A new discovery in Santa Cruz Mountains pumas reveals that after pollution enters the ocean, some toxins may return to land
Veronica Yovovich kneels beside a dead mountain lion. Her field tweezers tighten around a whisker and slowly pull. The skin of the cat’s cold snout stretches out and then silently snaps back as she plucks the hair free.
“Mountain lions don’t tend to live very long in the Santa Cruz Mountains,” says Yovovich, a wildlife biologist with the Santa Cruz Puma Project. “Many are hit by cars.” But when scientists took a closer look at the puma whiskers Yovovich has plucked in recent years, they found that the animals in the foggy Santa Cruz Mountains faced an unexpected problem: mercury.
In research presented this week in San Francisco at a meeting of the American Geophysical Union, environmental chemist Peter Weiss-Penzias and longtime Puma Project director Chris Wilmers showed that mountain lions in the woods and mountains around us are consuming potentially toxic amounts of mercury—carried from the ocean to the redwood ecosystem within the chilly droplets of coastal fog.

Something in the Fog

Weiss-Penzias is an expert in tracking the spread of mercury, a toxic chemical released mainly from coal plants.
“It’s iconic of the most reckless, unnecessary pollution,” he says.
Mercury sails on atmospheric currents around the globe, and some settles into the ocean—which is why Weiss-Penzias now watches fog reports regularly. Biking to work at UCSC one morning a few years ago, he noticed the heavy, wet air pressed against his temples and spider webs sagging under glowing water droplets. He realized the ocean was, in effect, all around him. He stopped pedaling and stared into the white horizon. “What is fog?” he recalls wondering. “What’s in it?” No one had ever looked for hazardous forms of mercury in coastal fog.
So Weiss-Penzias set up nets that condense fog into water, like the spider webs and their beaded droplets. His tests at the lab showed that every sample of fog water contained high levels of mercury.
“I didn’t believe it,” he says. “I was trying to figure out how we could have contaminated the sample somehow to get these high numbers.” So his team returned and collected more fog water, but the mercury was always there.
Still, he wasn’t sure what his findings might mean for the plants and animals in our foggy coastal habitats. “This was a source of a toxic compound to the environment that was new,” he says. “Here’s a mechanism that involves the air, the ocean and the land, and nobody knows anything about it.”
Coastal fog forms in Central California because the ocean is much colder here than our latitude and climate would suggest. The region’s spring winds cause cold, deep water to rise, cooling the surface water along the coast. When clouds from the Pacific encounter this cold coastal band, the chilled vapor forms heavier droplets that drape the Santa Cruz Mountains foothills, often reaching the summit. Mercury pollution from the surface ocean may hitch a ride.
“Coastal fog is basically an extension of the ocean,” Weiss-Penzias says. “Mercury settles into the ocean from the atmosphere, but it also finds a way back out.”

Tracing the Toxins

When Wilmers heard about Weiss-Penzias’ results, he wondered whether mercury could enter redwood ecosystems. The mercury in coastal fog is elevated but still dilute. For example, fog contains much less mercury than a can of tuna. But Wilmers also knew that trace metals can build up to much higher levels once living organisms get involved—like redwood trees.
In the Santa Cruz Mountains, redwoods survive the long rainless summer by using their needles like Weiss-Penzias’ fog nets. They sop up water directly from the wet air and trickle it along their needles. As redwoods drink, the toxic mercury builds up in the trees’ tissues. Eventually, redwoods shed their needles, along with the bound mercury, to the forest floor.
Researchers from the U.S. Geological Survey helped the UCSC scientists out. They gathered redwood needles and brought them back to Weiss-Penzias’s lab. Here, the team not only found mercury, but they also measured much more than was in the fog. “It seemed a little too simple to be actually occurring,” Weiss-Penzias says. The amount of mercury in the redwood needles was too small to threaten humans, but now the team wondered if they might find more elsewhere in the forest.
They soon had another opportunity—a graduate student in Weiss-Penzias’ lab was fond of collecting wolf spiders. “He would lay out cups in the forest, and the spiders would simply fall in,” Weiss-Penzias says. The team tested the spiders, and once again they found mercury—but this time at levels beyond the safe human health threshold.
The high mercury level in spiders doesn’t come from the fog droplets that bead on their webs. Instead, Weiss-Penzias says, spiders must consume mercury-laden prey. In a common ecosystem phenomenon called bioaccumulation, toxins become more concentrated as they move up the food chain. When predators consume prey, they burn through the fats, carbohydrates and proteins in their food, but the unusable toxins—often heavy metals—become trapped in their tissues.
Wilmers and Weiss-Penzias suspected that mercury might contaminate the entire redwood ecosystem. “Pretty much everywhere we look, there seems to be an enhancement,” Weiss-Penzias says. And bioaccumulation hints that mercury increases with each rung up the food chain. To know for sure, the team wanted to find out whether the redwood forest’s apex predators, mountain lions, were consuming mercury.

