Mercury Reduces Avian Reproductive Success and Imposes Selection: An Experimental Study with Adult- or Lifetime-Exposure in Zebra Finch

Abstract

Mercury is a global pollutant that biomagnifies in food webs, placing wildlife at risk of reduced reproductive fitness and survival. Songbirds are the most diverse branch of the avian evolutionary tree; many are suffering persistent and serious population declines and we know that songbirds are frequently exposed to mercury pollution. Our objective was to determine the effects of environmentally relevant doses of mercury on reproductive success of songbirds exposed throughout their lives or only as adults. The two modes of exposure simulated philopatric species versus dispersive species, and are particularly relevant because of the heightened mercury-sensitivity of developing nervous systems. We performed a dosing study with dietary methylmercury in a model songbird species, the zebra finch (Taeniopygia guttata), at doses from 0.3 – 2.4 parts per million. Birds were exposed to mercury either as adults only or throughout their lives. All doses of mercury reduced reproductive success, with the lowest dose reducing the number of independent offspring produced in one year by 16% and the highest dose, representing approximately half the lethal dose for this species, causing a 50% reduction. While mercury did not affect clutch size or survivorship, it had the most consistent effect on the proportion of chicks that fledged from the nest, regardless of mode of exposure. Among birds exposed as adults, mercury caused a steep increase in the latency to re-nest after loss of a clutch. Birds exposed for their entire lifetimes, which were necessarily the offspring of dosed parents, had up to 50% lower reproductive success than adult-exposed birds at low doses of methylmercury, but increased reproductive success at high doses, suggesting selection for mercury tolerance at the highest level of exposure. Our results indicate that mercury levels in prey items at contaminated sites pose a significant threat to populations of songbirds through reduced reproductive success.

Assessment of mitochondrial DNA damage in little brown bats (Myotis lucifugus) collected near a mercury-contaminated river

“…Baker et al (1985) reported that in the marine gastropod, Cerithium rupestre, Hg resistance is reinforced at Hg polluted sites, presumably by natural selection for increased resistance and that Hg resistance in their study was associated with sites of high Hg pollution. Selection for resistance to mercury has also been demonstrated in birds chronically exposed to high levels of mercury over their lifetime (Varian-Ramos et al 2014).…”

Adverse Effects of Mercury and Some Management Options to Reduce Dental Amalgam Waste

“…In birds and mammals, mercury adversely affects reproduction, growth and development, nervous system, and metabolism (Varian-Ramos et al, 2014, Dietz et al, 2012. In addition, mercury in aquatic organisms can also disrupt blood chemistry the ability for osmoregulation, and oxygen exchange.…”

“…In addition, organomercury is acutely toxic to aquatic microorganisms and invertebrates. Organic mercury compounds 10 -100 times more toxic than the inorganic forms making bioaccumulated mercury a significant risk to aquatic organisms and organisms that consume them (Varian-Ramos et al, 2014).…”

“…Studies have shown that piscivorous birds are particularly at risk due to their diet (Varian-Ramos, 2014); however, it seems songbirds are also vulnerable to mercury exposure (Varian-Ramos, 2014, Jackson et al, 2011. The Varian-Ramos study showed that many songbird populations are at risk and many of them suffer population declines due to decreased reproduction and survival from mercury toxicity.…”

Dietary mercury exposure causes decreased escape takeoff flight performance and increased molt rate in European starlings (Sturnus vulgaris)

“…Many studies have documented the effects of mercury (Hg) on the health and fitness of aquatic bird species, but studies focusing on terrestrially foraging birds are fewer (Seewagen 2010). Recent research suggests that songbirds can suffer reduced reproductive success Hallinger and Cristol 2011;Varian-Ramos et al 2014), altered stress hormone profiles (Franceschini et al 2009;Wada et al 2009), and disrupted immune functioning (Hawley et al 2009;Lewis et al 2013) in response to Hg exposure, but more research is needed to fully understand the potential impact of MeHg on the health and fitness of terrestrially foraging species.…”