Zoobiquity

  If Sam the koala had been born a few years later, she might have benefited from the kindness of not just the firefighter but also a biologist named Peter Timms. Timms, along with his colleagues at the Queensland University of Technology, has been developing a vaccine for koala chlamydia. Early trials of the vaccine have cut infection rates slightly and blunted the virulence of the disease. Timms hopes his research will someday not only save koalas but also inform a human chlamydia vaccine.

  It’s hard to imagine anyone in Australia objecting to vaccinating their national symbol against a disease that causes blindness, infertility, and death. It hardly seems the koalas’ fault that the disease that’s wiping them out happens to spread via sex. But the development of human vaccines against STDs from chlamydia to HPV to HIV has been hindered by some groups that believe offering protection against these diseases is the same thing as actively encouraging the “immoral behavior” that spreads them.

  But here’s where a zoobiquitous perspective helps. Looking at these diseases in animals allows us to see infection as infection—independent of the route of introduction. While thinking about a human with chlamydia may make us grimace or blush, koalas with chlamydia likely make us feel sympathetic, or at least impassive. Most of us don’t judge the koala for its sexuality. Decreasing the stigma of STDs can improve treatment.

  An evolutionary approach could inspire clinical solutions. As we’ve seen, studying the history of infections may give epidemiologists a head start in identifying those bugs that are getting ready to jump transmission pathways. Maybe there are “good” microorganisms that spread sexually and maintain genital health, the way certain “good” microorganisms maintain intestinal health.

  Finally, studying STDs in animals can enlighten us in ways that extend beyond our consideration of the illness, infertility, and death these pathogens can cause. Sexually acquired infections have played an oceanic, though microscopic, role in evolutionary biology. Sam the koala may have succumbed to chlamydia, but not all her sexual partners met with the same fate. In fact, despite their unprotected sexual free-for-all, a small percentage of koalas never became infected. Something enabled them to resist the infection … and that something is genetic variation. Every time egg and sperm meet, a new and unique combination of genetic material is created. Every once in a while, the mix is such that the creature that possesses it gains an infection-resistant advantage. This is why, although HIV is complex and deadly to most humans, infectious disease researchers have discovered that about 1 percent of humans (primarily Swedes) seem to be immune to it.‖

  In populations of clones—those with identical genetics—a single species of virus, bacterium, fungus, or worm can wipe out the whole group. But when individuals within a group each possess a slightly different genetic makeup, chances increase dramatically that some will survive. And nothing provides diversity as predictably and effectively as one particular act: sexual reproduction.

  And herein lies a central irony with insights for evolutionary biologists, infectious disease specialists, and sexually active humans. Today we protect ourselves against sex. But over the course of evolution, it has been sex itself that protected us.

  *Technically, the disease affecting the koalas is Chlamydophila (usually C. pneumoniae or C. pecorum). The genome of Chlamydophila is slightly larger than that of the closely related genus Chlamydia, from which it has been taxonomically split off. While acknowledging this difference, I will use the term “chlamydia” to describe the koalas’ infection, as the veterinarians do. Similarly, while “the clap” is a specific and historic reference to Neisseria gonorrhoeae, it is used colloquially to refer to STDs in general.

  †Birds have a combined reproductive and excretory opening called a cloaca.

  ‡Cloacal pecking may aid sperm competition in birds such as dunnocks, whose pre-copulatory displays include beak stimulation that induces females to eject sperm from previous partners.

  §For a lively account of sperm competition tactics across species (the ultimate existential battle), see Matt Ridley’s fascinating The Red Queen.

  ‖A dramatic recent example of genetic resistance to HIV infection occurred when an American man with AIDS developed leukemia while living in Germany. The bone marrow transplant the “Berlin patient” was given for his leukemia came from a donor with a mutation in the genes coding for the CCR5 molecule. The AIDS virus uses CCR5, which usually sits on the surface of the cell, as a “door” to enter and infect the cell. If CCR5 is faulty (there is a mutation), the virus cannot enter. An individual with this mutation is essentially immune to HIV infection. This genetic defect is seen primarily in individuals with European heritage. An estimated 1 percent of people descended from northern Europeans are virtually immune to AIDS infection, with Swedes the most likely to be protected. One theory suggests that the mutation developed in Scandinavia and moved southward with Viking raiders.

