From: Kathleen M. Wong, California Academy of Sciences
Published December 2, 2004 12:00 AM

Homing Pigeons Sense Magnetic Fields, and Other Stories

Homing Pigeons Sense Magnetic Fields


Homing pigeons can read the Earth's magnetic field, an ability that may help them find their way back to the roost. The findings, by Cordula Mora and colleagues at the University of Auckland, New Zealand, bolster the idea that the birds can map their locations by sensing subtle variations in the planet's magnetic fields, much the way people read map coordinates. The researchers placed the birds in a long box with a feeding perch at either end. An apparatus capable of producing a magnetic field gradient was placed on the roof of the box. They trained the birds to go to one perch when they sensed a field, and the other perch when the field was absent. The birds chose correctly more than half of the time. But when a strong magnet was placed on their upper beaks, their accuracy plummeted. The same occurred when their upper beak area was anesthetized, and when the trigeminal nerve that runs through the area was cut. However, cutting the birds' nearby olfactory nerve did not impair their performance, suggesting the trigeminal nerve may conduct signals from magnetite grains in the upper beak to the brain. The work was reported in the journal Nature.


Cicada Swarms Fertilize Forests


Colossal cohorts of cicadas give Eastern forests hefty doses of fertilizers. The research, Louie Yang of the University of California, Davis, suggests the 17-year cycle of the cicada swarm known as Brood X aids forest productivity. A Brood X hatch can cover the ground with between 30 to 300 adult cicadas per square meter. When the insects die shortly afterward, their bodies contribute huge quantities of nutrients to local ecosystems. To determine just how much cicadas are aiding forests, Yang littered some forest plots with an extra 140 dead cicadas per square meter, and compared the growth of vegetation to control plots. Within a month, plots with cicadas yielded American bellflowers with 12 percent more nitrogen in their leaves, and seeds nearly 10 percent larger, than no-cicada plots. Nitrogen is often a limiting factor in forest plant growth, suggesting the cicadas help jump-start growth. This "resource pulse" appears similar to that seen in Pacific Northwest forests after big runs of spawning salmon die, and the strong plant growth after a rain-rich El Nino winter.


Humans Played A Big Role In Deadly European Heat Wave


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Summer 2003 was the hottest in Europe for 500 years, and humans seem to be the cause. A new climate study demonstrates that human-generated greenhouse gases doubled the risk that such a heat wave would occur. According to Peter Stott of the Hadley Centre for Climate Prediction and researchers at Oxford University, both in the United Kingdom, computer models that included the last 100 years of human-produced greenhouse gases were twice as likely to predict such a heat wave as climate models without manmade climate inputs. With human additions, the models predicted an average warming of about 0.5 degrees Celsius. The increase shifted both the mean and the extremes upward, increasing chances of a devastating heat wave. The scientists were able to calculate human contributions to this event only because the temperature rise was so extreme. Most other shifts in global climate are more gradual and must be considered as part of trends. During the summer of 2003, thousands of people died from the heat, and fires destroyed unusually large tracts of land.


Evolution At Work On Swallow Tails


Male barn swallows are growing particularly luxurious tail feathers these days. According to a new study, the outer tail plumage of male swallows has lengthened by 10 percent over the past two decades. This anatomical alteration, noted by Anders Pape Moller of Pierre and Marie Curie University, Paris, and Tibor Szep of the College of Nyiregyhza, Hungary, could be the most dramatic evolutionary change observed in wild creatures yet. For example, the degree of change is more impressive than noted in studies of beak remodeling in Galapagos finches after drought. Previous experiments have demonstrated that female swallows prefer swains with luxuriously long tail feathers. Scientists suspect females use tail length as a way to find the best mates, as only healthy males can produce such plumage. By contrast, the scientists measured no changes in the length of the swallows' central tail feathers, which do not excite females. The scientists say that the spread of the Saharan desert may be responsible for remodeling the birds' posteriors. Europe's barn swallows migrate to South Africa each winter, and fly home over the Sahara in spring. The extra distance they must travel without food may mean that only the fittest males, which grow the longest tail feathers, are making it home to mate. The research was published in the Journal of Evolutionary Biology.


Climate, Not Hunters, Drove Alaskan Bison Extinct


On the eve of humanity's entrance into North America, the continent teemed with a strange array of mega-mammals. Giant cave bears, woolly mammoths, and saber-toothed tigers roamed the forests and plains. Scientists have long debated why they disappeared. A controversial study led by Alan Cooper and Beth Shapiro of Oxford University, United Kingdom, and colleagues supports the idea that climate change, not human hunters, was to blame. The researchers studied the ancestors of modern Great Plains bison, which ranged from what is now Siberia and the Bering Sea into Alaska and Canada's Northwest Territories. The researchers found that populations of these Beringia bison began to decline amid an ice age about 37,000 years ago, a time about 23,000 years before humans first inhabited the area. By 22,000 years ago, growing glaciers closed the route into more hospitable southern territory. The climate grew drier, altering habitat. By 8,000 years ago, the researchers report in the journal Science, the population was virtually extinct, perhaps finished off by human hunters. However, other scientists strongly refute the claim. Detractors point to the disappearance of many groups of large mammals dating back to the arrival of humans, and point to evidence of abundant human habitation and hunting in the region as far back as 13,400 years ago. Some researchers say the big mammals' demise was due to a combination of causes (California Wild, Summer 2004).


Scurvy Devastated Early North American Colony


A new study of bones from one of the first French colonies in North America indicates the population was decimated by scurvy. The colony was located on St. Croix Island, between Maine and New Brunswick, Canada. It existed from 1604 to 1605, three years before Jamestown and 16 years before Plymouth. Conditions during that first winter killed nearly half of the 79 settlers. Scientists from Mount Desert Island Hospital in Bar Harbor, Maine, scanned the colonists' bones using computed tomography and analyzed their condition. The skulls had unusually thick hard palates and the femurs and tibias bore an extra layer of bony tissue. These symptoms are consistent with the effects of internal bleeding caused by scurvy. Scurvy is caused by a lack of vitamin C, which is found in many fruits and vegetables. The colonists may have attempted to diagnose the cause of the disease with an autopsy, as one skull bore cut marks from a knife. Written records from colonists describe symptoms such as loose teeth and swollen gums, confirming the diagnosis. The findings were reported in Chicago at the annual meeting of the Radiological Society of North America.


Related Links


Homing Pigeons Sense Magnetic Fields: Discovery.com / National Geographic / BBC
Cicada Swarms Fertilize Forests: news@nature.com
Humans Played A Big Role In Deadly European Heat Wave: news@nature.com
Evolution At Work On Swallow Tails: The New York Times
Climate, Not Hunters, Drove Alaskan Bison Extinct: San Francisco Chronicle / Scientific American / New Scientist
Scurvy Devastated Early North American Colony: San Diego Union Tribune (Reuters)


Source: California Academy of Sciences


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