Juan Fernández fur seals are so poorly understood that they were considered extinct for nearly a century before a remnant population that had managed to evade generations of hunters was rediscovered in the 1960s.
Their enigmatic nature stems in part from their isolation on the same-named archipelago 600 kilometers off the Chilean coast.
These isolated islands are located in a protected national park, which is the last place you'd expect to find animals exposed to high levels of pollution.
But samples collected and analyzed by researchers tell us something different.
A recent study published in Royal Society Open Science examined fur seal poo and discovered cadmium and mercury concentrations among the highest ever reported for any mammal on the planet.
This species consumes extremely high levels of these toxic heavy metals through its diet, but the process by which they enter the food chain proved more complicated than expected.
Where are the heavy metals coming from?
PATRICK HERTZOG/AFP via Getty Images
Some metals, such as zinc and iron, are essential micronutrients that we're encouraged to include in our diet.
This is not the case with mercury and cadmium, as per The Conversation.
Mad Hatter's disease is a neurological disorder caused by mercury poisoning, whereas Itai-itai disease, which translates as "it hurts," is a condition that causes severe bone pain and weakening as a result of chronic cadmium poisoning that affected people working in contaminated rice fields in Toyama prefecture, Japan.
Even in small amounts, these heavy metals are highly toxic, and they have few known biological applications.
Heavy metals are found naturally in the Earth's crust and are emitted by volcanic eruptions or weather erosion of rocks.
They're also produced during mining, waste incineration, and steelmaking.
Because the Juan Fernández Islands are so remote from major industries, researchers were initially perplexed by the fur seals' seemingly toxic diet.
However, the archipelago is located on the edge of the South Pacific subtropical gyre, which has accumulated massive amounts of ocean plastic.
The plastic patch is estimated to be larger than in India.
Unfortunately, Juan Fernández fur seal mothers must travel long distances through this plastic cloud to find the nutrient-rich prey they require to sustain their pregnancies and produce enough fatty milk for their pups.
Plastic debris can leach heavy metals into seawater or accumulate them from other sources.
When marine animals ingest plastic or prey on animals that have done so, they can also ingest these pollutants.
This may explain why Juan Fernández fur seals have such high levels of mercury in their bodies.
Mercury can damage the nervous system, kidneys, and liver of mammals, while cadmium can cause severe bone damage and kidney failure, as per Phys.org.
Yet, surprisingly, these fur seals show no signs of ill health.
In fact, they have one of the highest reproductive rates among all seal species.
This indicates that this species has adapted to withstand the toxic heavy metal.
If this is the case, it could mean that Juan Fernández fur seals have been exposed to natural cadmium sources for a long time.
One possible source is hydrothermal vents - underwater openings where hot water rich in minerals gushes out from the Earth's crust.
These vents are known to emit cadmium and other metals into the surrounding water.
The Juan Fernández Islands are located near a tectonic plate boundary where hydrothermal activity is common.
It is possible that some of the prey species consumed by fur seals feed on organisms that live near these vents and accumulate cadmium in their tissues.
This would explain why fur seals have such high levels of cadmium in their poo, but not in their bones.
Scientists can learn about how polluted the environment is by studying these marine mammals.
Better yet, we might learn a thing or two from them about surviving in polluted environments.
