December 2006–January 2007

Persistence hunter Karoha and his kudu quarry Photo by Louis Liebenberg

Couch potatoes may disagree, but people are fairly well built to run in the heat. We sweat more per unit of body surface area than any other animal, and our upright posture exposes less body surface to the sun than would walking on all fours, and more surface to the cooling wind. On the hunt, those traits give people a distinct advantage over most quarry. In fact, Australian Aborigines and various Native American and African groups have traditionally practiced “persistence hunting,” chasing antelopes or other game in the midday heat, often for hours, until the animals overheat and collapse.
     During the past twenty years, Louis Liebenberg, an animal tracker and the owner of CyberTracker Software in Cape Town, South Africa, has observed the only persistence hunters still left, the !Xo and /Gwi bushmen of the central Kalahari in Botswana. He reports a success rate as high as 80 percent—and a meat yield that beats hunting with bow and arrow, club, or spear. Only hunting with dogs proved superior.
     Conditions have to be just right: the days must be long and hot, and the terrain must slow down the quarry. Furthermore, the hunters must be terrifically fit—the runs Liebenberg observed lasted as long as six-and-a-half hours and covered as many as twenty-two miles. And the hunters’ tracking skills must be exquisite; finding and following the quarry every time it bolts out of sight or mingles with a herd is no easy task—teamwork helps. But done right, Liebenberg says, persistence hunting is so effective that it may have helped select for the excellent thermoregulatory system, bipedal posture, and long strides that we all possess. Perhaps sadly, the practice is dying out, though the physical skill endures in those who shun couches and run for fun. (Current Anthropology)

——Stéphan Reebs

	Eastern elliptio mussels such as the ones pictured above were studied for the effects of fluoxetine exposure on breeding. Photo by Chris Eads, North Carolina State University

The freshwater mussels of North America are in trouble. Of 300 native species, some 70 percent are extinct, endangered, or declining. Invasives such as the zebra mussel have been, er, muscling them out of lakes and streams; development and pollution are also threatening their habitats. Now the beleaguered bivalves must add yet another peril to their list of woes: a new study shows that the widely prescribed antidepressant Prozac, a common pollutant, interferes with their reproduction.
     Like other drugs, Prozac often ends up in lakes and streams after being excreted and making its way through a wastewater-treatment plant. Fluoxetine hydrochloride, its active ingredient, boosts the concentration of the neurotransmitter serotonin in the brain.
     To test its effect on mussels, Rebecca M. Heltsley, a biologist at the National Institute of Standards and Technology in Charleston, South Carolina, and several colleagues placed female eastern elliptio mussels in tanks of water laced with serotonin, or with fluoxetine hydrochloride at various concentrations, some matching the levels that commonly occur in bodies of freshwater. All the mussels were carrying larvae; within forty-eight hours, mussels in each tank had prematurely released their larvae, which were often too immature to survive. The greater the concentration of fluoxetine hydrochloride, the more larvae were released. Better filtering of sewage would give freshwater mussels a less depressing outlook, says Heltsley. (Presented at the national meeting of the American Chemical Society, September 2006)

—Rebecca Kessler

An immature assassin bug of the genus Acanthaspis has camouflaged itself with a coat of debris and an ant corpse. Photo by Christiane Weirauch

You may feel more secure when your skeletons are hidden in the closet, but a young assassin bug is safest when the carcasses of its victims are conspicuously arrayed across its back. Investigators have interpreted such “corpse camouflage” as a strategy to help vulnerable juveniles of certain assassin-bug species, which prey on other insects, avoid predation themselves. Sensing nothing but a pile of corpses and other debris, a predator—perhaps a gecko or an adult assassin bug—moves on.
     Now Christiane Weirauch, an entomologist at the American Museum of Natural History in New York, has figured out how the juveniles don their camo: they have minute brushes on their hind legs with which they sweep corpses, soil, sand, or plant material onto their backs. “Glue,” which oozes from hairlike structures on their backs, sticks the debris in place. When a juvenile molts to a larger—but still juvenile—size, it retains its leg brushes and glue, but it must renew the camouflage. Once it molts to its adult stage, though, both brushes and glue disappear, and the assassin bug faces the world camouflage-free. (American Museum Novitates)

