitting them with collars to estimate.

The exploratory spatial models added further texture. The team evaluated whether distance to cropland, distance to water, and canopy height explained variation in fecal metal concentrations, using collinearity diagnostics, false discovery rate correction for multiple testing, and influence diagnostics to guard against spurious inference. While the models were framed as exploratory rather than confirmatory, the overall conclusion is that fecal measurements provided species- and element-specific exposure-related signals within a generally low-contamination environmental context. In other words, even where soil and vegetation screening suggests the landscape is not heavily polluted, the animals themselves register measurable exposure, and that exposure varies in legible ways with their ecology.

The choice of feces as the monitoring matrix is the methodological heart of the study. Traditional wildlife toxicology often relies on invasive sampling—blood, liver, or hair from hunted or necropsied animals—which is ethically fraught, impractical for endangered species, and biased toward dead individuals. Fecal biomonitoring sidesteps these problems entirely: samples can be gathered during routine field surveys, verified to species using molecular or morphological scat identification methods, and analyzed repeatedly over time and space. Feces have already proven effective as pollution biomonitors in birds, marine animals, polar bears, and terrestrial mammals, and the approach is gaining traction precisely because it is compatible with the logistics of monitoring rare, wide-ranging carnivores. For an Amur tiger population that numbers only a few dozen individuals on the Chinese side of the Sino-Russian border, a method that requires no capture, no sedation, and no harm is essentially the only viable option.

The study region itself amplifies the significance. The Tumen River Basin straddles the borders of China, Russia, and North Korea and anchors the Northeast China Tiger and Leopard National Park, established to secure a transboundary meta-population of Amur tigers and Amur leopards. Previous research in the area has documented heavy metals in farmland soils of the Hunchun basin, and separate work has found microplastics in tiger habitat, underscoring that anthropogenic contaminants are reaching even core conservation zones. Tigers in this landscape move between forest corridors and agricultural fringes, prey on wild boar and deer that feed in croplands, and drink from waterways that may carry runoff from settlements and agriculture. The new fecal data provide the first integrated, multi-trophic view of how those exposures register in the animals’ own excreta.

For conservation managers, the implications are concrete. Fecal biomonitoring can now complement conventional soil, vegetation, and water monitoring to build a surveillance system for contaminant threats to endangered species—one that requires only continued sample collection during existing survey efforts. The baseline concentrations reported here for tigers, deer, and boar can serve as reference points against which future changes are judged, whether those changes stem from expanding agriculture, industrial development, transboundary pollution, or remediation successes. Given that cadmium is a recognized endocrine disruptor, that lead and mercury are potent neurotoxins, and that chronic exposure can impair reproduction in wildlife, the ability to detect rising burdens early—without harming a single animal—could prove decisive for the long-term health of one of the world’s most charismatic endangered predators.

The authors caution that their models are exploratory and that the dataset, while substantial, involves sensitive information about endangered species and protected areas; the underlying data are available from the corresponding author upon reasonable request. Still, the study demonstrates that a humble pile of scat, properly collected and analyzed, can carry a rich dossier of environmental information: which metals an animal has encountered, how those metals compare across species and trophic levels, and where in the landscape exposure risks concentrate. In the coexistence zone where tigers and people share the Tumen River’s forests, that dossier may become one of conservation’s most valuable—and least invasive—tools.