National Wildlife: December 2007 Archives


THE DRY SEASON grips the highland savanna of eastern Africa between late May and early June. As the relentless sun drives the last bit of moisture from the soil, the verdant highlands shrivel to yellow and brown. Herds of antelope, gazelle, wildebeest, elephant and buffalo gather by the hundreds of thousands, chafing to begin their annual migration to the lowlands to find water. Lions, hyenas and other predators likewise ready themselves to follow their prey on the seasonal journey.

But none can leave the depleted highlands until the plains zebra departs--and readies lowland grasses for the grazers that follow. "They start the whole grazing succession," says Princeton University ecologist Daniel Rubenstein, who has been studying the species for the past 15 years. Because zebras have an unusual hindgut digestive system rich in fermenting microbes, they can gorge on the low-quality coarse straw available at the start of the rainy season. Buried beneath this sunlight-blocking thatch lies the tender new growth upon which all other grazers depend. This means that from the vast, acacia-dotted savannas of the Serengeti, south through Zambia's scrubby woodlands and the treeless veldt of South Africa, the plains zebra plays an unmatched role in maintaining some of the most biologically diverse habitat on the planet.

Consequently, the long-term conservation of Africa's tropical grasslands relies on the wise stewardship of this linchpin species, says Rubenstein. In partnership with the African Wildlife Foundation and Earthwatch Institute, he currently heads a broad-based project aimed at integrating zebra conservation with the rapid growth of agriculture, livestock and game-ranching industries in eastern Africa.

To the tourist on safari, the plains zebra certainly looks to be thriving. Indeed, where these zebras still occur, their inimitable stripes dominate the landscape, with a population of nearly 665,000, dwarfing that of its two gravely endangered cousins--the Grevy's zebra of Kenya and the mountain zebra of Namibia and South Africa.

"But if you actually look at the details," says Rubenstein, "you see that the plains zebra has been extirpated from much of its historic range." Today, 70 percent of its population concentrates in just two countries, Kenya and Tanzania. Ecotourism dollars have enabled these nations to protect large swaths of lowland zebra habitat inside national parks. But just outside park borders, farms and ranches have sprung up to block many of the age-old migration routes that zebras need to cross between dry season refuges and lusher uplands. Though the lengths of these migration routes vary widely, many populations travel hundreds of miles a year.


In Tanzania, for example,"there used to be nine migratory routes out of Tarangire National Park," says Rubenstein. "Now there are only two or three left, all on privately owned land." Because of the low quality of their grasses, dry-season refuges such as Tarangire can't support large populations of zebras unless the animals can fatten up in adjacent highlands during the wet season.

How this changing landscape will affect the plains zebra is unknown. But what is certain, given Rubenstein's studies, is that human activities are affecting the species' unique multilayered social structure.

At the core of plains zebra society is a breeding group, or harem, with a single breeding stallion, up to six mares and their young. Rubenstein has discovered that although the stallion defends his harem from danger and harassment, the mares generally call the shots. "The males that let their females do what they want--organize behind the scenes and choose the kind of grazing they want--those are the stallions that get the most females," he says.

As with many social mammals, female plains zebras form strong, long-term bonds with one another and stay together even after the death of a stallion. What's unusual is that the females in a zebra harem are not related. And unlike many other large mammals--such as lionesses and female elephants, which form alliances with sisters and daughters--plains zebra mares cast out all of their offspring. "In fact, it's usually the young females who get the message first and take a hike," says Rubenstein. In her second year, a maturing filly goes in search of a young stallion. A young male leaves by his third year, either to join a bachelor group or begin recruiting his own harem.

Adding more complexity, stallions and their harems often band together to form a much larger, loose-knit herd of several hundred, where the females freely comingle. Yet the stallions generally resist pilfering each other's mares. The purpose behind these superherds appears to be defense against raiding parties of bachelor males. "A single stallion set upon by a large group of bachelors has little chance of defending his females. As he barrels into the mob, taking on five or six of the marauders, the others just stream around him and head for the females," says Rubenstein. By contrast, a coalition of stallions can present a united front to keep the interlopers at bay. Rubenstein has observed that the more bachelors in a given area, the larger the combined-harem herds.

