Why Do Animals Do That?
The fertile field of ethology never runs short of surprising discoveries; here are just a few of our recent favorites
- Jessica Snyder Sachs
- Oct 01, 2005
JAMES HARE stands in a grassy Midwestern pasture, tossing a tan fedora over the head of a female gopher (a.k.a. Richardson's ground squirrel) as she emerges from her burrow. Hare, a zoologist with the University of Manitoba, knows that the swooping hat will trigger the gopher's instinctive alarm response. She flicks her tail, opens her mouth and heaves her chest as if to give the sharp chirrup that announces "Hawk!" to the surrounding colony. Instead, she appears struck dumb, nothing but a faint whisper of air rushing from her gaping mouth.
The first time Hare observed this behavior several years ago, he came away puzzled. His fascination grew as he noticed other gophers opening their mouths as if to bark a warning, only to exhale with a barely audible call. An outbreak of ground squirrel laryngitis, perhaps? Hare's early, lost-voice theory crumbled when closer study revealed gophers switching back and forth between their usual alarm calls and the strange, open-mouthed whisper.
Like Hare, scholars as far back as the ancient Egyptians and Greeks have puzzled over mysteries like these. Today the study of animal behavior, or ethology, is still one of biology's most productive fields, with scientific journals brimming with reports on everything from the selection of gender-specific parental roles in cichlid fish to the importance of courtship song in fruit flies (seriously).
While much of this scholarly minutiae holds little draw beyond specialist circles, every year scientists make discoveries that overturn widespread assumptions or reveal previously unknown animal abilities. Here are just few of the more recent noteworthy finds.
SILENT SCREAMS AND JUNGLE RUMBLES
Determined to solve the mystery of the whispering gophers, Hare returned to his university, where he borrowed an ultrasonic "bat detector." Such devices lower high-frequency sounds--such as a bat's echolocation shrieks--into human range by slowing down their pitch. Back out on the prairie, he pointed the device at emerging gophers as they reacted with visible but noiseless alarm to his presence. Sure enough, these were screams--at frequencies around 48,000 hertz. By contrast, the limits of human hearing reach only half as high, around 20,000 hertz. Hare suspected that he had discovered a "silent" alarm call.
To test the idea, graduate student David Wilson recorded silent screams from over a dozen gophers. He and Hare then replayed the recordings for gophers miles away. The ultrasonic cries produced much the same reactions as did recordings of audible alarms, with one clear difference. The ultrasonic recordings triggered alarm only in gophers close to and directly in front of the speakers.
The researchers' conclusion: While the gopher's silent alarm does not travel as far or wide as does the species' familiar warning bark, a gopher can direct it, with laser-beam accuracy, to a nearby pup or other close relative, all without endangering itself by drawing attention from a predator. "This sort of thing may be far more common than we realize," says Hare, who speculates that we live in a world filled with wildlife chatter beyond our senses.
That view gets support from a discovery at the opposite end of the sound spectrum. In Papua New Guinea, biologists from the New York-based Wildlife Conservation Society have recorded scarcely audible rumblings coming from the direction of large, ostrichlike birds called cassowaries. "It's hard to describe, but you more feel it than hear it," says study leader Andrew Mack.
He and his students have documented cassowaries producing "boom" calls at infrasound frequencies at the lower limit of human hearing--around 20 hertz and, by far, the deepest bird calls ever recorded. Mack suspects the solitary birds use their infrasonic booms to locate mates in the dense underbrush of the rain forest, where the vibration can carry for at least a mile.
In the mid-1800s, scientists began to speculate that some migratory birds rely on an internal compass to guide their journeys along Earth's magnetic fields. Since this idea was accepted in the 1970s, they've found evidence for similar compasses in a wide range of other animals, including crustaceans, insects, fish, amphibians and mammals.
More recently, zoologists have shown that, in some birds, this navigational sense goes beyond a mere compass by providing maplike information about position--akin to today's global positioning systems. Since then, the race has been on to find similar abilities in animals outside the bird family.
