September 2005

According to the most reliable recent estimates, the monstrous earthquake that rocked the northwest coast of Sumatra and the nearby Andaman and Nicobar island groups on December 26, 2004, released the energy-equivalent of a 250-megaton bomb, shaking every point on Earth’s surface half an inch or more. It also launched a tsunami at more than a million coastal dwellers, killed nearly 300,000, and wrecked many fragile local economies.

Earthquake of December 26, 2004, led to slippage (jagged, bright-red line) along a thousand miles of the boundary between the Indo-Australian tectonic plate, which pushes steadily northward (red arrows), and the Eurasian plate. The quake caused a devastating tsunami in the Indian Ocean. In March 2005 an adjacent segment (orange line) of the plate boundary ruptured. Map © Ian Worpole

So just what took place in the Earth’s crust to generate such force and cause such devastation? Four dozen earth scientists—among them Charles J. Ammon of Pennsylvania State University in University Park; Roger Bilham of the University of Colorado in Boulder; Thorne Lay of the University of California, Santa Cruz; and Jeffrey Park of Yale University—have now given a detailed answer to that question.
     Along the coast of Sumatra and the islands to its north, the Indo-Australian tectonic plate is steadily pushing northward and, at the same time, thrusting under the Eurasian plate. The two plates are converging here at a rate of an inch or two a year, building up immense stresses at the boundary. Large earthquakes have been recorded here since 1797, though none so massive as the one late last year.
     On that morning the plate boundary, which is normally locked up by the pressure and friction of the rock masses, gave way near the northern tip of Sumatra, about twenty miles below the surface of the sea. It was almost as if a zipper had suddenly become unzipped. Within the next minute, a sixty-mile stretch of seafloor north of the initial rupture slipped rapidly beneath Sumatra. In the next eight or nine minutes the rupture raced northward for nearly a thousand miles along the plate boundary. The seafloor instantly buckled: above the rupture a vast area rose up several feet, while areas to the east and west of the rupture sank. This contortion of the seafloor displaced millions of tons of seawater, initiating the tsunami.
     Then, as vibrations spread through the surrounding rock, the rate at which the two plates had been slipping past each other slowed dramatically. About half an hour later the slippage stopped. Altogether, the total displacement of the seafloor made for an event of magnitude 9.3 (by some estimates)—so powerful that its vibrations triggered small earthquakes as far away as Alaska. But its effects did not stop there: This past March, another 200-mile-long segment of the plate boundary failed, adjacent to the December rupture. Around the world, detectable vibrations continued for months. Even the global sea level is now half a millimeter higher than it was before the temblor. (Science 308:1126–44, 2005)

—Dave Forest

It’s hardly news that women, on average, live longer than men. So, in fact, does the average female of many other mammalian species. José Viña and several other biochemists at the University of Valencia in Spain have long been investigating the physiological causes of this common gender gap.
     Inside each animal cell, within the minuscule structures called mitochondria, oxygen reacts with the by-products of digested food to yield energy. Few animals can live without oxygen, which, alas, also forms oxidants—compounds that strip electrons from other essential molecules, thereby disabling them. Fortunately, animal cells produce various antioxidants, which neutralize most oxidants. But the oxidants that survive, especially the very reactive ones known as free radicals, can damage DNA.
     According to one popular theory, aging is the result of a lifelong buildup of damage caused by free radicals. One culprit is hydrogen peroxide (H2O2), which is commonly produced from and also transformed into more potent free radicals, and so its presence signals trouble. Viña and his colleagues recently determined that the mitochondria in females have only about half as much hydrogen peroxide as the ones in males. Furthermore, they discovered that female hormones—estrogens such as estradiol—are to thank for this healthy restraint. Estradiol binds to a receptor in the cell membrane and triggers a cascade of cellular reactions that activate genes and eventually give rise to more antioxidants.
     So what’s a guy to do? Because estrogen could cause feminization in males, gulping estradiol pills is probably not the solution. But Viña and his colleagues have been investigating plant compounds that resemble estrogens. One such compound in soybeans lowers hydrogen peroxide in isolated human cells; within the coming decade, tests of the compound in people are expected. (FEBS Letters 579:2541–45, 2005)

—Stéphan Reebs

Not long ago in Shark Bay, off the coast of western Australia, a female bottlenose dolphin broke a chunk of sponge off the seafloor and wore it as a mask over her snout while she probed the sediment for fish. Today “sponging” is a foraging fad among dolphins in Shark Bay—but, with one exception, exclusively among females. Moreover, though the Shark Bay dolphins adopt a dozen foraging tactics, sponging is the only one that involves a tool.