This elaborate social system complicates the task of conserving and managing the plains zebra. Many zebras live on private game ranches, where hunters target established stallions--the male's brighter coat makes for the most desirable trophy and the "homebody" stallions tend to be easier to find than the farther-ranging, vagabond bachelors. But increasing the proportion of bachelors causes surviving stallions and mares to band together even more, producing an unnatural herd size. "It's creating a whole new population structure," says Rubenstein, "It's not clear whether this is natural or sustainable, or what it might mean for the future."

More clear-cut is the impact that stallion hunting has on the zebra birth rate. Remove a harem's stallion and the fertility of the surviving mares drops dramatically for at least two years while young and inexperienced bachelors vie for control. "From a game management point of view, that means fewer babies," says Rubenstein, "a fact that has to be taken into consideration when setting hunting quotas if zebra populations are going to remain self-sustaining."

That said, wild populations as large as the plains zebra's can adapt to new pressures. But only if they have enough space. "Free run is the issue," says Rubenstein. "The zebras can take care of themselves."

Jessica Snyder Sachs, a regular contributor to National Wildlife magazine, is the author of Good Germs, Bad Germs: Health and Survival in a Bacterial World (Hill&Wang/FSG) and Corpse: Nature, Forensics, and the Struggle to Pinpoint Time of Death (Perseus/Basic Books).

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Copyright Jessica Snyder Sachs, as first published in National Wildlife (photo courtesy

SOMETHING SHINY caught my eye when I glanced at the old thermostat the repairman left behind. I was about to toss it in the trash, when I realized that it contained a glass bulb filled with mercury. I had done enough reading to know that I had enough toxic metal in my hand to contaminate a large lake, rendering its fish unsafe to eat for years, if not many decades, to come. The challenge would be to safely and responsibly dispose of it. Unfortunately, thermostats are just one example of the scores of mercury-containing products found in a typical American home. Few bear any indication of their status as hazardous materials. Some contain enough mercury to pose an immediate danger if they break and release toxic vapors. All of them present homeowners with the same challenge I faced: How do you dispose of these items safely so the mercury doesn't get into the environment and return in a more dangerous form inside a can of tuna or bite of sushi?

As for the food products containing mercury that are already in our pantries and refrigerators, consumers today have good reason to think hard about what kinds of fish they're feeding their family, and in what quantity (see "Dietary Cleanup" box). "We need to look at the mercury problem through two lenses," explains Felice Stadler, national policy coordinator for NWF's Clean the Rain Campaign. "The first lens concerns what we can do to minimize our exposure today. The other lens concerns identifying sources of mercury so we can minimize environmental contamination and reduce exposure through tomorrow's food supply."

Though mercury occurs naturally in the environment-in rocks, soil, oceans and air-only in modern times has it become a pervasive and pernicious threat to health. Indeed, scientific research indicates that the rate at which mercury enters our food chain has increased considerably in recent decades.

What makes this dangerous is mercury's extreme neurotoxicity. That is, its devastating effects on the brain and nervous system. The "mad hatters" of Victorian times suffered from mercury poisoning when they rubbed the metal into felt cloth to preserve it. Mental confusion, trembling and eventually death can result from inhaling the vapors of this liquid metal, which readily evaporates at room temperature. As the dangers of mercury exposure became known in the early 20th century, acute poisonings became rare. But they still occur, as when children find and play with mercury or when someone eats large quantities of contaminated fish over a relatively short period of time.

Of far greater concern today is the harm that is done to developing fetuses, infants and young children exposed to even low levels of methylmercury. A recent report by the National Academy of Sciences concluded that the amount of methylmercury (an especially problematic form) consumed by pregnant women in this country results in at least 60,000 children born at risk of developmental harm. The risk of permanent injury continues for at least the first five years of life, as the brain and nervous system continue to develop and children consume mercury through breast milk and food.