Surprisingly, the first clear evidence has turned up in a lowly invertebrate: the spiny lobster, which migrates up to 120 miles across the Gulf of Mexico and Caribbean. To shed light on how the animals accomplish this feat on the bottom of the sea floor--without sunlight or other visual clues--University of North Carolina researchers began studying young lobsters in the coastal waters off the Florida Keys. After capturing the crustaceans, the scientists transported them (inside closed containers) along circuitous routes of about 20 miles to one of two test sites, one north and the other south of their home range. To ensure the animals could not use visual cues to orient themselves, the researchers placed rubber caps over the lobsters' eyestalks. Despite this, the lobsters all began walking in the direction of home--be that north or south of their drop-off site.
To test whether the lobsters were using geomagnetic navigation, the researchers returned to the home site and used powerful magnets to simulate the geomagnetic field conditions that exist some 250 miles either north or south. Sure enough, the lobsters exposed to the northerly magnetic field immediately began walking south, just as those exposed to the southerly field turned to walk north.
JUST HUMAN NATURE?
Sarah Brosnan had long marveled at the apparent generosity of the capuchin monkeys she studies at Yerkes National Primate Research Institute in Atlanta, where the animals live in two large and highly social colonies. If Brosnan left a bowl of food within reach of one capuchin but not another--separating them with a mesh partition--the first invariably shared by passing food through the divider.
As an anthropologist, Brosnan has a special interest in the roots of such behavior, specifically in mechanisms that encourage humans to cooperate when they receive no immediate benefit. Is the Golden Rule something we learn, or might it stem from an instinctual sense of fair play that we share with other primates?
At Yerkes, Brosnan designed an experiment to explore the question. First, she taught the monkeys a bartering game: She would hand one of them a small stone, then offer it a grape or a cucumber slice in trade. The capuchins eagerly bartered for either treat, though they clearly preferred the sweeter fare.
Next, Brosnan began giving one capuchin a grape, offering its partner a cucumber. The monkeys' reaction to such unequal treatment could only be described as outrage, says Brosnan. "They'd literally throw their cucumber slices at me, something I'd never seen in all the years I worked with them." The capuchins reacted even more negatively if they witnessed another receive the preferred treat for less work--if Brosnan handed their partner a grape, for example, without receiving a token exchange. In that situation, those offered cucumbers often threw away their stones or turned their backs on the scientist.
"People often turn down a reward because it's not what they think is fair," notes Brosnan. "Such self-defeating behavior may not seem rational, but our research suggests it traces to the kind of emotional sense of fairness that may promote the high level of cooperation needed in species that hunt or otherwise work closely together."
Elsewhere in the primate world, researchers have documented another eerie mirror of human behavior: Girl chimpanzees appear to study harder than do their rambunctious brothers. Working in Tanzania's Gombe National Park, University of Minnesota scientists tracked how young chimps learn to fish for termites using tools they make from sticks and stems. While both sons and daughters accompanied their mothers on termite-fishing expeditions, the daughters spent far more time watching and imitating, while the more-easily-distracted sons spent most of the time wrestling with each other.
As a result, most young females had perfected the skill by 30 months of age, while their brothers didn't catch on until they were nearly twice as old. That's not to say the boys were wasting their time, says study leader Elizabeth Lonsdorf. Their rough-and-tumble play may be important to sorting out dominance--a key factor in a male's later reproductive success.
New Jersey journalist Jessica Snyder Sachs wrote about the natural history of water in the June/July 2004 issue. To read about more animal behavior discoveries, see this month's "Web Exclusives" at www.nwf.org/nationalwildlife.
Pollutants Tied to Behavioral Oddities
In the 1970s, ornithologists began noticing a dramatic increase in same-sex California gull couples, with an abundance of female pairs attending nests brimming with both birds' eggs. While most of the gay gulls did just fine raising their chicks, thank you, the abrupt change in behavior proved an early warning of something amiss. In the following years, the gulls began to show signs of acute poisoning by the pesticide DDT.