	The latest in foraging attire Photo © Amanda K. Coakes (www.monkeymiadolphins.org)

     Biologists have resisted giving the label “culture” to the perpetuation of the practice of sponging. But Michael Krützen, a molecular ecologist formerly at the University of New South Wales in Sydney, Australia, and his colleagues maintain that the term fits, and they’ve ruled out alternative explanations. Both spongers and nonspongers forage in deep water, so sponging is not a response to habitat. And samples of nuclear DNA from adult spongers rule out the possibility that a propensity for sponging is a genetic trait.
     The only remaining explanation, Krützen and his colleagues argue, is that sponging is socially learned—the first established example of the cultural transmission of tool use in marine mammals. But if it’s social learning, it remains (almost) all in the family: according to an analysis of the spongers’ mitochondrial DNA, all but one of them are descended from a single matriarch. Thus they most likely learned the practice from a female relative, probably Mom. The single sponging male examined by the investigators is kin, and would have spent the same amount of time with his mother as a daughter would have. So why don’t other males sponge? That’s still a puzzle. (PNAS 102:8939–43, 2005)

—Graciela Flores

In spite of its many symmetries, the body is subtly asymmetric: the heart, for instance, lies to the left of center and the liver to the right. But how does the asymmetry arise? Investigators in the laboratory of Nobutaka Hirokawa, a cell biologist at the University of Tokyo, have discovered that the rapid gyrations of cilia, or microscopic hairs, on the underside of eight-day-old animal embryos are responsible.
     Embryos manufacture protein-rich fluids filled with chemical cues. As the cilia whip around clockwise (the direction is always clockwise because of the cilia’s asymmetric internal structure), they circulate the fluids. Stem cells pick up the cues and act accordingly. Surprisingly, the cilia’s gyrations don’t create a tornado-like vortex of fluids. Instead the result is a linear, leftward flow.
     By doing some fancy camera work with fish, mouse, and rabbit embryos, Hirokawa’s group discovered that the fluids flow leftward because the cilia sprout off-center from a domelike membrane, and so the axis of the cilia’s gyrations tilts toward the embryo’s rear end. During the right-to-left phase of its clockwise swing, each cilium is perpendicular to the membrane and pushes fluid unimpeded toward the surface of the embryo. The “recovery” phase of the swing, which begins in a trough, close to the surface of the embryo, gives less of a push to the fluid. Consequently, the proteins don’t get distributed evenly, the chemical cues vary, and asymmetry develops. (Cell 121:633–44, 2005)

—S. R.

Honey possum Photo © Petroc Sumner (www1.imperial.ac.uk/medicine /people/p.sumner.html)

To see color, your retina has to have cone cells; the more kinds of cones it has, the more colors you can differentiate. Like us, several species of Australian marsupials have three kinds of cones. One such animal, the honey possum—a wee creature that tips the scales at less than a tenth of an ounce—feeds exclusively on the nectar and pollen of colorful flowers, particularly those of the tree Banksia attenuata. And one of the possum’s cones is sensitive to a much longer wavelength—a redder color—than the corresponding cone in other marsupials.
     Petroc Sumner, a neuroscientist at Imperial College London, and two biologist colleagues measured the composition of light reflected by the flowers and leaves of fifteen species of plants in the possum’s habitat, figured out what would register on every possible kind of cone, and computed the ideal cone sensitivity for a honey possum to have. It turns out that the possum’s oddly tuned cone is perfect for distinguishing the mature, yellow flowers of B. attenuata from the similarly shaped, but green, immature ones and the brown, senescent ones. Makes sense: instead of climbing all over a tree just to reach a nectarless flower, the possum can judge from a distance whether the climb is worth the effort. (Journal of Experimental Biology 208:1803–15, 2005)

—S. R.