  ELEVEN

  Leaving the Nest

  Animal Adolescence and the Risky Business of Growing Up

  A bend in the central California coastline shelters a stretch of soft, white sand ideal for a family day at the beach. The waves shimmer. The sun warms. Kite-perfect, brine-scented breezes gust over sandy dunes, keeping aloft strings of seabirds that glide effortlessly over the gentle breakers.

  Go ahead. Slather zinc oxide on the kids. Force them into their swim shirts. Remind them to stay within eyeshot. But before they race, Boogieboards bouncing behind them, into the water, I should warn you about one thing. Just a few miles out, stretching south of San Francisco toward the Farallon Islands, is a place sea otter researchers call the Triangle of Death.

  Great white sharks prowl the chilly waters. Sneaker waves, riptides, and treacherous undertows sweep the shores. The barren seafloor cannot support plant growth, so it’s devoid of the sheltering, protective kelp forests found in other coastal regions farther south and north. The depths here teem with higher-than-normal levels of Toxoplasmosis gondii, the feared, infection-causing microbe found in some cat feces and uncooked meat.

  You will not spot female sea otters in this dangerous location. Otter pups don’t go there, either. Dominant, mature males seem to know better than to venture into these dangerous waters and rarely do. Even scuba divers, hired by the U.S. Geological Survey to radio-track sea otter movements, refuse to submerge themselves in this perilous location.

  But one intrepid type of otter makes frequent forays into the Triangle of Death—even though shark attacks and unexplained disappearances occur commonly here. They’re adolescent males, the daredevils of the otter world.

  The concept of animal adolescence may surprise you, as it did me. Of course we’ve all seen gangly young dogs just out of puppyhood who haven’t quite matched their oversized paws to their less-advanced motor skills. But the drama, awkwardness, and peril of teen life seems unique to our species. And, indeed, it probably is one of a kind, if you associate adolescence with teenagers’ matchless ability to wound their parents with a strategic eyeroll or to ruin the family photo with a moody slouch. But while the details may differ, a larger truth ties human teens to the vast majority of other species. They all must pass through a fraught transition: the period between leaving the care of adults and becoming adults themselves.*

  We often call adolescence the teenage years, for the obvious reason that the transition roughly corresponds with that segment of a human life span. In other animals, the gradual shift from child to adult can last anywhere from about a week for a housefly to fifteen years for an elephant. For zebra finches, it lasts about two months, starting forty days after they hatch. In vervet monkeys, the journey from their mother’s side to motherhood (or fatherhood) happens over four years. Even lowly, single-celled paramecia have an adolescent phase—a don’t-blink-or-you’ll-miss-it fifteen to twenty-four hours in which their cell nucleus and plasm change as well as, believe it or not, their behavior.

  We human doctors have dealt with the unique and vexing trials of this period the sam
e way we have with especially complicated organs or diseases—by creating a new specialty. “Adolescent medicine” caters to an in-between population: patients who’ve outgrown their pediatricians but aren’t quite ready for an internist. It addresses the hormonal shifts of puberty and the physical challenges of emerging sexuality. Practitioners in this nascent field work vigilantly to keep at bay a chilling list of threats to young humans: traffic accidents, STDs, alcohol and drug abuse, traumatic injury, teen pregnancy, date rape, depression, and suicide. Much of what we associate with adolescence involves behavioral changes, and lately research has focused on brain changes that help explain those behaviors—like risk taking, sensation seeking, and the somewhat perplexing compulsion to fit in with a group.

