In the heart of the Peruvian jungle, inside a camping tent, four dedicated scientists gathered around a diminutive patient – an Amazonian rodent small enough to rest comfortably in a human hand. Their mission was clear: to examine the effects of mercury contamination on terrestrial mammals in one of the world’s most vital ecosystems, the Amazon rainforest.

With utmost care, they placed the small-eared pygmy rice rat inside a plastic chamber and gently administered anaesthetic gas until the creature succumbed to sleep. Once the rodent was anaesthetised, they equipped it with a miniature anaesthetic mask and meticulously measured its body parts with a ruler. Then, using delicate tweezers, they extracted tiny hairs from its back, carefully collecting them in a minuscule plastic bag. These hairs held the key to uncovering whether this rat was yet another victim of mercury contamination.

Los Amigos Biological Station, situated in the southeastern Peruvian Madre de Dios region, serves as the backdrop for this groundbreaking research. In this region, approximately 46,000 miners are fervently searching for gold along the riverbanks, making it the epicentre of small-scale mining in the country.

The preliminary findings from this pioneering study, which represents the first extensive investigation into the impact of illegal and poorly regulated mining on terrestrial mammals in the Amazon rainforest, are both concerning and enlightening.

Mercury, a highly toxic substance, has long been known to pose significant health risks to humans and some avian species when absorbed or ingested through contaminated water or food. It can lead to neurological illnesses, immune disorders, and reproductive failures. However, the full extent of its impact on the diverse range of forest animals in the Amazon, where over 10,000 plant and animal species are at high risk of extinction due to rainforest destruction, remains largely unknown.

The research team, comprising scientists from the San Diego Zoo Wildlife Alliance, the California nonprofit Field Projects International, and Peruvian partner Conservación Amazônica, embarked on this ambitious mission to collect fur and feather samples from more than 2,600 animals, representing at least 260 species. This comprehensive study zone encompassed a 4.5 square kilometre (1.7 square mile) area surrounding the Los Amigos station.

While the scientists initiated their mercury testing efforts at Los Amigos in 2021, some of the collected samples date back to 2018. Among the most striking findings is that, of the 330 primate samples tested thus far, almost all exhibited mercury contamination, with some reaching “astounding” levels, as revealed by biologist Mrinalini Erkenswick Watsa of the San Diego Zoo Wildlife Alliance. Specific readings were not disclosed, as the findings await publication in peer-reviewed journals.

A study conducted last year by biogeochemist Jacqueline Gerson of the University of Colorado Boulder, based on data generated at Los Amigos, demonstrated that songbirds residing around the station had mercury levels up to 12 times higher than those in a forest further removed from gold mining activities.

During Reuters’ visit to Los Amigos, scientists employed various methods to capture animals for testing, using metal traps baited with peanut butter for rodents and deploying mist nets to capture birds and bats in the forest.

A critical issue at hand is the rapid expansion of mining in the Amazon rainforest over the past 15 years, which is viewed by regional governments as both an environmental and health hazard. Colombia has proposed a regional pact to combat illegal mining, although it has not yet established a deadline for achieving this goal. The prevalence of small-scale miners operating illegally in protected areas or informally in government-designated mining corridors with minimal regulatory oversight exacerbates the problem.

The rise of artisanal mining, which accounts for approximately one-fifth of global gold production, has created a complex challenge for monitoring and regulation. This sector, valued between $30 billion and $40 billion, has seen substantial growth, with around 500 metric tons of gold produced annually as of 2023, up from approximately 330 metric tons in 2011, according to data from the nonprofit Artisanal Gold Council (AGC). Peru, as the largest gold producer in Latin America, contributes approximately 150 metric tons of artisanal gold each year, as reported by the AGC.

In Madre de Dios, a 2022 USAID report estimated that around 6,000 miners operate with formal permission, while roughly 40,000 work informally or illegally. This challenging landscape prompted the Peruvian government to declare a state of emergency in Madre de Dios in 2019 and deploy 1,500 police and soldiers to the region to crack down on illegal mining. While this operation led to the displacement of many miners from protected areas into government-designated mining corridors, satellite monitoring projects indicate that illegal miners still contribute significantly to the region’s mercury contamination problem.

The process of informal mining often involves the use of toxic liquid mercury to separate precious metal from sediment. Mercury is mixed with fine river silt in oil drums, binding to gold fragments to form amalgams. Subsequently, burning these amalgams vaporizes the mercury, leaving behind only the gold. This gaseous mercury infiltrates the forest through plant leaves’ pores, as demonstrated by research published in Nature Communications last year. The vaporized mercury adheres to dust and aerosol particles, drifting down through the forest canopy and settling on leaves. When it rains, this mercury is washed down to the forest floor, further exacerbating contamination.

The intricate web of mercury contamination in this environment has raised significant concerns about its impact on the health of animals. For example, in the early hours of the morning at Los Amigos, biologist Jorge Luis Mendoza Silva carefully untangled a vibrant band-tailed manakin bird from a fine-mesh net. Back at the sampling tent, scientists delicately plucked clusters of feathers from the manakin’s breast for analysis before returning the bird to its natural habitat. Advanced machinery was then used to incinerate the feathers at extremely high temperatures, allowing for the measurement of emitted mercury.

Animals in this ecosystem may ingest mercury through their diets, which primarily consist of plants, insects, or other animals. As mercury accumulates in organisms higher up the food chain, animals at the top are more likely to exhibit higher levels of contamination.

However, scientists at the Los Amigos station face a perplexing challenge in determining the source of mercury contamination in monkeys, as traditional mercury-laden foods, like fish, are not typically part of their diet. Consequently, animals may be accumulating mercury through the water they drink or the air they breathe, posing significant uncertainties regarding its potential health effects. The long-term consequences of mercury contamination could manifest in population size and reproduction rates, raising concerns about the sustainability of these vital species.

The findings from this groundbreaking research highlight the urgent need for action to address the environmental and ecological consequences of illegal and poorly regulated mining in the Amazon. As leaders from the eight countries surrounding the Amazon convene in Brazil to discuss strategies for combating illegal gold mining, it is evident that the fate of this irreplaceable rainforest and its unique biodiversity hangs in the balance. Efforts to curb mercury contamination and protect the Amazon’s fragile ecosystem are paramount, as the consequences of inaction reverberate through both the natural world and human societies that depend on the riches of this extraordinary biome.