Samplings—News from Nature

April 2007

Astatotilapia burtoni, one smart fish Photo by Logan Micail Grosenick

“Pick your battles wisely” is sound advice that people forget all too often. We could learn a thing or two from Astatotilapia burtoni, a little cichlid fish from the shallows of Lake Tanganyika in central Africa. New research shows that A. burtoni possesses surprising powers of logic—for a fish. The males can deduce the pecking order among their rivals after watching only some of them fight each other.
     Logan Grosenick and his adviser, Russell D. Fernald, a biologist at Stanford University, along with a colleague, placed “bystander” fish in the central part of an experimental tank. There the bystanders could watch staged, one-on-one fights between five rival males in compartments around the tank’s perimeter. To establish a dominance hierarchy among the rivals, the investigators predetermined the outcome of each fight by handicapping one contender—removing it from the water to stress it, then placing it in the other’s home tank. Only closely ranked rivals were pitted against one another. Thus the bystanders watched fish A fight and beat fish B, B fight and beat C, and so on through fish E.
     After exposing eight bystanders to either two or four of the fights each day for eleven days, the investigators tested whether the bystanders had been able to infer the complete hierarchy despite the gaps in their knowledge. Each bystander was shown two males that had never fought—A and E or B and D—in compartments on opposite sides of the tank. In nearly all the trials, the bystander clearly identified the lower ranking of the two males, visiting him first and spending longer near him (a sensible preference, considering a bystander’s improved odds at beating a low-ranking rival). That cognitive leap is roughly equivalent to the reasoning abilities children attain around age four. Not bad for a fish! (Nature)

—Nick W. Atkinson

	Carbonado and its glassy, melt-like surface Photos by Stephen E. Haggerty

Carbonado diamonds, also called black diamonds, are nothing like their flashy cousins: they’re an opaque black or gray, with a porous, sometimes charcoal-like texture. Conventional diamonds, moreover, form under pressure deep within the Earth and are shot toward the surface in hot, volcanic pipes, whereas the origin of carbonados has remained as dark as their color. They’re found only in the Central African Republic and Brazil, and even there, they never occur in volcanic formations. A new study has finally illuminated the mystery, showing that carbonados came not from far below, but from far above—from outer space.
     Jozsef Garai and his former graduate adviser, Stephen E. Haggerty, a geoscientist at Florida International University in Miami, along with two colleagues, analyzed the chemical bonds in carbonados by studying how they absorb infrared light. The technique, commonly employed with conventional diamonds, had been impossible to apply to carbonados because of silica impurities that mask absorption of relevant wavelengths of infrared light. The team devised a way to remove the impurities from crushed carbonados, then made the standard infrared analysis. They also aimed a much brighter infrared beam at impurity-free areas of carbonado slices. The result was the first complete infrared analysis of the carbonado diamond.
     The study revealed the presence of hydrogen—a sign that the carbonados formed in a hydrogen-rich environment, such as outer space—and a lack of nitrogen clumps, which form only under pressure, deep in the Earth. Those features, along with others, indicate an extraterrestrial origin, possibly in a supernova explosion. Garai and Haggerty say carbonados probably landed on Earth some 3 billion years ago, perhaps as a single, mile-wide asteroid that broke apart before landing. Now that would have been a lot of carats. (Article in Astrophysical Journal)

—Stéphan Reebs

Pregnant mole-rat queen is shown in false color X-ray image. Image by Erin Henry

In a colony of naked mole rats, as in a beehive, only one female—the queen—gets to breed. A mole-rat queen is easy to recognize because her body is the longest one in the colony. Furthermore, once she dies, the other females fight to replace her, and the victor grows longer with time. What’s causing all the stretching?
     Working with captive animals, Erin C. Henry, a postdoctoral fellow, and Kenneth C. Catania, a neurobiologist at Vanderbilt University in Nashville, together with a colleague, took weekly X rays of mole rats. Concentrating on one representative vertebra, they measured the length of the fourth lumbar in “new queens” (females recently paired with a mate) and in a mole-rat bachelorette over a period of two and a half years.
     Among the queens that underwent five or more pregnancies during the study period, the lengths of the fourth lumbars increased, on average, by 34 percent; in the bachelorette, the increase was less than 14 percent. Thus a longer body is probably a consequence of pregnancy. Most of the growth in the queens took place during the second half of each pregnancy, when gestation hormones peaked, suggesting that the hormones induce the growth spurts.
     The average litter of naked mole rats numbers twelve pups, but it can reach twenty-eight, the largest known for any mammal. Sure, it takes a big belly to accommodate that many fetuses, but why grow long, not wide? According to the investigators, queens must remain slim enough to patrol the narrow, subterranean tunnels of their colonies, in part to prevent other females from breeding. Mole-rat moms must literally go to great lengths for their babies. (Journal of Experimental Biology)

—S.R.