The vast majority of methylmercury in our diet turns up in large, commercially caught fish, such as tuna and swordfish, and in smaller species pulled from contaminated waters by recreational anglers. Scientists have traced the source of the contamination primarily to coal-burning power plants, certain industrial processes and the incineration of mercury-containing products.

Once mercury enters the planet's water cycle as a pollutant, bacteria convert it into methylmercury, which bioaccumulates (or concentrates) in the tissues of organisms that consume it. So it occurs at greater concentrations as it moves up the food chain. In top predators such as tuna and swordfish, methylmercury can be present at concentrations 10,000 to 100,000 times that found in surrounding waters.

The good news is that mercury-free alternatives now exist for many of the metal's uses in household and automotive products. This has encouraged a handful of states to bar the sale of easily replaced mercury devices such as thermometers and switches. Only two, Vermont and Minnesota, require manufacturers to label mercury-added devices. Until such labeling requirements become widespread, ferreting out mercury in the home takes some detective work. Understanding the metal's useful properties can help you recognize where it's likely to be found.

Thermometers and thermostats: Elemental mercury expands and contracts evenly with changes in temperature. So it has long been used in glass thermometers and the mercury-bulb switches in thermostats. Mercury switches can likewise be found inside heating elements that shut off when they reach a set temperature-such as those inside clothing irons and older microwave ovens. The most easily recognized nonmercury alternatives have digital displays-an indication of electronic sensors.

Vapor lamps: Fluorescent, neon and high-intensity-discharge (HID) light bulbs contain mercury vapor. Fluorescents more than make up for their added mercury by conserving energy and so reducing mercury emissions from coal power plants. Until mercury-free alternatives are available, the best options are low-mercury bulbs as long as they are disposed of properly; they are recognized by their green end caps.

Batteries: Over the last 20 years, U.S. manufacturers have reduced the amount of mercury in household batteries by 99 percent. Small amounts can still be found in "button" batteries, such as those used in watches and hearing aids; as a result, they should be properly disposed.

Barometers and blood-pressure gauges: Mercury expands and contracts with pressure. So it is used in conventional barometers and blood-pressure gauges. Indeed, a typical home blood-pressure device contains a whopping 1.5 pounds! Mercury-free alternatives include "Bourdon tube" barometers and "aneroid" blood-pressure gauges.

Old pesticides and latex paint: Mercury's toxicity makes it an effective preservative and pesticide. Fungicides and herbicides produced before 1994 and latex paint made before 1992 release significant amounts of mercury vapor during application. Though the sale of such items is now phased out, countless half-used containers remain in America's garages and basements.

Toys and novelties: Old children's chemistry sets (1960s vintage or earlier) often contained vials of liquid mercury. Until recently, athletic shoe manufacturers used mercury in the blinking heels of light-up sneakers. Toy importers still occasionally sell novelties that contain a drop of mercury that rolls through a maze.

Tilt switches: Mercury conducts electricity and flows when you tilt it. So it is used in switches that stop motors or turn on lights when you open a lid. Think washing machines, top-loading freezers, car hoods and trunks. Alternatives include electronic sensors and nonmercury mechanical switches (check with the manufacturer). In 1995, the International Automobile Manufacturers Association announced that it had completely eliminated mercury switches from foreign vehicles. U.S. carmakers pledged to do the same by 1997, but have been slow to fulfill their promise, says Clean Car Campaign spokesman Dean Menke.

More mercury in cars: Contrary to their pledge, U.S. automakers have actually expanded the use of mercury in cars, says Menke, who calculated more than 10,000 pounds of mercury in model-year 2000 passenger vehicles. This poses a colossal problem as neither manufacturers nor car recyclers want to take responsibility for safe disposal. Until they do, Menke advises car shoppers to ask for evidence that light switches, headlamps, antilock brakes, convenience lighting and active ride-control systems have been manufactured without mercury.

