Enhanced reproduction in mallards fed a low level of methylmercury: An apparent case of hormesis

Abstract

Breeding pairs of mallards (Anas platyrhynchos) were fed a control diet or a diet containing 0.5 microg/g mercury (Hg) in the form of methylmercury chloride. There were no effects of Hg on adult weights and no overt signs of Hg poisoning in adults. The Hg-containing diet had no effect on fertility of eggs, but hatching success of eggs was significantly higher for females fed 0.5 microg/g Hg (71.8%) than for controls (57.5%). Survival of ducklings through 6 d of age was the same (97.8%) for controls and mallards fed 0.5 microg/g mercury. However, the mean number of ducklings produced per female was significantly higher for the pairs fed 0.5 microg/g Hg (21.4) than for controls (16.8). Although mercury in the parents' diet had no effect on mean duckling weights at hatching, ducklings from parents fed 0.5 microg/g Hg weighed significantly more (mean = 87.2 g) at 6 d of age than did control ducklings (81.0 g). The mean concentration of Hg in eggs laid by parents fed 0.5 microg/g mercury was 0.81 microg/g on a wet-weight basis. At this time, one cannot rule out the possibility that low concentrations of Hg in eggs may be beneficial, and this possibility should be considered when setting regulatory thresholds for methylmercury.

Toxicity Reference Values and Tissue Residue Criteria for Protecting Avian Wildlife Exposed to Methylmercury in China

“…When lesser doses of MeHg were injected into mallard eggs, hormesis was observed at 0.05 m g MeHg/g (wwt) (least dose) (Heinz et al 2011 ) , which agrees with a similar observation, in which mallards were exposed to a single dose of 0.5 m g MeHg/g (dwt) (Heinz et al 2010b ) . However, the mean concentration of MeHg in eggs of hens fed MeHg in the diet was 0.81 m g MeHg/g (wwt) (Heinz et al 2010b ) , a value greater than the 0.05 m g/g (wwt) of Hg that was injected.…”

“…However, the mean concentration of MeHg in eggs of hens fed MeHg in the diet was 0.81 m g MeHg/g (wwt) (Heinz et al 2010b ) , a value greater than the 0.05 m g/g (wwt) of Hg that was injected. The exposure pathway was regarded to be the major reason for the difference.…”

“…Although the mechanism for this discrepancy is not well understood, it is probably due to a difference in dose vs. dose rate, or from the binding of MeHg, which results from differences in biological activity between the two vectors of exposure. The results of a single dose MeHg exposure (0.5 m g MeHg/g (dwt)) (Heinz et al 2010b ) seemed to contradict the results of the multigeneration, dietary exposure in which the same dose was used. This might result from differences between forms of MeHg and the sources of mallards tested (Heinz et al 2010b ) .…”

“…The results of a single dose MeHg exposure (0.5 m g MeHg/g (dwt)) (Heinz et al 2010b ) seemed to contradict the results of the multigeneration, dietary exposure in which the same dose was used. This might result from differences between forms of MeHg and the sources of mallards tested (Heinz et al 2010b ) . Thus, additional research is needed on "lowdose" effects of MeHg to the mallard.…”

Comparison of chronic low-dose effects of alpha- and beta-emitting radionuclides on marine bacteria

“…The stages of bioluminescence activation in Figure 2 can be ascribed to hormesis phenomenon [29][30][31]. Activation of vital functions of various organisms is a wellknown effect, common to all living organisms.…”