During the most recent ice age, some 20,000 years ago, a thick ice sheet covered Canada and parts of the northern United States. The climate then was obviously quite different than it is today—but was it so different that the prevailing winds south of the ice sheet blew in a different direction? Xiahong Feng, an earth scientist at Dartmouth College in Hanover, New Hampshire, and several colleagues came up with a clever way to answer that question.
     Feng’s team analyzed pieces of ancient wood collected across the continent for two rare, heavy isotopes of the elements that make up water, deuterium (hydrogen-2) and oxygen-18. The isotopes came from the rainwater taken up by the trees when they were alive. Because rainwater molecules made of heavy isotopes fall to Earth before those made of light isotopes do, the higher the proportion of heavy isotopes in the ancient wood, the closer the rain clouds were to their origin, the ocean.
     The investigators discovered that the relative amounts of deuterium and oxygen-18 in North American wood from the most recent ice age decline from east to west. Hence the winds prevailing across the continent blew from the east. After the ice age, beginning about 10,000 years ago, the wood shows a different pattern: the levels of the two isotopes reach minimum in the Midwest and rise toward both coasts, the mark of modern prevailing westerlies and the storms that take place on both coasts. (Geology)

—S.R.

Had Napoléon Bonaparte escaped or been released from his exile on the South Atlantic island of Saint Helena, some historians believe, European history might have taken a decidedly different course. But a recent investigation into the cause of the former emperor’s death there in 1821, at age fifty-two, suggests otherwise.
     The autopsy report for Napoléon listed stomach cancer as the cause of death. But in 1961 investigators discovered elevated levels of arsenic in his hair, spurring theories that he had been poisoned, perhaps by his supposed friend, the Comte de Montholon. Moreover, accounts of Napoléon’s obesity in his later years seemed to refute the stomach-cancer hypothesis.
     In a recent study, a team led by two pathologists—Alessandro Lugli of University Hospital of Basel in Switzerland and Robert M. Genta of the University of Texas Southwestern Medical Center in Dallas—took another look at the emperor’s death. They evaluated his clinical history and autopsy reports, his physician’s memoirs, and other pertinent historical documents in accord with the methods of modern pathology. They also compared his case to 135 recent confirmed cases of stomach cancer. Their conclusion: Napoléon had an advanced, debilitating stomach cancer that would have prevented him from altering the balance of European power had he left Saint Helena.
     The team found no evidence that arsenic poisoning caused Napoléon’s death (though he might well have been exposed to arsenic, perhaps innocuously), or that he had a familial predisposition to cancer. As for the emperor’s weight, the same group showed in an earlier study, based on a collection of his trousers, that he lost some twenty-four pounds in his last year of life. Such weight loss is consistent with stomach cancer. The team thinks Napoléon was infected with the bacterium Helicobacter pylori, which could have caused an ulcer and ultimately led to his fatal cancer. (Nature Clinical Practice Gastroenterology & Hepatology)

—Graciela Flores

Lions, like other large carnivores lions, don’t waste energy on small prey. Photo by Jakob Bro-Jorgensen

Land mammals that eat meat fall into two camps: the kiddie-menu crowd and the supersize-me set. Small carnivores usually go after bite-size prey, such as worms and mice, which don’t take much effort to hunt. But even the most proficient hunter can catch only so many small prey a day. As species get bigger, the energy gained from catching small prey soon lags behind rising metabolic needs and hunting costs. For carnivores weighing more than about forty pounds, the size of a coyote, it pays to switch to large prey, so large carnivores hunt animals closer to their own size.
     A new model developed by Chris Carbone, a biologist at the Zoological Society of London, and two colleagues explains how the balance between gains and expenditures in energy determines—and limits—the carnivores’ size and their prey selection. According to the model, as body size surpasses forty pounds, the metabolic costs of hunting rise more steeply than the energy gained. A carnivorous mammal weighing more than about a ton couldn’t catch enough prey—no matter how large—to survive.
     That’s a good fit with reality: the largest known mammalian predators, such as the extinct short-faced bear, max out at around a ton. Those and even somewhat smaller meat-eaters probably live close to the edge in maintaining their delicate energy balance. Indeed, they have advanced energy-conservation tactics, such as lions’ long bouts of inactivity or bears’ hibernation. Carbone thinks that explains why more large carnivores run a bigger risk of extinction than small carnivores or vegetarians: they’re more vulnerable to changes in the availability of prey. As nature’s self-appointed custodians, we should take note. (PLoS Biology)

—S.R.

