Samplings—News from Nature

May 2007

Watch a wallaby dig, mate! Photo by Sean Doody, University of Canberra, Australia

For wallabies in Australia’s Northern Territory, putting a nose in the river often leads to a lot more than just a refreshing sip of water. In some areas, saltwater crocodiles (which can also live in brackish waters) are common, lying nearly submerged in the water to ambush the thirsty and unwary. But the agile wallaby has found a way to get a safer drink, according to a study by J. Sean Doody, an ecologist at the University of Canberra, and two colleagues. Not only do they visit the river at times of the day when the “salties” are relatively inactive; the cunning marsupials have also figured out that it’s safer to dig a drinking hole in the riverbank than to sip from the river directly.
     A shallow pit in the soil a yard or more away from the river quickly fills with water. By recording wallabies’ behavior at the drinking holes with motion-sensitive cameras and studying footprint patterns on the riverbank, Doody found that they much preferred the holes to the river.
     He also discovered that the wallabies appear to respond to variable risk: where the crocodiles were numerous, the wallabies sited their holes farther from the water’s edge and dug them deeper than where the crocodiles were scarce. Wallabies, it seems, are happy to invest extra energy to avoid becoming a hungry reptile’s dinner. (Ethology)

—Nick W. Atkinson

Not all parasites are created equal; biologists have struggled to explain why some are more contagious than others. One well-regarded theory suggests that parasites are most infectious when their hosts move around a great deal and come into frequent contact with one another. But when hosts roam less, highly infectious parasites soon find themselves surrounded by infected individuals, which limits their further advance. Under such conditions, parasites do well to become less infectious, thereby increasing the odds of encountering new victims. Michael Boots, an evolutionary ecologist at the University of Sheffield in the United Kingdom, and Michael Mealor have now corroborated the theory in an unusual experiment with breakfast cereal and virus-ridden, cannibalistic caterpillars.
     In the laboratory, Boots and Mealor infected caterpillars—larvae of the moth Plodia interpunctella—with a virus that spreads naturally when an uninfected caterpillar cannibalizes an infected one. They placed infected caterpillars among healthy ones in bins of food, a honey-sweetened cereal. Some caterpillars lived in dry, crunchy cereal and could mingle readily. The others lived in the same cereal, but moistened and sticky, which hindered their movement and reduced contact between them.
     After forty weeks (about eight P. interpunctella generations) Boots and Mealor fed the viruses from each group to a fresh, healthy batch of caterpillars, then measured the rate of infection. Sure enough, viruses in slow-moving, gruel-dwelling caterpillars had evolved to be a third less infectious than the viruses in mobile, crispy-cereal caterpillars.
     The results probably hold outside the cereal box, too. As extensive travel and trade bring people and wildlife into ever more frequent contact, parasite strains may become more infectious. (Science)

—Corey Binns

	Top row: Color photographs of a sunlit Arctic sky under foggy conditions (left), clear conditions (middle), and partly cloudy conditions (right). In the circular photographs the horizon is along the perimeter and the zenith is in the center. Corresponding to the photographs in the top row, the middle row shows patterns of the degree of linear polarization and the bottom shows the angle of polarization, as measured by full-sky imaging polarimetry in the blue part of the spectrum. Images courtesy of Gábor Horváth and Proceedings of the Royal Society A

Navigating under clear skies is relatively straightforward, but the ancient Vikings sailed northern seas that are frequently shrouded in fog and clouds. Their sagas mention enigmatic “sunstones,” held aloft on overcast days to locate the position of the Sun. Such sunstones could have been useful for navigation, but given their obvious romantic appeal, they may have been just literary inventions—none have ever been found. In 1967, however, the late Danish archaeologist Thorkild Ramskou pointed out that cordierite—a crystal common among pebbles on Norwegian coasts—changes color and brightness when rotated in polarized light. Cordierite stones, he suggested, might have enabled Vikings to perceive polarized light in the sky, from which they could reliably deduce the Sun’s position.
     It’s possible to detect polarized light in patches of open sky; many insects rely on it to find their way if the sun is invisible. It has been unknown, however, whether light that has passed through fog or clouds is strongly enough polarized—or makes an appropriate directional pattern across the sky—to serve as a sun compass. Now Ramón Hegedüs and his graduate adviser, Gábor Horváth, a biophysicist at Eötvös University in Budapest, and two colleagues have confirmed that foggy and cloudy skies at northern latitudes exhibit a polarization pattern similar to that of open skies.
     That makes the use of sunstones plausible. Still, the polarized light under foggy skies is extremely weak; under cloudy skies it’s stronger, but whether cordierite (or another natural material, such as tourmaline or calcite) are sensitive enough to reveal it needs further study. For now, how Vikings navigated in gloomy weather remains obscure. (Proceedings of the Royal Society A)