The Puma Connection

To study redwood ecosystems, Wilmers had been collecting mountain lion whiskers for years and storing them in a lab freezer. “It’s hard to catch a mountain lion,” says Yovovich, “but every time we capture one we want to gather as much data as we can, so we pull a whisker.”
Like a human hair or a tree ring, a mountain lion whisker is a chemical archive that traces the cat’s health as the whisker grows. Mercury in particular sticks to hair-like tissues, Weiss-Penzias says, such as whiskers. As a mountain lion whisker grows, tiny amounts of the mercury from within the cat attach to the whisker.
The team pored through the Puma Project’s whisker collection. More than one-third of Santa Cruz–area mountain lions had mercury levels well above the human health threshold. Of the 88 pumas sampled from the Santa Cruz Mountains, three cats had whiskers with four times the human health threshold—with one animal carrying 12 times more of the metal than is considered safe in humans.
Meanwhile, technicians at the California Department of Fish and Wildlife sent the team whiskers from Sierra Nevada pumas—inland cats that live hundreds of miles from coastal fog. When the scientists tested the whiskers of these fog-free cats, the high level of mercury wasn’t there. Only cats from foggy coastal climes, they found, had whiskers rich in mercury.
Mercury pollution can come from other sources, such as abandoned mines. However, for the mercury that reached pumas here, fog is the most likely source. Cats in the Santa Cruz Mountains that live near old mines are about as likely to have high mercury as those that live far away from them. “We do see some high mercury levels near old mines at the summit,” Weiss-Penzias says. “But we also see comparable levels at some coastal sites.” And abandoned mines exist in the Sierra Nevada, too.
Weiss-Penzias points to another important clue that coastal fog is the most likely source. “Mercury from old mines exists in a form that is hard for organisms to take up,” Weiss-Penzias says. “I’m not sure how they could even do it.” The mercury in coastal fog, on the other hand, is very dilute, but it’s in a form that is more easily bound by plants and animals.
Scientists don’t know exactly how mercury gets into fog, but the idea that it trickles back into coastal ecosystems is new. And for pumas, the potential effects of mercury haven’t been examined before. “We didn’t really think mercury was a threat to mountain lions,” Yovovich says. Now scientists need to understand whether the mercury that pumas are consuming poses risks to them.

Mapping Mercury’s Path

Researchers don’t yet know how mercury might be affecting mountain lions, but research has clearly shown that it harms other large mammals. Mercury tangles up the enzymes that make important “cleaning molecules” in cells. These molecules are necessary to prevent natural but harmful byproducts from building up. In small doses, an animal can cope with the damaging clutter. But if it consumes too much mercury, the rogue byproducts damage tissues in an irreversible, cascading cycle. Still, the researchers emphasize, mercury’s effects—and the levels that should be deemed dangerous—are unknown in mountain lions.
The team’s next step will be to understand where the pumas are consuming the mercury-rich prey. “In general, the majority of what mountain lions eat in this area is mule deer,” says Yovovich. The researchers plan to use deer fur, like the puma whiskers, to find out whether deer are the source of the mountain lions’ mercury. They will also look for mercury in the plants that Santa Cruz mule deer graze. Their tests should help map out mercury’s path through the redwood ecosystem.
Yovovich points out that increasing residential developments in San Francisco’s Bay Area may also affect how mercury reaches pumas. “In more urbanized areas, mountain lions tend to eat less deer, and more small mammals,” says Yovovich. Unlike mule deer that graze meadow grasses, small mammals may feed on mercury-contaminated prey such as spiders. If more pumas eat small mammals, their exposure to mercury may increase.
Mercury from fog may be the latest and most surprising threat to wildlife in the Santa Cruz Mountains, and knowing that coastal fog can carry toxins from the ocean into the hills may complicate how scientists track pollution. But the biggest struggle for pumas in the Santa Cruz Mountains is still simply a lack of space. “There’s a challenge of pumas being pumas,” Wilmers says.
For mountain lions, living in a crowded world isn’t new. But the protected cats are doing better here than elsewhere. “We don’t have a lot of older pumas,” Wilmers says, “but we still do have a lot of pumas in the Santa Cruz Mountains.”
As they hunt and roam through the foggy hills, these wild cats have to make do in the tight margins created by our towns and highways. “There are tendrils of development interspersed with the open spaces,” Yovovich says. “It’s a mosaic of land uses that the cats have to negotiate in order to just be cats.”

The Puma Project

The Santa Cruz Puma Project is a collaboration between UC Santa Cruz researchers and the California Department of Fish and Wildlife that tries to understand how pumas and people coexist in the Santa Cruz Mountains.
Pumas help maintain a healthy balance of animal life in the redwood forest ecosystem. Ecologists call animals like pumas keystone species, because their activities help shape their habitat. For example, pumas prevent deer from overgrazing plants, or trampling stream corridors. But although pumas play a pivotal role in the redwood ecosystem, making space for them has never been easy.
The Santa Cruz Mountains border one of the densest urban areas in the country. Highways and roads cut through pumas’ redwood homes, and everywhere people and pumas compete for scarce space. Neither people nor pumas are to blame, say the researchers, but the more we know about how pumas adapt and respond to human development, the better we can design our communities to help share the space.
The Puma Project uses tracking collars to map out how pumas move through our local mountains. Knowing how pumas travel, and where they choose to spend their time, helps us learn what regions are most important to conserve, and what highway crossings are most dangerous.
The Puma Project’s whisker library, which provided the samples used to test for mercury, also helps researchers understand food chain position and foraging behavior.

Follow the Santa Cruz Puma Project’s online blog at santacruzpumas.com, or search santacruzpumas on Twitter to hear about what Chris Wilmers and other researchers are up to each week.