—Mary Knight

All sea-turtle eggs can develop into either male or female hatchlings; which gender depends on the temperature of the sand where the eggs are buried to incubate. Now, it seems, tourist development is leaving so much hot sand in its wake that batches of sea-turtle hatchlings are tipping heavily female—which could contribute to the endangerment of the various species.
     Stephanie Jill Kamel and Nicholas Mrosovsky, both zoologists at the University of Toronto, studied hawksbill-turtle nesting sites on the Caribbean island of Marie-Galante in Guadeloupe. Among hawksbills the “pivotal” temperature is 84.6 degrees Fahrenheit: nests in warmer sand give rise mostly to females; nests in cooler sand yield mostly males. In many sea-turtle populations female hatchlings heavily outnumber the males, which could impair reproduction. But could nest location explain it?
     Not surprisingly, Kamel and Mrosovsky determined that sand amid low-growing vegetation is warmer than the hawksbills’ pivotal temperature, and so the nests there are likely to produce females. Sand in the shade of forests high on the beaches is cooler, and so friendlier to males. Trouble is, when Caribbean beachfronts under development are cleared of their native forest, palm trees are often planted instead. Kamel and Mrosovsky found that the sand beneath palm trees gets as hot as it does in deforested areas—3.6 degrees warmer than under native forest, and well above the hawksbills’ pivotal temperature. Palm-fringed beaches could therefore produce few or no males—and spell more problems for the hawksbills. (Ecological Applications)

—S.R.

	Migratory dragonfly, radio-ready for takeoff Photo by Christian Ziegler (www.naturphoto.de)

Think of a migratory flier, and chances are a bird comes to mind. But at least nine species of dragonfly in North America head south in the fall, too. Until recently, though, migratory dragonflies could not be tracked because radio transmitters were too big for them to carry. A new study has changed all that, and determined that dragonflies and birds abide by some of the same rules of flying.
     Martin Wikelski, an ecologist at Princeton University, and several colleagues glued minute, custom-made radio transmitters to the undersides of fourteen dragonflies known as common green darners, which the investigators captured in New Jersey. Then they tracked the darners for twelve days by car and light aircraft. The insects, like birds, began their flights only after several nights of plunging temperatures (a herald of winds from the north that assist their southward journey). Also like most birds, the dragonflies alternated days of flying and resting, stayed put on overly windy days, and avoided crossing large bodies of water.
     Unlike birds, though, dragonflies that fly south in the fall probably do not come back in the spring; instead, investigators think, their progeny use the return ticket. Future studies, perhaps aided by satellite tracking and longer-lasting transmitters, may solve some enduring mysteries. For example, Wikelski showed that the dragonflies advanced thirty-six miles in six days, but no one yet knows how far the little migrants ultimately travel, or their final destination. (Biology Letters)

—S.R..

Epidermal peel of tomato, with stomata closed after exposure to bacteria Photomicrograph by Maeli Melotto.

Plants have pores on their leaves called stomata, which let carbon dioxide in and oxygen and water vapor out during photosynthesis. That function would seem to make each stoma a portal for invaders, too, such as disease-causing bacteria. Indeed, botanists have long assumed that plants cannot bar entry to pathogens, and so must fight them internally. But a new study shows that stomata function more like the portcullis of a medieval city: plants close them when under attack, and invaders pry them open to gain entry.
     Maeli Melotto, Bill Underwood, and Sheng Yang He, plant biologists at Michigan State University in East Lansing, and colleagues deposited virulent bacteria on the leaves of Arabidopsis plants, then observed that most of the stomata closed tight within two hours. The plants, they determined, detect and respond to certain molecules on the surfaces of the bacteria. Thus the “portcullises” are probably the plants’ first line of defense against bacterial invaders. But the bacteria kept up the attack, producing a chemical called coronatine that forced the stomata to reopen within a few hours. (Remarkably, the plant biologists also noted that the bacteria selectively swarmed open stomata.)
     How have such a dramatic defense and counterattack remained unknown until now? It was simply a case of misguided assumptions. So prevalent has been the idea that stomata can’t prevent invasion that investigators studying plant disease often injected pathogens directly into the leaves, thus bypassing the stomata entirely. (Cell)

