The Thing with Feathers (8 page)

It was time for Bachman to shut down his backyard vulture studies. He feared that the vultures “might become offensive to the neighbors.” Like Audubon and Darwin, his early vulture experiments spurred him on to greatness: He later founded Newberry College, tended dying soldiers during the Civil War (and had an arm permanently paralyzed by Sherman’s army),
published Audubon’s folio of American mammals, and dedicated his life to Lutheran ministry, living to the ripe old age of eighty-four—no doubt tickled to the end by his memories of that gruesome skinned-sheep painting in his backyard garden.

—

WORLDWIDE, THERE ARE TWENTY-THREE
different species of vultures, divided into two main groups. Old World and New World vultures are quite distinct, but similar in appearance and habits—an example of convergent evolution. Like bird wings and bat wings, they developed independently to accomplish the same function.

Seven vultures live in the New World: Andean and California condors, respectively of South and North America; the clownlike king vulture of South and Central American rainforests; lesser and greater yellow-headed vultures of South America; and the widely distributed black and turkey vultures so familiar to North Americans. Black vultures keep to the eastern half of the continent, while turkey vultures range all over the map, from southern Canada to the tip of Chile and the Falkland Islands.

Of these, the turkey vulture is best known and most reviled. It is a common bird even in areas inhabited by humans, often seen circling overhead as if waiting for something to die (the spiraling flight actually helps them gain altitude in narrow columns of rising air). Audubon, for one, appreciated their movements: “The flight of the Turkey Buzzard is graceful . . . It sails admirably either high or low, with its wings spread beyond the horizontal position, and their tips bent upward by the weight of the body.” Close up, turkey vultures are distinctive. They get their name from a superficial resemblance to that familiar dinner-table animal—with bare heads and black bodies—but,
to my eye, the vulture is more arresting than any farm fowl. Perhaps it takes an unusually sympathetic view to detect beauty in a vulture, but I think they look crisp and clean. The turkey vulture’s scientific name,
Cathartes aura
, literally translates to “purifying breeze”—more household freshener than landfill scavenger.

It’s an apt scientific name, as turkey vultures serve the unenviable task of cleaning up our world. Their dirty reputation is undeserved—rather than spreading germs, they gobble them up. To be able to eat animals that have often perished of disease, together with the attendant microbes of rotting flesh, turkey vultures have stomachs of cast iron.

Vulture digestion has lately caught the interest of medical professionals for exactly this reason. Vulture excrement is, amazingly, completely sterile. One delectable fact of life for vultures is that they habitually defecate down their own legs, which serves two practical benefits: It cools them off through evaporation (vultures can’t sweat), and the feces help sterilize the bird’s legs, which have often just been dragged through bacteria-filled carcasses. Recent evidence indicates that vulture stomachs can process and sterilize anthrax spores without ill effect. They can also eat botulism-infected carcasses and kill the bacteria while their immune systems deal with associated toxins. It’s even possible that vultures can disinfect dead rodents containing hantavirus—the virus goes in, but it doesn’t come out. The best way that humans have found to deactivate hantavirus, by contrast, is to either soak it in chemical detergent or blast it with at least 115 degrees of heat. If we could figure out exactly how these scavengers handle such serious infections and poisons, perhaps we might find a way to apply that knowledge to humans—with major implications for preventing biological warfare and epidemics. The answer to why vultures have this
ability to cleanse and prevent disease probably lies in their remarkably efficient digestion and immune systems, and in the fact that what makes one species sick will not necessarily affect another. Vultures have evolved to survive—thrive, even—on things that would kill us (and many other animals).

All vultures share this gift of a strong stomach, but different vulture species may rely on different methods to find their food—and these species’ differences may help explain why the early vulture experiments seemed to provide mixed results. It would be up to later scientists to get to the bottom of this pungent mystery.

