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Samplings—News from Nature

May 2008

	Peacock displays for a peahen Photo by Mariko Takahashi

Show-off males are common among birds, but peacocks stand out for their extravagantly long tail feathers adorned with dramatic spots (let’s not call them “eyespots”). There is general agreement that long and elaborate trains have evolved as a result of female preference, but a new study suggests that this selection may not be operating anymore.
     Mariko Takahashi of the University of Tokyo and three colleagues studied the sexual behavior of a feral population of the Indian peafowl, Pavo cristatus, in Izu Cactus Park, Japan. After seven years, the team concluded that peahens are not sweeter on males with longer trains, more symmetrical arrangements of tail spots, or greater numbers of tail spots. Perhaps, say the ornithologists, the differences between tail features have become too small for females to notice.
     Females did, however, spend more time with males that shook their tails at them—a display called shivering. Is a shivering male more attractive to a peahen these days? Takahashi’s group warns that high rates of shivering may be a consequence of a female having already chosen a male, not a cause of it. So figuring out what’s hot to peahens and what’s not may need more study. (Animal Behaviour)

Web links:

     Toshikazu Hasegawa Laboratory

     Mariko Hasegawa Laboratory

—Stéphan Reebs

Pirates may not come across as the most civic-minded folk, but a new study shows that pirate crews operating in the late seventeenth and early eighteenth centuries adopted a democratic constitution and a sophisticated system of checks and balances before France, Spain, and the United States—possibly even before England’s Bill of Rights was drawn up in 1689.
     Peter T. Leeson, a professor of capitalism at George Mason University in Virginia who is currently studying the economic organization of pirate societies, says that becoming a pirate was a rational choice: merchant marines could make a mere fifteen to thirty pounds per year, whereas an individual pirate could net hundreds and sometimes thousands of pounds in one successful attack. With such high stakes, pirates needed an effective system to keep their captains from becoming tyrants—an all too common phenomenon on the high seas. So crews adopted written constitutions that dictated how laws and leaders were chosen. Those systems typically operated on the basis of “one pirate, one vote.” To keep the captain’s power in check, crew members elected an independent quartermaster who had much of the authority that captains would hold in times of battle, while simultaneously looking out for the interests of the crew.
     Today’s pirates, who operate mostly off the coast of Africa and in the South China Sea, are not nearly as civil, because they spend little time together at sea and therefore do not require an elaborate system to govern their offshore criminal behavior. (Journal of Political Economy)

Web links:

     "An-arrgh-chy: The Law and Economics of Pirate Organization"

     Peter T. Leeson

—Brendan Borrell

	An artificial “butterfly” marked with conspicuous wing spots, as used in the experiment. Photo by M. Stevens

Some moths and butterflies bear circular, high-contrast marks on their wings that have long been thought to scare off predators by mimicking the eyes of the predators’ own enemies.
     Not so, say Martin Stevens and two colleagues at the University of Cambridge in England, who argue the marks work simply because they are conspicuous. (Predators are wary of prey with striking patterns, as those patterns often warn of toxic substances.)
     To test their claim, the team created artificial prey using pieces of gray paper marked with black-on-white spots in various shapes, sizes, and numbers; they attached the paper “wings” to dead mealworms, pinned the worms to trees, and waited. Two days later, they found that the worms affixed to “moths” with eye-mimicking pairs of spots had been devoured by wild birds in numbers equal to those associated with eye-catching shapes: rectangles, single large spots, and trios of small spots. It was conspicuousness that was doing the trick.
     Why, then, do wing marks look so much like eyes? The answer may lie in the process of wing formation. During moth development, molecules that cause wing cells to produce pigment can easily radiate from a central point, resulting in circular designs. Eye-like marks in the animal kingdom are often called “eyespots.” Now, Stevens recommends that the words “wing spots,” “tail spots,” or “fin spots” be used to designate them instead. (Behavioral Ecology)

Web Link:

     Martin Stevens

—S.R.