Link: Biooptics Laboratory

—Stéphan Reebs

“Missing” rock lies under the surface region marked in red on this composite satellite image. Image from NASA’s Earth Observatory

The Plateau of Tibet is a geological puzzle. Comprising nearly 900,000 square miles and rising 16,000 feet above the surrounding terrain, it is the largest and highest plateau on Earth. It also has the thickest crust—at an average thickness of more than forty miles, the crust is double that of most landmasses. How did the plateau come into being? Geologists have floated numerous hypotheses over the decades, but strong evidence either for or against them has been sparse.
     According to one hypothesis, when the Indian and Eurasian plates plowed into each other 55 million years ago, the Eurasian plate’s lithosphere (the outer crust plus an underlying layer) crumpled and pushed the Plateau of Tibet upward into being. Then, about 15 million years ago, a massive block of rock at least 60,000 square miles in area detached from the bottom of the Eurasian plate. As the rock sank, the plateau above it buoyed upward another mile, until it reached its present height.
     For decades, investigators have been looking for signs of the sunken hunk of lithosphere without success. But recently Tai-Lin Tseng and her graduate adviser, Wang-Ping Chen, a geophysicist at the University of Illinois at Urbana–Champaign, demonstrated that the missing rock is just where everybody expected it to be, centered some 350 miles north of the border between Nepal and Tibet and 400 miles beneath the Earth’s surface.
     Tseng and Chen made the discovery after collecting seismic signals from some 300 monitoring stations in India, Nepal, Tibet, and beyond, which indicated that seismic waves traveling beneath the plateau were moving at high speed. Seismic waves move relatively fast in cold materials. Because the missing rock was predicted to be colder than the surrounding mantle, the investigators knew they’d found it, providing firm evidence to support the detachment hypothesis. (Journal of Geophysical Research)

—Graciela Flores

As any textbook of biology (or sex education) will tell you, inbreeding is a big no-no. But at least one species of fish apparently cannot read. A team of behavioral ecologists led by Timo Thünken at the University of Bonn in Germany has discovered that members of a species of cichlid, Pelvicachromis taeniatus, prefer to mate with their brothers and sisters. One possible reason: closely related parents do a better job of raising their young than unrelated parents do.
     P. taeniatus is a colorful fish, between two and three inches long, that lives in the streams of Cameroon and Nigeria. Mom and dad cooperate to repel predators that attack their eggs and young fry.
     When Thünken’s team gave captive fish the choice of spawning with a stranger or with a sibling, three times as many chose the sibling. That was the case even though siblings were unfamiliar with each other because they had been separated shortly after hatching. In the resulting pairs of inbreeders, males spent more time near their eggs and young and quarreled less with their mates than did males in outbreeding pairs.
     Inbreeding among animals is rare because offspring are often severely handicapped by harmful, recessive genes. Yet both inbred and outbred fry in Thünken’s experiments grew and survived equally well. The species may have few harmful recessive genes, in which case the genetic cost of inbreeding may be easily outweighed by the twin benefits of passing along all the genes shared with one’s mate and providing one’s offspring with two caring, cooperative parents. (Current Biology)

—S.R.

	Bacillus subtilis that can be used as a living medium for long-term data storage Image by Yoshiaki Ohashi

Today’s exploding volume of data resides on stacks of paper, reels of magnetic tape, piles of compact disks, or banks of silicon chips. But those media are fairly fragile and last, at most, a few thousand years. For truly long-term storage, something nearly indestructible is needed. A new study suggests an intriguing possibility: the DNA of bacteria.
     Nozomu Yachie and his graduate adviser, Yoshiaki Ohashi, a molecular geneticist at Keio University in Tsuruoka, Japan, together with several colleagues, encoded the message “E=mc^2 1905!” in the DNA of Bacillus subtilis, a tough bacterium that lives in soil. After assigning the letters and symbols to specific sets of DNA nucleotides, they prepared coded nucleotide sequences, which they inserted into the genomes of bacteria. A few days—and numerous bacterial generations—later, they extracted the DNA and decoded the sequences to read the message.
     Certain bacteria, including B. subtilis, form resistant spores that can revive after millions of years of dormancy. And living bacterial populations can survive for eons, too. Of course, their DNA can mutate, but the Japanese team developed a simple way to encrypt and store redundant—yet distinct—versions of the data. As the technology for replicating and sequencing DNA becomes cheaper, faster, and more accessible, bacterial DNA might someday replace the silicon chip. (Biotechnology Progress)

—S.R.