  Of course, all animals have different things to learn while traversing the arc that takes them from sexually immature, vulnerable child to reproductively capable, developed adult. In our case, those include advanced language skills and critical thinking. But there’s one feature that defines adolescence in species from condors to capuchin monkeys to college freshmen. It’s a time when they learn by taking risks and sometimes making mistakes.

  A surprising and sad fact of life is that just being a human teenager—especially a boy—is very risky, and often deadly. In the United States, once children have survived infancy and early toddlerhood, most will enjoy a brief period of relative safety, until they hit age thirteen.† At that moment, however, the death risk climbs abruptly, mostly because of traumatic injury. The Centers for Disease Control and Prevention (CDC) reports that “among teenagers 12–19, death rates increase with every year of age. This pattern is stronger for males.” At about age twenty-five, rates of fatal injuries seen so commonly in adolescents taper off. In adult years, cancer, heart problems, and other long-term diseases emerge as the main health risks.

  These stark statistics parallel death trends in the animal world. “Young [animals] suffer higher rates of predator-induced mortality than adults,” according to Tim Caro, a UC Davis biologist and author of Antipredator Defenses in Birds and Mammals. Risk tapers off as the infant survives early challenges. But as animals’ bodies grow in anticipation of transition, so do the dangers. Consider an adolescent warthog foraging for the first time without the protection of his mother. Because he lacks his full complement of defensive horns and thick hair and doesn’t yet have the adult stamina to outrun a predator, his odds of survival would be low if a cheetah came upon him. Since they can’t run as fast, fly as high, or otherwise outmaneuver threats as skillfully as adults, young animals fall to predators more often. Less experienced, they misjudge situations and blunder into danger.

  Of course, on the whole, modern human teens aren’t being picked off by mountain lions or the other hungry predators that threatened our distant ancestors. What kills adolescents disproportionately in many countries around the world is a different lethal presence: motor vehicles. The CDC reports that 35 percent of deaths in the twelve-to-nineteen age group in the United States come from traffic accidents.

  Other sudden, violent causes of death threaten teens, too. According to the World Health Organization, interpersonal violence claims the lives of hundreds of ten- to twenty-four-year-olds every day. And gun accidents, suicide, homicide, drowning, burns, falls, and warfare are also leading killers of adolescent humans worldwide.‡

  Adults recognize this behavior so well that it’s enshrined in both law and supposedly forward-thinking parenting strategies. It’s why it’s harder to rent a car before you’re twenty-five and why auto insurance rates are highest for adolescents. Why we set drinking ages and driving ages. Some states and locales dictate how many teens can be in a car at one time. New Jersey forbids all teens, not just the driver, to use electronic devices. And a scarlet rectangle must adorn their license plates, marking them as younger drivers.

  Some parents prefer to take safety into their own hands, setting curfews and stocking living rooms with teen bait—gaming consoles, junk food, even alcohol. “If he’s going to drink, I’d rather he do it safely in his own home,” the thinking goes.

  And then there’s “choice.” A core tenet of teen risk-intervention strategies focuses on teaching teens to make “smart choices.” But extensive, new neurological research shows that risk taking at this age isn’t really a “choice.” Profound changes deep in the adolescent brain allow impulsive action to override prudent inhibition. Transitioning teens thrill to novelty. They’re drawn to groups of peers. More than adults, they search for ways to stimulate their senses. Their emotional responses are more extreme.

  Rest assured that if adolescent rats drove cars, they, too, would have exorbitant insurance rates. Researchers from Rome’s Istituto Superiore di Sanità ran a mixed-age group of rats in a maze that ended with a tasty treat. To reach the reward, the rats had to scurry across a narrow plank, suspended high above an open space, with no protective side walls.

  Half the rodents flat-out refused to enter that section of the maze. However, of the half that dared, every single one was an adolescent. No babies or elders took the risk.

  Adolescent rats display several other common behaviors. When placed in new surroundings, they have lower base levels of anxiety compared to other age groups. They are more impulsive about approaching unfamiliar objects. Novelty doesn’t just interest them. It attracts them. They seek it out.

