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Mercurial Effects of a Toxic PollutantMallard ducks exhibit unexpected benefits of eating traces of methylmercury.Jörg Hempel/Wikimedia Commons

A pinch of methylmercury is just ducky for mallard reproduction, according to a new federal study. The findings are counterintuitive, since methylmercury is ordinarily a potent neurotoxic pollutant.

Over a two-month feeding trial, treated adults produced more offspring — and young that at least initially grew faster — than did mallards dining mercuryfree.

No one was more surprised at the data than Gary Heinz, who led the study for the U.S. Geological Survey at the Patuxent Wildlife Research Center in Beltsville, Md. Especially since the new data fly in the face of a study he conducted three decades ago — also with mallards and diets that had been laced with the same concentration of mercury: half a part per mllion.

Writing in the March Environmental Toxicology & Chemistry, his team concedes its findings represent “an apparent case of hormesis.” That’s a poorly understood but well-recognized phenomenon whereby trace concentrations of a poison sometimes prove beneficial.

Gary Heinz and his colleagues randomized more than 100 breeding pairs of the ducks to receive either normal chow or a doctored recipe containing between 0.5 and 8 parts per million methylmercury chloride. Only 20 of the pairs received the lowest concentration of mercury.

Roughly a month into the feeding trial, the scientists began collecting eggs daily from the birds’ nests. On select days over the 33-day laying period, an egg from each nest was retained for chemical analysis (which confirmed methylmercury made it into the egg). The rest were incubated until hatching. The USGS scientists will show in a followup paper (wending its way through publication now) that all methylmercury doses except the smallest one elicited some signs of toxicity.

Birds in the lowest treatment group, however, appeared healthy throughout the trial and laid the same number of eggs as mallards getting clean chow. And the hatch rate was significantly higher for the low-dose mercury group: almost 72 percent compared to 57.5 percent in the ducks scarfing down clean chow. Although ducklings from both groups weighed the same amount at hatching, those from mercury-fed parents grew faster. By six days old, those ducklings weighed roughly 8 percent more than little quackers that came from mercuryfree parents.

The USGS group acknowledges that its findings would appear to contradict a host of earlier studies — including one that Heinz conducted with mallards, albeit using a slightly different chemical formulation (methylmercury dicyandiamide). Then again, the latest study is not the first to highlight an apparent hormetic effect of methylmercury. The Patuxent team cites one study in African clawed frogs (the lab rat of the amphibian world) that showed low doses of methylmercury improved survival of tadpoles while higher doses increased death rates.

Still, Heinz acknowledges niggling doubts about his findings.  

He wonder, for instance, whether the flock he studied might have harbored some low, subclinical infection that nobody picked up on. If so, mercury — used for many decades as an antimicrobial (albeit a toxic one) — might, at low doses, have knocked out the infection that otherwise hurt reproduction in the untreated birds. No one will ever know. But fueling his suspicions was the “poor” hatching success in untreated birds. Their rate should have been closer to that seen in mallards exposed to the low dose of mercury.

Unless and until someone repeats this experiment and gets a normal rate of hatching in the control group, Heinz and his teammates conclude, “one cannot rule out the possibility that low concentrations of mercury in eggs may be beneficial, and this possibility should be considered when setting regulatory thresholds for methylmercury.”

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