The Warming Earth

Besides being of major economic importance, mussels represent an important food source for birds and act as ecosystem engineers, creating the right underwater climate for other life along the shorelines. Photo by CNRS Photothèque / Philippe Garrigues

Oysters on the half shell are considered a delicacy, but what about mussels on the three-quarter shell? A new study shows that human emissions of carbon dioxide (CO₂) could reduce bivalves’ ability to build their shells by as much as 25 percent.
     In addition to warming the Earth, excessive CO₂ is making the oceans more acidic, which decreases the concentration of dissolved carbonate in seawater. Without carbonate for building their shells, numerous minute organisms—including corals and species of phytoplankton and zooplankton—are showing alarming signs of distress. Now Frédéric Gazeau, a marine biologist at the Netherlands Institute of Ecology in Yerseke, and several colleagues have shown that the phenomenon propagates up the food chain.
     In the laboratory, Gazeau exposed mussels and oysters to water with various levels of CO₂ for periods of two hours, measuring the water’s average pH and the change in its alkalinity, which is proportional to its concentration of carbonate. From alkalinity levels he calculated the mollusks’ rate of shell construction, or calcification. Sure enough, the higher the water’s CO₂ concentration and the lower its pH, the slower the mollusks’ calcification.
     If atmospheric CO₂ reaches the levels expected by 2100, Gazeau predicts the calcification of oyster shells could decline by 10 percent and that of mussel shells by a quarter. As the declines in calcification affect the development of juvenile shellfish, and as adults become more vulnerable to predation, both aquaculture and marine ecosystems are likely to change. Gazeau stresses that his findings are preliminary; he measured only short-term responses to high CO₂ and low pH. But his next experiment will test their responses over several months. (Gazeau, F., C. Quiblier, J. M. Jansen, J. Gattuso, J. J. Middelburg, and C. H. R. Heip [2007], “Impact of elevated CO₂ on shellfish calcification,” Geophysical Research Letters 34, L07603, doi:10.1029/2006GL028554)

Links:

Netherlands Institute of Ecology (NIOO) Laboratoire d’Océographie de Villefranche (LOV)

—Rebecca Kessler

Kueppers’s modeling of the effects of vegetation indicates that average high temperatures are lower (in blue) in areas converted to irrigated agriculture. (White areas are areas that did not experience statistically significant climate difference. Conversion to irrigated agriculture was confined to areas within the gray outlines.) Illustration from Geophysical Research Letters, © 2007 American Geophysical Union

Cool Acres

Irrigation might be cooling local climates, and in the process, hiding the true magnitude of global warming. Many temperature-monitoring climate stations happen to lie near irrigated agricultural land; new research shows that the temperatures measured there might be skewed downward, making the actual warming much higher than estimates show.
     Lara M. Kueppers, an ecosystem scientist now at the University of California, Merced, and two colleagues ran a computer model to estimate what temperatures would have been in California between 1980 and 2000 if irrigated areas had not replaced natural vegetation. For comparison, they ran another model that made the estimate on the basis of actual land-use patterns in 1990.
     On average, the team found, daytime high temperatures in summer were about thirteen Fahrenheit degrees cooler in the irrigated areas than they would have been if natural vegetation still covered the land. (Irrigation made little difference in nighttime or winter temperatures.) Irrigated farmland occupies 8 percent of California’s land, causing the state’s overall temperature to drop by slightly less than one degree. The cooling effect of irrigation probably stems from increased evaporation from soils and plant leaves on summer days.
     The effects are almost certainly not limited to California. Around the world, more than 650 million acres are irrigated, and more than half the temperature-monitoring stations in at least one important global temperature dataset also lie in such areas. (Geophysical Research Letters)

—S.R.

As the Earth warms, life in big cities is getting tougher. Abundant dark, sun-absorbent surfaces and heat emitted by cars and buildings, among other factors, push temperatures as much as twenty-two Fahrenheit degrees higher than those in the surrounding countryside. One way or another, people manage to avoid the excessive heat. But what about the rest of the urban fauna?
     Michael J. Angilletta Jr., a thermal biologist at Indiana State University in Terre Haute, and a team of investigators argue that so-called urban heat islands are excellent natural laboratories for testing the possible effects of climate change on organisms. In many species, populations from warm habitats tolerate heat better and cold worse than populations from cooler climes. Angilletta and his colleagues predicted that the same would hold true for urban and rural populations of the leaf-cutter ant Atta sexdens.
     To test the prediction, the investigators collected A. sexdens in the megacity of São Paulo, Brazil, and in rural areas nearby. Then they exposed the insects to a stressful temperature of 108 degrees F. and compared the time it took the two groups to lose mobility. Finally, they chilled members of both groups for twenty minutes, then timed the ants’ recovery. As predicted, the urban “Paulistanos” survived the heat 20 percent longer than their rural counterparts. But their greater heat tolerance came at no obvious expense of cold tolerance: both groups of ants recovered from “chill coma” in nearly identical times.
     Angilletta and his colleagues can’t tell whether the different responses of urban and rural ants come from genetic adaptations or are simply the result of physiological acclimatization. In either case, their study hints that ants, at least, might be able to beat the heat of a warming Earth. (PLoS ONE)

—G.F.

Copyright © Natural History Magazine, Inc., 2007