—S.R.

The Warming Earth

Planets outside the solar system keep popping up in the Milky Way. Two "exoplanets" were discovered 26,000 light-years away, the farthest yet detected—by a team of astronomers led by Kailash C. Sahu of the Space Science Telescope Institute in Baltimore. The team also identified fourteen more possible exoplanets in the same region. Another two exoplanets were identified by a team led by A. Collier Cameron of the University of St. Andrews in Scotland. The new discoveries bring the total number of known exoplanets to more than 200.
     Most of those have been inferred from periodic wobbles in the motion of their parent stars, which can be detected from shifts of their spectra. The wobbles could be caused only by the gravitational pull of an unseen giant planet.
     But detecting the spectral shifts is tricky, and the four new exoplanets, as well as the fourteen candidates, were all discovered with a much more efficient technique. Telescopes are programmed to search for stars that dim at regular intervals; the dimming could result when an orbiting planet transits, or partly eclipses, the star. The method is so rapid that instruments can repeatedly scan millions of stars for the telltale sign.
     All four newfound exoplanets are gas giants, like Jupiter, and at least that big. Unlike Jupiter, though, they are so close to their central stars that they orbit in periods of between forty-three hours and four days (Jupiter orbits the Sun every twelve years). Sahu estimates that some six billion more exoplanets are scattered across the Milky Way, just waiting to be detected. (Monthly Notices of the Royal Astronomical Society; Nature)

—S.R.

One-half of the Hubble Space Telescope’s field of view employed in the Sagittarius Window Eclipsing Extrasolar Planet Search (SWEEPS), top image, contains approximately 150,000 stars, down to 30th magnitude. Half are bright enough to monitor for dips in brightness caused by the passage of an exoplanet passing in front of the star. The green circles identify nine stars that on that basis appear to be orbited by planets with periods of a few days. The bottom frame identifies one star whose back-and-forth wobble due to the pull of the planet was measured spectroscopically, confirming the existence of a planet whose mass was less than 3.8 the mass of Jupiter. (See more detailed information and additional images.) Images courtesy of NASA, ESA, K. Sahu, and the SWEEPS Science Team

Naturally bleached colonies of Acropora millepora at Middle Island in the Keppel Island group along Australia’s Great Barrier Reef. The organisms (zooxanthellae) in the bleached colonies succumbed to warm waters because they belonged to the sensitive C2 type; all that remains is their white coral “skeleton.” The more thermally tolerant D type survived, as is evident from their normal, dark color. Photo by Ray Berkelmans, Australian Institute of Marine Science