—

ON MARCH 18, 1937,
a school building in the oil community of New London, Texas, exploded without warning while classes were in session. Witnesses stated that the walls bulged and that the school’s roof lifted before everything crashed down, killing more than 295 students and teachers—ranking the disaster today as the third deadliest in Texas history (after a 1900 hurricane and a 1947 ship explosion). Further investigation showed that the school had used piped-in natural gas for heating, and that a leak had filled the building with a deadly, odorless mixture. When an instructor flipped on an electric sander at about three in the afternoon, the tiny spark caused the whole building to blow.

Within weeks, new legislation decreed that all natural gas in Texas should be mixed with low concentrations of malodorants—extremely smelly chemicals, such as mercaptans, that immediately alert anyone to the presence of gas. It was an easy solution; any leak could then be detected by the human nose with plenty of warning. The practice of adding smelly chemicals to natural gas lines quickly spread worldwide.
Although the idea had been around for a while, it took a major disaster to precipitate its implementation.

Mercaptans are a class of organic compounds known mostly for their foul smell. They’re toxic in big doses but are typically found in harmless dilution. One particular arrangement called ethyl mercaptan was once named the “smelliest substance” in existence by
The Guinness Book of World Records
; the human nose can detect it at concentrations of less than one part per billion—about a thousand times lower than the threshold for sulfur dioxide (the penetrating smell of pollution and volcanic gas). Mercaptans help give cooked cabbage, onions, flatulence, cheese, bad breath, and feces their fragrant bouquet. Also present in animal blood and brains, mercaptans are released as a carcass decays, helping to give corpses their smell.

It wasn’t long before workers at the Union Oil Company noticed a peculiar side effect of the scented gas. Whenever one of their remote lines sprang a leak, a group of turkey vultures would soon collect overhead, evidently detecting the mercaptans percolating in the atmosphere. The workers began looking for circling vultures when tracking down a leak, a technique that is apparently still used today.

This bit of information went unnoticed for several decades by those studying turkey vultures, who tended to have their own noses buried in labs nowhere near remote gas lines. Though more than a hundred years had passed since Audubon and Bachman conducted their classic experiments, it was an open question as to whether vultures discover their food by sight or smell. Stories kept popping up about turkey vultures following their noses. They’d been seen investigating mushrooms and flowers that gave off strong, carrion-like fragrances. Also, a couple of anatomical studies showed that vultures probably had the capacity to smell out their food, even if they
didn’t use it. One investigation of 108 bird species found that, of the ten birds with the largest olfactory bulbs relative to their own brain size, nine were seabirds (also known for possessing a keen sense of smell). The other was the turkey vulture, which ranked eighth overall.

In the 1960s, Kenneth E. Stager, then the senior curator of ornithology at the Natural History Museum of Los Angeles County, happened to be talking to a Union Oil worker, who relayed the company wisdom about turkey vultures collecting over gas leaks. Despite his interest in vultures, Stager had never heard of any such thing, and the story stuck in his head. When he decided to tackle the question of vulture smell for his Ph.D. thesis, rerunning some of Audubon’s experiments along with other tests, he realized that mercaptans—a common denominator of natural gas and carrion—could be a key to the whole puzzle.

Stager was a hands-on, maverick field ornithologist who had literally proved himself under fire. As a foot soldier in the jungles of Burma during World War II, he once came across a couple of ethnic Kachin people who had just killed a beautiful silver pheasant. Stager struck an on-the-spot deal for the bird’s skin and sat down to strip out its meat, but before he could finish, Japanese shells began landing in camp—so the enterprising ornithologist jumped in a trench to finish preparing the specimen. This incident led to his quick transfer to a division studying scrub typhus disease, which in some areas of Southeast Asia was killing five times as many troops as weapons were, and he spent the rest of the war happily collecting bird and mammal specimens in remote parts of China. That silver pheasant is still displayed in the Natural History Museum, and Stager went on to participate in scientific bird-collecting expeditions to Australia, Brazil, Africa, India, Mexico, and various
far-flung islands, adding considerably to our current knowledge of the birdlife of those areas.

Inspired to study vultures at home in California, Stager brought his usual straightforward intensity and commitment to the project. He made an exhaustive literature search on vulture smell, still the most encompassing review anyone has ever published on the subject. Then he designed experiments to demonstrate exactly where previous studies had gone wrong.