Physicists once thought gamma rays—the most energetic form of electromagnetic radiation—originated primarily from distant celestial sources. But new technology has been registering gamma rays in Earth’s own atmosphere, at a rate of at least fifty bursts a day. Now Steven A. Cummer, an electrical engineer at Duke University in Durham, North Carolina, and his colleagues have analyzed twenty-six of those so-called terrestrial gamma-ray flashes (TGFs) and discovered another big gamma-ray surprise.
     A process called runaway breakdown seems to set off TGFs: A cosmic ray, or high-speed atomic nucleus, strikes an ordinary air molecule within a strong electric field. The collision energizes and dislodges one of the molecule’s electrons, which the electric field then accelerates to nearly the speed of light. The highly energetic electron strikes other air molecules, energizing yet more electrons. When a beam of such energetic electrons collides with an atom, gamma rays burst forth.
     It used to be thought that TGFs were generated far above thunderstorms, immediately after a very strong bolt of lightning. What Cummer and his colleagues found, however, is that TGFs precede lightning bolts by a split second, and are associated with lightning several hundred times weaker than anyone expected—or even with no lightning at all. To the investigators, the link with moderate lightning implies that TGFs develop at surprisingly low altitudes: near the tops of thunderclouds, rather than some twenty miles above them, as had been predicted. And so runaway breakdown, they say, seems to be connected to the cause of both lightning and TGFs. Apparently not only gamma rays, but also lightning itself, are still a bit of a mystery. (Geophysical Research Letters 32:L08811, 2005)

—Rebecca E. Kessler

A pair of spectacled parrotlets sitting in close contact: this behavior is typical for mated pairs. Photo by Ralf Wanker

For people, it’s second nature to refer to a friend or family member by name, but there’s scant evidence that other animals do the same. Some species do communicate information on the whereabouts of food or the presence of a predator, and animals can clearly recognize individual members of their own group. But even dolphins, clever as they are, have apparently not invented tags for one another.
     But spectacled parrotlets, brilliantly colored Central and South American birds, may be a different matter. Ralf Wanker and two other biologists at the University of Hamburg in Germany report that, in an experimental setting, the birds made different “contact calls” for different members of their family. Furthermore, they responded more often to recordings of calls that had originally been directed toward them rather than toward another family member. That is strong evidence, say Wanker and his colleagues, that some nonhuman species label their social companions individually. People will just have to learn that spectacled parrotlets are not all called Polly. (Animal Behaviour 70:111–18, 2005)

—Nick W. Atkinson

When did people become the only animals that regularly wear shoes? The oldest well-dated surviving footwear—North American sandals made of plant fibers or leather—is 9,000 years old. Earlier shoes have decayed; their existence must be extrapolated from figurines, footprints, and remnants of burial goods. To peer farther into footwear’s past, says Erik Trinkaus, a physical anthropologist at Washington University in Saint Louis, one must look at feet. A bare foot in direct contact with the ground depends more on the four small toes for traction and weight distribution than it does when supported by a shoe. Thus the small toes of the habitually unshod become stronger and bigger than those of the habitually shod.
    Trinkaus examined toes from the remains of Neanderthals between 75,000 and 40,000 years old, Middle Paleolithic modern humans about 100,000 years old, and Upper Paleolithic modern humans between 28,000 and 20,000 years old. He found that, compared with the remains of several groups of recent humans whose footwear habits are known, Neanderthal toes were several times more robust than modern toes. Robustness declined most rapidly between the Middle Paleolithic and the midpoint of the Upper Paleolithic. As early as 28,000 years ago, Trinkaus concludes, people had begun to wear shoes on a regular basis. (Journal of Archaeological Science 32:1515–26, 2005)

—Caitlin E. Cox

Copyright © Natural History Magazine, Inc., 2005