Good news is rare in research on global warming, but here’s a hopeful discovery. Certain species of coral may be able to cope with warming seawater with a little help from their microscopic friends.
     In return for a safe place to live, in the tissue of hard coral, single-celled algae of the genus Symbiodinium supply their hosts with photosynthesized sugars, and help calcify the coral’s hard skeleton. When rising sea temperatures kill the algae or cause them to become toxic to their hosts, hard corals suffer bleaching and may die. But some corals harbor several strains of Symbiodinium, which differ in their response to light and temperature, and in some of their metabolic products. Investigators suspected that the algal strains might also alter the thermal tolerance of their hosts.
     To test that idea, Madeleine J.H. van Oppen, a marine geneticist, and Ray Berkelmans, a coral ecologist, both at the Australian Institute of Marine Science in Townsville, transplanted colonies of Acropora millepora, a common Indo-Pacific hard coral, from their home waters on Australia’s Great Barrier Reef to warmer sites on the reef. They also tested the colonies’ thermal tolerance in the laboratory and genetically identified the strains of algae living inside. After a year, the investigators discovered, the transplanted corals increased their heat tolerance, a direct result of shifting the strain of Symbiodinium that dominates their tissues. Apparently, the corals initially take up an assortment of strains; if a strain with low heat tolerance is lost during high-temperature stress, a more heat-tolerant strain takes over.
     Will hard corals survive the next century’s hike in sea temperatures, predicted at between one and three Celsius degrees? Shuffling their Symbiodinium strains will probably not be enough to save the corals, say van Oppen and Berkelmans. But it may buy enough time to save them by reducing emissions of greenhouse gas. (Proceedings of the Royal Society B)

—Graciela Flores

Sunspots (dark areas) and faculae (bright areas) dot the solar surface. Image courtesy of the Royal Swedish Academy of Sciences

The theory that the Sun, not human activity, is responsible for most of the warming of the Earth in the past century has been debated for many years. According to that theory, the Sun has increased in brightness, and the brightening accounts for most of the warming. A new study puts the theory to rest.
     Astrophysicists Peter Foukal of Heliophysics, Inc., in Nahant, Massachusetts, and Hendrik C. Spruit, of the Max Planck Institute for Astrophysics in Garching, Germany, together with two colleagues, analyzed records of variation in solar luminosity caused by changing dark and bright areas on the Sun—sunspots and faculae.
     The team began by examining twenty-five recent years of precise solar-luminosity records gathered by radiometers on spacecraft. To peer further back in time, they scrutinized historical records of sunspots and faculae from the past century, as well as isotope ratios in the Greenland and Antarctic ice sheets, which register changes in solar activity for the past 1,000 years. They correlated those data with reconstructions of how temperatures have varied in the Northern Hemisphere during the past millennium, giving a fine-grained picture of the effects of changes in the Sun’s brightness on climate.
     The Sun did get brighter during the past 200 years, they discovered, but only by about 0.04 percent. That variation, they conclude, is far too small to have contributed substantially to the accelerated global warming observed since the mid-1970s. Although other solar traits––variable ultraviolet rays or solar winds, for instance––may yet be discovered to play a role, people burning fossil fuels are responsible for the bulk of the recent warming. (Nature)

—G.F.

	Gray jays Photo by Dan Strickland

In the forests of Ontario, Canada, rising temperatures have caused a decline in gray jays and may eventually eliminate the species in the southern parts of its breeding range.
     Thomas A. Waite, an ecologist at Ohio State University in Columbus, and Dan Strickland, a naturalist at Ontario’s Algonquin Provincial Park, analyzed population data collected between 1980 and 2006 in the park, which stands at the southern edge of the gray jay’s range. Waite and Strickland discovered that in the past twenty-five years the gray jays’ breeding season has advanced by about a week. More disturbing, the number of nestlings has declined, on average, by half a bird, and the overall population has plummeted by half. Moreover, Waite and Strickland determined statistically that warm fall temperatures, which have risen by about five Fahrenheit degrees in the past thirty years, probably caused those changes. But how?
     Most likely by affecting the food supply: the team found that the breeding of gray jays that received food from people throughout the winter was unaffected by fall temperatures. To prepare for winter, a gray jay hoards hundreds of times its body weight in perishable food—as many as fifty pounds of berries, fungi, insects, and vertebrate flesh. The stores are particularly important because the jay breeds before winter’s end. Warm autumns could cause the stored food to rot, Waite and Strickland hypothesize, which would probably delay and compromise breeding. (Proceedings of the Royal Society B)

—G.F.

Copyright © Natural History Magazine, Inc., 2006