Fast-forward to 2004, and a flood of reports from field biologists documenting a veritable freak show of bizarre animal behaviors, from fearless mice, stupid frogs and paranoid ducks to spastic egrets, hyperactive doves and sluggish salmon. Add to these an especially disturbing increase in clueless mating behaviors and neglectful parenting in species from crustaceans to kestrels.
In these and scores of other cases, biologists have now traced the eccentricities to low-level environmental contamination with behavior-bending chemicals known as endocrine disrupters. "We've documented that hundreds of man-made pollutants have profound effects on animal behavior," reports Amherst University biologist Ethan Clotfelter.
These chemicals include hundreds of biologically active pesticides, heavy metals and pharmaceutical products spread by runoff and seepage from farms, factories and municipal sewer systems. Worse, the low-level contamination has now pervaded every known ecosystem, from the tropics to the Arctic Circle. But with environmental testing geared to monitor for acute toxicity--that is, animals falling down dead--these behavioral disruptions have gone largely unnoticed, says Clotfelter, whose report on scores of such behavioral effects appeared in a recent issue of the journal Animal Behavior.
Clotfelter and other researchers who study endocrine disrupters are urging field biologists and laboratory toxicologists to recognize even subtle changes in animal behavior as a red flag that a particular chemical or an environmental cocktail of contaminants is approaching deadly levels.
"Like the DDT-poisoned gulls in the 1970s, you may see behavioral effects long before you see a population crash," he says. And the weird behaviors alone may threaten the survival of species already on the brink of extinction. Examples include the loss of predator-evasion instincts in endangered Pacific salmon in streams contaminated with trace levels of pesticides from bordering farm fields.--Jessica Snyder Sachs
Not Like Other Birds
Bird-watchers have long assumed that hummingbirds fly more like insects than like other birds. A new study in the journal Nature says that's true--to a point. Most birds use the downstroke of their wings to get 100 percent of the lift that keeps them in the air. Insects get half of their lift on the downstroke. Using a high-tech, high-speed device to measure the birds' flight, researchers found that hummingbirds get about 75 percent of their lift on the downstroke. Making use of the upstroke, too, is what allows them to hover almost indefinitely in front of a flower--or next to your feeder.--Hannah Schardt
Dolphins may be joining humans and some other primates in the cultural elite: A new study of bottlenose dolphins in Australia's Shark Bay indicates that they pass cultural knowledge--not merely genetics--from one generation to the next.
A few Shark Bay dolphins were first spotted more than a decade ago wearing harvested sea sponges on their snouts. Researchers surmised that the sponge could protect the animals from injury while they foraged for food. Now, after years of observation and DNA testing, researchers have determined that those dolphins are not merely acting on instinct governed by their genetic makeup but are sharing cultural knowledge.
"If the behavior were based on genes, it should follow certain patterns throughout the population," says lead researcher Michael Krützen of the University of Zurich. "We think we can pretty much rule out a genetic explanation."
Just 13 out of 185 bottlenose dolphins in the study use the sponges. All but one of the "spongers" are female and all, including the solo male, are descendents of a single female dolphin. That means that mother dolphins in that family line have passed down their knowledge from generation to generation, says Krützen--fulfilling the basic definition of culture.
The researchers believe other male dolphins may have been taught to use the sponges but later dropped the practice once other activities--like finding a mate--became more important.
"We're pretty sure that once they're weaned, they stop using this behavior," he says.
Krützen says the study could have a profound effect on the way people understand animal life--and their own place in the world. A few other nonprimate animals use tools; crows, for example, sometimes forage for food with twigs. But scientists believe those animals are acting on instincts buried in their genetic codes. Until now, only primates were thought to pass on knowledge to their offspring. That's why the "spongers" come as such a surprise.
"With primates, it's more expected because they're very close to humans," says Krützen.--Hannah Schardt