This month, delegates from over 140 countries gathered in Geneva and finalized the first international treaty to reduce emissions of mercury. The treaty — four years in the works and scheduled for signing in October — aims to protect human health from this very serious neurotoxin.

But barely considered during the long deliberations, according to those involved in the treaty process, was the harm that mercury inflicts on wildlife. While mercury doesn’t kill many animals outright, it can put a deep dent in reproduction, says David Evers, chief scientist at the Biodiversity Research Institute (BRI), who serves on a scientific committee informing the process. “It is a bit of a silent threat, where you have to kind of add up what was lost through studies and demographic models.”

Harmful levels of mercury have turned up in all sorts of animals, from fish and birds living around the world to pythons invading the Florida Everglades and polar bears roaming far from any sources of pollution. In recent years, biologists have been tracking mercury’s footprints in unexpected habitats and species. Their research is illuminating the subtle effects of chronic exposure and is showing that ever-lower levels cause harm.

Coal burning, gold mining, and other human activities release mercury into water bodies or the atmosphere, where it can travel great distances before settling back to Earth. Mercury contamination is ubiquitous and hotspots are common around the world, with fish and human hair collected in 14 countries regularly exceeding U.S. Environmental Protection Agency (EPA) standards, according to a BRI report released just before the Geneva negotiations. And while mercury emissions are declining in North America and Europe they are rising quickly in the developing world, according to the United Nations Environment Programme, the treaty coordinator.

Exposed animals have trouble ridding their bodies of mercury and it accumulates in tissue.

The new global treaty bans the production, import, and export of certain mercury-containing products, requires governments to create plans to reduce mercury in small gold mining operations, and puts some controls on industrial facilities — but some environmental groups warn that it is too weak. The U.S. is going further. On January 1, an export ban on elemental mercury took effect, and the EPA is finalizing new limits on coal plant emissions.

“In the end the treaty will reduce mercury that’s being released into the environment. And I think the question will be, as we move along, ‘Is it enough?’ — especially for areas that are sensitive to mercury input. And then ‘Is it enough for wildlife conservation purposes?’ which really wasn’t addressed all that well,” Evers says.

Exposed animals have trouble ridding their bodies of mercury, and it accumulates in tissue with every link in the food chain. Long-lived predators tend to carry the heaviest loads. Research and public attention have largely focused on contaminated fish, the main route of human exposure. In water, mercury converts quickly to methylmercury, its most toxic and bioavailable form, so for many years wildlife biologists trained their sights on aquatic, fish-eating birds and mammals, says Bill Hopkins, a Virginia Tech physiological ecologist.

Lately, though, Hopkins and others have uncovered mercury in reptiles, amphibians, insects, spiders, terrestrial songbirds, and a wider variety of mammals than expected. “All these different groups can be exposed to mercury and pass it on to their babies,” says Hopkins.

Mercury is also turning up in strange places, he says. Invertebrate-eating songbirds living in the floodplain bordering a contaminated Virginia river had as much mercury in their blood as the river’s fish-eating birds, and sometimes more, showing that mercury pollution doesn’t stay put in aquatic habitats. Scientists have found mercury-laden food chains in mountainous forests, and shown that methylmercury forms in the woods, as well as in water. BRI scientists and collaborators discovered high levels in many invertebrate-eating songbird and bat species living in varied habitats across the U.S. Northeast and Mid-Atlantic states, including remote uplands. The pollutant has also emerged as a serious problem in the Arctic.

Mercury plays havoc on vertebrates’ development and their neurological and hormonal systems.

Mercury plays havoc on vertebrates’ development and their neurological and hormonal systems, and doses too low to kill can cause problems that aren’t always obvious in the wild, experts say. “Methylmercury is one of most toxic environmental pollutants we’ve ever come upon,” says Gary Heinz, a recently retired federal wildlife biologist who studied it over four decades.

In the earliest studies of these sublethal effects in the 1970s, Heinz reported that captive mallards fed mercury-laced food laid fewer eggs than control ducks and laid them outside the nest. Also, their ducklings didn’t respond well to their calls. Numerous examples have accumulated since. Fish form loose, sloppy schools and are slow to respond to a simulated predator. Several bird species sing different songs. Loons lay smaller eggs, and they incubate their nests, forage, and feed their chicks less. Salamanders are sluggish and less responsive to prey, Hopkins and colleagues found. Egret chicks are similarly lethargic and unmotivated to hunt.

Changes like these could be grave for wild animals, says Peter Frederick, a University of Florida ecologist who was part of the egret study. “Getting lunch or a mate depends on milliseconds and millimeters. You have to perform that courtship dance just right. You have to make the calls just right. You have to stab your prey to within a millimeter. If you’re off by a microsecond, it’s gone,” he says.