	Dicanthelium lanuginosum grows in Yellowstone National Park at high soil temperatures. Photo by Regina Redman

Hot geothermal soils aren’t particularly hospitable to plant life, yet that’s where Dichanthelium lanuginosum, a species of grass, thrives. It owes its survival to a symbiotic fungus, Curvularia protuberata, that lives in its tissues—or so plant biologists thought. But new research has uncovered a third party to the affair: a virus.
     Grown separately, neither the grass nor the fungus survives temperatures above 100 degrees Fahrenheit; when grown together, the two do just fine at a sweltering 149 degrees. But the fungus, it turns out, is protective only when a virus is infecting its tissues. The discovery was made by Luis M. Márquez, an ecologist, and Marilyn J. Roossinck, a viral evolutionary ecologist, both at the Samuel Roberts Noble Foundation in Ardmore, Oklahoma, along with two colleagues.
      Grasses inoculated with a virus-free form of the fungus acted just like grasses that were entirely fungus-free: when grown in soil warmed daily to 149 degrees, they became shriveled and pale, and eventually died. Then, to confirm that the virus was responsible for the increased thermal tolerance, the investigators reintroduced the virus into the virus-free fungus. Sure enough, the newly infected fungus conferred the same level of heat tolerance in the grass as the naturally infected fungus did. Moreover, the infected fungus had a similar protective effect when transferred to the tomato plant, Solanum lycopersicon. Tomato is only distantly related to grass, suggesting that the virus affects a physiological mechanism for heat tolerance common to many plants.
     All plants harbor fungal symbionts, some of which carry viruses, but this example is the first known case of a virus, a fungus, and a plant living together cooperatively. Others surely remain to be discovered. (Science)

—G.F.

The Warming Earth

Cornfield in Maryland, one of sites where corn leaves were sampled for testing of CO2 isotopes. Photo by Diana Y. Hsueh

Reading the Leaves

There’s nothing corny about global warming, perhaps the twenty-first century’s most serious environmental and economic challenge. But a new study shows that corn can help map emissions of carbon dioxide (CO₂), an important greenhouse gas.
     CO₂ released by burning fossil fuels includes much less of the isotope carbon-14 than does naturally occurring CO₂. Furthermore, plants incorporate carbon from atmospheric CO₂ into their greenery during photosynthesis. So Diana Y. Hsueh and her adviser, James T. Randerson, an earth scientist at the University of California, Irvine, and colleagues argue that plants provide a cost-effective means of sampling the CO₂ derived from fossil fuels. And what better plant for that job in North America than abundant, ubiquitous corn?
     Hsueh and Randerson’s team analyzed the carbon-14 in corn leaves gathered from sixty-seven locations across the United States and Canada, then made a map of North American CO₂ emissions derived from fossil fuels. What they discovered was hardly surprising: plenty of emissions in California and the eastern U.S. (the Ohio Valley in particular), both densely populated regions, and relatively few in the less-populous Rocky Mountains and the Great Plains. The agreement with known patterns of fossil-fuel use shows the technique is a reliable mapping tool that could help track emissions over time and pinpoint high-level sources. (Geophysical Research Letters)

—S.R.

	UCI postdoc Steve Franks (left) and former student Sheina Sim, now a laboratory technician at the Max Planck Institute in Halle, Germany, measure Brassica rapa plants to determine the genetic basis of flowering time and the evolutionary changes in this trait following drought. Photo by Eric Weiss

Some plants take decades to mature before reproducing; others complete their entire life cycles in a year. A new study shows that when it comes to global warming, fast-maturing plants might have a “leaf up” on slow-maturing plants because they can evolve quickly in response to climate variations.
     Steven J. Franks, an evolutionary biologist at the University of California, Irvine, www.uci.edu, and two colleagues studied field mustard (Brassica rapa), a weedy annual plant common throughout North America. The team collected field-mustard seeds from two sites in California in 1997, after several years of heavy rainfall, and again in 2004, after a five-year drought. They grew plants from the seeds, then experimentally subjected the plants’ offspring to dry, moist, or wet growing conditions. Members of the postdrought lineage were clearly adapted to a parched environment: under dry conditions they had a higher survival rate than predrought-lineage members, and under all three growing conditions they flowered earlier. (During a drought, early flowering gives plants a better chance of reproducing before they wither.)
     Franks’s team then crossed pre- and postdrought plants, and grew the hybrid offspring alongside other pre- and postdrought plants. Sure enough, the timing of the hybrids’ flowering was intermediate, confirming that flowering time is hereditary and changes with selective pressure from drought. In short, the field-mustard populations had evolved.
     Franks did not test the adaptive responses of slow-maturing plants, which include many species of trees. Nevertheless, he reasons, the demonstrated adaptability of fast-maturing plants, such as field mustard, to climate variability gives them an advantage over slow-maturing plants, which simply have fewer generations, hence fewer chances to adapt, in a given time. Of course, Franks warns, if climate change becomes extreme, even the weeds won’t evolve fast enough to keep up. (PNAS)

—Rebecca Kessler

Project funded by the National Science Foundation

Copyright © Natural History Magazine, Inc., 2007