Disposal: Mercury sealed inside solid devices poses no immediate danger to the user. The key is proper disposal at the end of the product's useful life, followed by replacement with mercury-free alternatives when possible. (The Clean Car Campaign's national "Switch-the-Switch" exchange program, for example, enables vehicle owners to exchange mercury for nonmercury light switches. More information can be found at

Mark as "containing mercury" any device you suspect may contain the metal. Then contact your local solid waste department to determine your disposal options: Many communities have disposal sites or neighborhood pickup dates. Collected mercury in most cases is sent to recycling facilities where it is then reused in new mercury products. Environmental advocates are working on national policies to get mercury used in products or in manufacturing completely out of circulation.

Should a mercury spill occur in your home, don't panic. Elemental mercury does not readily absorb through the skin or even the digestive tract, says Lynn Goldman of the Johns Hopkins Bloomberg School of Public Health. The health danger is vapor inhalation. So in the case of a small spill-say, a broken thermometer or thermostat bulb-move children out of the room and open doors and windows to ventilate. Never try to sweep or vacuum mercury. Doing so can contaminate an entire building by scattering toxic particles through the air. Use adhesive tape or an eyedropper to collect small amounts and seal them in a plastic container. Dispose of contaminated clothing, carpeting and upholstery in sealed plastic bags. Place contaminated materials outside and inaccessible to children and animals, until they can be disposed as hazardous waste. In the case of a large spill, such as that from a blood-pressure gauge, leave the area immediately, notify your local public health agency and call your physician for possible treatment.

NWF recently published a guide to products that contain mercury, alternatives available and local actions being taken to remove mercury products from the marketplace. For copies, contact Kathleen Eales at 734-769-3351,, or see New Jersey writer Jessica Snyder Sachs is a regular contributor to this magazine.

Dietary Cleanup
Sadly, what should be one of the most healthful foods in the American diet has become potentially one of the most dangerous. Persistently high mercury levels in streams, lakes and oceans render many kinds of fish unsafe for children and women of childbearing age. Currently, 43 states have advisories against eating some or all fish caught from local waters. But don't count on finding the warnings publicized or posted. Check with your state environmental agency before eating any recreationally caught fish, and check for new advisories each season.

As for fish from the grocery store, in 2001 the Food and Drug Administration (FDA) advised that young children and women of childbearing age not eat shark, swordfish, mackerel and tilefish because such fish were found with levels of methylmercury greater than 1 part per million (ppm). That the FDA did not warn against tuna proved controversial. Though most tuna falls below the 1-ppm mercury limit, Americans tend to eat more of it.

Methylmercury studies by the EPA suggest that levels as low as 0.25 ppm may be unsafe for consumption. In recent tests by Consumers Union, canned tuna averaged about 0.31 ppm; light tuna averaged about 0.16 ppm. Based on these test results and EPA guidelines, a 132-pound woman should eat no more than 9 ounces of light tuna or 5 ounces of white tuna a week. A 44-pound child should eat no more than a third that amount. (This assumes that no other fish is consumed by the child during that time.) The FDA recommends eating no more than 12 ounces of cooked fish per week. Far less mercury turns up in smaller commercially caught fish such as flounder, haddock, butterfish, herring and sardines.

Mercury in Medicine
Medical studies have not documented any harm from the mercury used in amalgam dental fillings, but a problem nevertheless exists with disposal of mercury wastes by dentists. Currently, no federal requirements mandate safe disposal techniques. Research also has not uncovered problems resulting from the trace amounts of mercury preservatives (thimerosal, phenylmercuric acetate and phenylmercuric nitrate) used in some vaccines, nasal sprays and contact lens solutions. Importantly, thimerosal contains a form of mercury that does not bioaccumulate in the body. In any case, mercury-free options exist for all of these products. An increasing number of physicians and dentists recommend using these alternatives. (Ask your physician before having your children vaccinated.)

Jessica Snyder Sachs, a regular contributor to National Wildlife magazine, is the author of Good Germs, Bad Germs: Health and Survival in a Bacterial World (Hill&Wang/FSG) and Corpse: Nature, Forensics, and the Struggle to Pinpoint Time of Death (Perseus/Basic Books).

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About this Archive

This page is a archive of entries in the National Wildlife category from December 2007.

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