Frederick discovered a remarkable example in white ibises from the Everglades. There, mercury levels are low but constant, and ibises seem to nest less and abandon their nests more often than elsewhere. To see if chronic mercury exposure was responsible, Frederick captured 160 ibis nestlings and fed them food with mercury levels similar to their wild fish prey. He and his team observed the birds for three years to see if it affected their breeding behavior.

As expected, the dosed birds produced far fewer offspring than undosed controls. There were the usual reasons: eggs didn’t hatch and chicks died under lousy parenting. But Frederick was wholly surprised to see widespread homosexual pairing among the dosed males and to find this caused much of the reproductive deficit. Avian homosexuality usually occurs with stark sex imbalances — which wasn’t the case here, Frederick says.

‘We can be essentially neutering populations by cutting off reproduction through the endocrine system.’

No one had ever reported homosexuality as an effect of mercury, or any other contaminant for that matter, Frederick says. Moreover, the effects appeared in ibises he’d fed as little as 0.05 ppm of mercury in their food — one-tenth of what Heinz fed his mallards. Further work indicated that hormonal changes wrought by mercury’s effects on the ibises’ endocrine systems were at work. In a 2011 paper, Frederick and a colleague estimated that out in the Everglades, mercury could cut the number of ibis fledglings by half — easily enough to curtail the population.

No one has checked wild ibises for poor parental behavior or homosexuality, which might lay the blame more squarely on mercury, he says. (Different species react to mercury differently, and Frederick stresses that for many reasons his results in no way suggest that mercury might play a role in human homosexuality.) Nevertheless, the broader implications for chronically exposed wildlife are chilling. “We can be essentially neutering populations by cutting off reproduction through the endocrine system,” he says. “This could easily be going on in the wild with many kinds of contaminants. Mercury is not the only endocrine disruptor.”

Like Frederick’s study, much of the research on mercury’s sublethal effects has been conducted on captive animals. In nature, it’s very difficult to get the large sample sizes and control groups needed to identify subtle differences statistically, says Erick Greene, a conservation biologist at the University of Montana.

‘They may look all right, but I don’t know if would recognize a mentally impaired osprey chick.’

Studying ospreys living near Montana’s polluted Clark Fork River, Greene and two colleagues found that about half the eggs laid by high-mercury birds fail to hatch. But they’ve been puzzled as to whether the surviving chicks are affected. In humans, blood levels around .005 ppm can cause cognitive deficits, Greene says. But his osprey chicks commonly have levels 100 — and even 1,000 — times higher. The chicks seem to do fine in the nest, he says. “They may look all right, but I don’t know if I would recognize a mentally impaired osprey chick.”

Once they’re fledged they soon migrate south, out of sight. Greene suspects they may have trouble making the demanding migration to Central or South America (where mercury flows freely in small gold mining operations), or just figuring out how to survive on their own. His team has begun outfitting fledglings with satellite transmitters to determine how far mercury-loaded birds get compared to their normal peers, and how long they live.

It’s one thing to show that wild animals are exposed to harmful levels of mercury, but solid evidence that whole populations are harmed is harder to come by, experts say. A notable exception is loons. Evers and more than a dozen colleagues amassed an impressive 18-year data set of nearly 5,500 mercury measurements from loons on 700 lakes across 17 U.S. states and Canadian provinces. They showed that when mercury in loon blood hits 3 ppm, the number of young fledged drops by 41 percent — and that enough loons are affected to set back some New Hampshire and Maine populations.

In a forthcoming paper, Hopkins and another researcher go a step further with a population model they developed based on four years of field data on American toads. Toads readily move between small populations scattered throughout the landscape. Mercury exposure can kill eggs and tadpoles, and survivors are often small and slow to mature. The model revealed that not only can mercury kill enough tadpoles to wipe out small populations, but that nearby uncontaminated populations can also drop or go extinct because there are too few toads around to replenish them if their numbers happen to dip for other reasons. Hopkins says he thinks the paper will change biologists’ understanding of contaminants. “Contaminant effects in one population can actually affect adjacent populations that aren’t being exposed to that contaminant,” he says.

Whatever its weaknesses, the new treaty represents a “great step forward,” says Evers, and the good news is that once local sources are controlled, mercury in nearby wildlife can drop quickly. The bad news is that mercury from coal burning can travel great distances — for instance, from China to North America — before settling.

Overall, Evers says the forecast for wildlife is cloudy. When it comes to mercury, “the more we look the more we find, and the more we find the lower that toxicity level is going,” he says. “Right now at a global level, mercury is just being released more and more in the system. Those trend lines are going in the wrong directions.”