  Similarly, when primatologists place unfamiliar items near vervet monkeys, adolescents are the fastest to rush over to investigate. Whether the objects are simply neutral, such as a cardboard box, or unusual but unthreatening, like a tree covered in lights and tinsel, or carry a degree of danger, like a fake tarantula or stuffed snake, adolescents are the ones that eagerly approach, gesture, call out alarms, and try to touch.

  Even at great personal risk, adolescent animals seem almost to delight in exploring new things. Preadult zebra finches will approach humans, even sit on an offered finger, when the adults have long since fled. Transitioning sea otters begin venturing into new territories, like the Triangle of Death. Animal behaviorists and human neurologists agree that this sudden lowering of the fear threshold in humans and nonhuman animals stems from specific brain changes.

  Said another way: risk taking is normal.

  And not only is it normal; it’s necessary and serves very specific purposes. For example, to survive on their own, animals need to know how to recognize predators. While the ability to spot threats is to some extent inborn, some of it must be learned during adolescence. For animals, Sun-tzu’s classic military advice to “know your enemy” includes studying how he smells, hides, runs, and attacks. And one important way to gain this knowledge is to get close enough to see him in action.

  One seemingly suicidal, but actually very effective, way of learning about predators is to gallop, swim, or fly right up to them … and live to tell the tale. As Tim Caro, the U.C. Davis biologist, writes in Antipredator Defenses in Birds and Mammals, “Young animals may approach and inspect a predator when they see it and perhaps learn about its characteristics, including its motivation and behavior.”

  For example, instead of hiding from prowling cheetahs and lions, immature Thomson’s gazelles often stroll right toward them. Sometimes the young gazelles will trail their hunters for an hour or more, as though the big cats, not the small gazelles, are the quarry. Astonishingly, the discomfited predators frequently slink away, but not before the adolescent gazelles have gotten a good eyeful and whiff of the creature that someday might try to kill them. This practice, however, comes with casualties. According to a Cambridge University study carried out in Tanzania, the curious young gazelles die from feline fangs once in every 417 approaches (compared to one in 5,000 for adult animals). What animal behaviorists call “predator inspection” is seen widely in guppies, gulls, and other fish and birds. While predator inspection often continues into adulthood, the learning begins in animal adolescence—when inexperience also makes it more dangerous.

  Luckily for animals, humans aren�
�t the only species whose adults will show the younger generation the ropes. A widespread teaching technique used in species from birds and fish to mammals is something researchers call “mobbing.” Moving together in one big group while vocalizing threateningly, a whole group of animals, including both experienced adults and developing adolescents, can intimidate a hunter into seeking its meal elsewhere. Mobbing is an effective antipredation strategy. But, as U.C. Davis animal behavior expert Judy Stamps pointed out to me, it has another crucial, though often overlooked, function.

  “Mobbing is a way to impress upon the whole community that something dangerous is near,” she told me. “If the whole group is making a huge racket, that helps younger animals learn the community’s predators.” Mobbing, she continued, is also safer than solo inspection. Young animals, she said, “aren’t very good at evading predators.” Moving toward danger in the protection of an adult-led mob provides youngsters with a safe and educational close-up view.

  When I was in high school, I went through a quintessentially American rite of passage: I took driver’s ed. Several decades later, the physical skills of steering, scanning the road, and signaling have become so engrained in my muscle memory, I really don’t even remember learning them. But one part of driver’s ed remains seared into my mind. Along with generations of neophyte drivers in California, I was made to watch a film called Red Asphalt. Produced by the California Highway Patrol, the movie shows scene after gory scene of traffic accidents. Blood rushes down gutters. Bodies lie akimbo under cars. Motorcyclists’ limbs are smeared across the pavement. Drivers who spent their teen years outside California might remember being traumatized by other pieces of edu-propaganda, such as The Last Prom, which featured a crushed and bloody corsage on the side of the road, or videos with cautionary titles like Wheels of Tragedy, Mechanized Death, and Highways of Agony.