First, Stager repeated Audubon’s basic test of concealing a carcass to see whether turkey vultures would find it. He baited cage traps with carrion and hid them, always out of sight of the sky. When Stager checked his traps, he’d caught some vultures—which must have followed the reek of decay to discover the bait.

Then he hid a carcass and watched where vultures appeared in the sky. The birds tended to fly low, as you might expect if they were searching for ground scents, and usually approached the bait from downwind. He figured, by crude visual estimation, that they could detect the hidden carcass from at least a couple hundred yards away.

But Stager’s most interesting test involved the odor of natural gas spiked with mercaptans. Spurred by the stories of oil workers, he devised an ingenious experiment to attract turkey vultures with scent alone. The results were clear: When Stager puffed pure foul-smelling gas across the hills of California, vultures materialized and circled overhead. Interestingly, the shy birds wouldn’t land without a decoy carcass placed nearby. Stager reported that in his odor-only trials, turkey vultures would circle for about twenty minutes before wandering away, which suggested their need for visual confirmation of the source of the smell. He concluded that turkey vultures are able
to use their noses to find food, even if the final approach is often visual.

Stager and many others have accordingly speculated about flaws in Audubon’s and Bachman’s early experiments. Audubon conducted many of his tests with black vultures, and the current wisdom is that only turkey vultures possess a good sense of smell—black vultures don’t (they often follow turkey vultures to find carcasses). Audubon may have misinterpreted his results by assuming that all vultures act the same. Also, he seemed to believe that vultures like their meat rotten, which is definitely not the case—we now know turkey vultures prefer fresh carcasses—so perhaps the birds smelled Audubon’s hidden corpses but just didn’t want to eat them. And when Bachman covered his pile of garden offal with canvas to see whether the vultures could find it, he didn’t factor in how weak the birds’ beaks and claws are; they might have sniffed the food but couldn’t reach it through the thick canvas.

Modern experiments have added substantial evidence to support the idea that turkey vultures possess an extraordinary sense of smell. In one study, a researcher painstakingly set out seventy-four chicken carcasses in the understory of a Panamanian rainforest (he purchased freshly slaughtered, fully feathered chickens from a Panama City market, then hustled out to the jungle before they could rot) and monitored how long it took for turkey vultures to discover each one. They found seventy-one of the dead chickens within several days, with no measurable difference between those left on the open forest floor and those covered loosely with leaves. A curious finding was that the birds were more attracted to two-day-old than either one- or four-day-old carcasses. The researcher hypothesized that his chickens weren’t ripe enough the first day—not
emitting enough gases, including mercaptans, to be smelled from a distance—but that by the fourth day they were too putrid for even a turkey vulture’s taste.

If turkey vultures smell their way to food, we still don’t know exactly what they sniff that attracts them to a decaying carcass. Two scientists at Humboldt State University tested turkey vultures’ reactions to mercaptans to see whether the Union Oil Company workers’ story could be verified experimentally—and whether, as Ken Stager once hypothesized, mercaptans are the key to the whole puzzle. Vultures get excited when they smell food, so the researchers exposed captive vultures to increasing concentrations of the gas while measuring the birds’ heart rates; when the heart rate spiked, a threshold had been reached. The lab test, somewhat disappointingly, showed that vultures tended to react only when mercaptan concentrations reached about one part per million—impressive for a bird, but not even close to our own ability to detect the same chemical at one part per
billion
.

When the scientists then modeled that concentration of gas dispersing from a single point, they calculated that a turkey vulture would have to fly within seventeen centimeters of the ground to smell a decaying carcass. Either the calculations were incorrect (as some critics have suggested) or turkey vultures are sniffing something else. Mercaptan isn’t the only gas released from decaying carcasses; the same study also tried butyric acid (the odor emitted by rancid butter) and trimethylamine (a fishy odor), with similar results. Rotting meat gives off a whole suite of delectable compounds, including such aptly named chemicals as cadaverine and putrescine. Which of these are most inhaled—and enjoyed—by turkey vultures, nobody has any idea.