Male dwarf chameleon, Bradypodian transvaalese, displays for females. Photo by Devi Stuart-Fox

In popular culture, chameleons are considered masters of disguise. They can swiftly change their body coloration and thus blend in with their environment. But chameleons can also alter their appearance when expressing aggression and, in the case of males, when courting a female. So, which has driven the evolution of the chameleon’s ability to change its livery—camouflage or communication?
     If camouflage, one would predict that species living in environments with greater color variation—richer in shades of browns and greens—should have a greater capacity to switch colors. But when measuring color change in twenty-one species of dwarf chameleons, Devi Stuart-Fox at the University of the Witwatersrand and Adnan Moussalli at the University of KwaZulu Natal, in South Africa, found little support for that prediction.
     The investigators staged contests between males within each species and, using algorithms that model visual systems, measured how conspicuous a chameleon would appear to other chameleons—and to hungry wild birds. As seen through chameleons’ eyes, as well as through the eyes of their avian predators, the greater a chameleon’s color spectrum, the more it stood out.
     There is little doubt that chameleons in the presence of predators can adopt the hue of their background and thus hide themselves, but this study suggests that, at least in some species, the lizards’ striking color changes are first and foremost signals, not cloaks. (PLoS Biology)

—S.R.

	Three-dimensional reconstruction of a primitive feather © Perrichot/European Synchrotron Radiation Facility

A two-inch piece of amber discovered among thousands of others at the muddy bottom of a French quarry is helping scientists bridge a gap in their knowledge of the early development of feathers. The fossil dates to the Middle Cretaceous, around 100 million years ago, and encases seven beautifully preserved primitive feathers of a type never before seen, together with various arthropods and several types of microorganisms.
     Vincent Perrichot of the Museum of Natural History in Berlin, Germany, and four colleagues carefully cut the piece into twenty-two fragments. From one of the fragments, they isolated the ancient feathers and observed them in three dimensions, using scans, microtomography, and other sophisticated X-ray imaging techniques at the European Synchrotron in Grenoble, France. The feathers’ primitive feature—a flattened central shaft composed of not-yet-fused barbs—differs from those of all other feathers, both modern and fossilized, providing evidence for a key step in the evolution of feathers and thus the ability to fly.
     So whose feathers were they? Teeth from two species belonging to a group of feathered dinosaurs known as theropods were found in the same quarry as the amber, raising the possibility that the feathers came from a dinosaur. But the researchers would rather not make that call based on isolated feathers alone, since the feathers may well belong to a primitive bird. (Proceedings of the Royal Society B)

Web Link:

     “ESRF X-rays reveal clues about life 100 million years ago trapped in opaque amber”

—Harvey Leifert

This summer, storm chasers looking for the heaviest average rainstorms in the southeastern United States should venture out midweek in the afternoons.
     So suggests a study by NASA meteorologist Thomas L. Bell and five colleagues, who analyzed data gathered between 1998 and 2005 by the Tropical Rainfall Measuring Mission satellite, a NASA spacecraft that uses weather radar and microwave imaging to estimate U.S. rainfall south of latitude 40 degrees north. The team reports that in the southeast—where rainstorms cover more ground and disgorge more water than anywhere else in the U.S. within the satellite's purview—rainfall follows a familiar daily cycle: downpours in the afternoon.

Average rainfall for each day of the week is shown for rain falling between noon and midnight (“Afternoon”) and between midnight and noon (“Morning”).  Storms tend to grow most vigorously during the afternoon hours, and the authors suggest that pollution causes these storms to grow even more vigorously than they would in “clean” air.  It is possible that these extra-strong afternoon storms during the middle of the week deplete the energy for storm formation so that less is left for storms that might form the next morning, and that this is why the morning storms tend to be weaker during the middle of the week. Figure by Thomas Bell, NASA/Goddard

Intriguingly, the rain also follows a weekly cycle. Maximum rainfall occurs in the afternoons Tuesday through Thursday—when it’s almost double that of Saturday, the day of least rain.
     Now, guess what? Air pollution—aerosol particles released by traffic and industrial activities—also peaks midweek, according to monitoring programs by the Environmental Protection Agency. Correlation does not prove causation, Bell duly notes, but he points to the fact that aerosols keep cloud droplets small, allowing updrafts to lift them to greater heights. Higher in the atmosphere they freeze, releasing more heat, and thus intensifying storms. (Indeed, storm clouds grow higher in the middle of the week.) This phenomenon is particularly likely where air is moist and warm—exactly the conditions that prevail on summer afternoons over the southeastern U.S. (Journal of Geophysical Research)

Web link:

     “Midweek Increase in U.S. Summer Rain and Storm Heights Suggests Air Pollution Invigorates Rainstorms”

—S. R.

	Female sockeye salmon swim in the Skeena River watershed, British Columbia. Photo by Allen S. Gottesfeld

Building Their Own Beds

When a female sockeye salmon is ready to spawn, she finds a spot in stream gravel and sets about digging with her tail. The result is a shallow but large depression, about ten inches deep and three feet in diameter, wherein the female deposits her eggs. A recent study reports that all those excavations add up to rival water erosion as a shaper of streambeds.
     A team of biologists and geologists led by Marwan A. Hassan of the University of British Columbia, in Canada, studied the effect of salmon digging in four mountain watersheds of the Fraser River Basin, using sediment traps, magnetized particles as tracers, and detailed channel maps. The researchers found that, on average, salmon activity displaces a third to a half of the total sediments that move within the streams in the course of a year. Water currents, especially during floods, are responsible for the rest. All that hoeing has an indirect, beneficial effect: it loosens up streambeds, letting in oxygen.
     Salmon streams are often the targets of restoration projects. Yet such efforts may miss the mark unless landscaping by the streams’ inhabitants is taken into account. (Journal of Geophysical Research Letters, Vol. 35, L04405, doi:10.1029/2007GL032997)

—S.R.

Scraping Bottom for Blue

The underside of copal incense that filled a Maya tripod pottery bowl shows evidence that helped solve mysteries surrounding the ancient production of pigment known as Maya Blue. Fine blue and white grains were removed for analysis and indicated the presence of Maya Blue and palygorskite, indicating that the pigment was being made near Chichén Itzá's Sacred Cenote, into which the bowl (containing this copal) was thrown. Photo by Linda Nicholas; Courtesy of The Field Museum

Almost 2,000 years ago the Maya concocted a pigment, since dubbed Maya blue, which has endured the harsh climate of southern Mexico on murals, pottery, and sculptures. Archaeologists, however, never determined how or where the pigment was manufactured, even though they ascertained its chemical composition in the 1960s. Now researchers suggest that residents of Chichén Itzá, an archaeological site on Mexico’s Yucatán peninsula, ritually produced the pigment right before offering human and material sacrifices to the rain god Chaak.
     While looking through artifacts at the Field Museum of Natural History in Chicago, Dean E. Arnold, an archaeologist at Wheaton College in Illinois, discovered a clay bowl, five inches in diameter, that had been retrieved in 1904 from the Sacred Cenote, a ceremonial well at Chichén Itzá. The bowl was flecked with bits of palygorskite clay and indigo dye—the chemical constituents of Maya blue—and was filled to the brim with hardened copal, an incense made from tree sap.
     Arnold and four colleagues realized that burning the copal could have provided the low, steady heat required to form the durable chemical bonds in the pigment—and that priests may have daubed it on sacrificial victims prior to cutting their beating hearts from their bodies. Along with incense and more than a hundred human skeletons, the well contains a fourteen-foot layer of blue sediment. (Antiquity)

Web links:

     Field Museum Expeditions

     Dean E. Arnold

     Gary M. Feinman

—B.B.

The Warming Earth

In June 1991, Mount Pinatubo in the Philippines erupted and spewed out an estimated 17 million tons of sulfur dioxide into the stratosphere, creating a global layer of sulfuric acid haze that veiled sunlight and temporarily dropped worldwide temperatures by about 1 Fahrenheit degree. That hiccup in the modern trend toward global warming has made some visionaries sit up and take notice. Couldn’t mankind, they asked, counteract the warming effect of greenhouse gases by deliberately injecting sulfur-based aerosols into the stratosphere—above and beyond what we already contribute by air pollution?
     Curious, Philip J. Rasch, of the National Center for Atmospheric Research in Boulder, Colorado, and two colleagues calculated how much particulate matter would be needed for the job. Running various computer models of climate change, they found that 1.5 million tons of sulfur released every year—15 percent of the annual U.S. production—could keep global temperatures stable in the face of a doubling in carbon dioxide levels. The released particles would have to be small, however. Larger aerosols—of the size produced by volcanoes—do not scatter or trap sunlight energy as well, and would require twice the dosage.
     Alas such a geoengineering endeavor would contribute to ozone depletion. Therefore, as Rasch’s group recognizes, reducing greenhouse-gas emissions remains society’s best strategy to slow down climate change. (Geophysical Research Letters)

—S.R.

Copyright © Natural History Magazine, Inc., 2008