October 2006

Capacity of T. rex for binocular vision is demonstrated in a reconstruction of eyeball position in relation to bony structures of the head. Photo by Neal Larson / Black Hills Institute of Geological Research, Inc., Hill City, South Dakota (www.bhigr.com)

Every schoolkid knows that Tyrannosaurus rex was really scary. Now a new study adds some frightening detail in answer to the question, How scary? Kent A. Stevens, a computer scientist at the University of Oregon in Eugene analyzed reconstructed heads of seven dinosaur species to discover how well they could see. In particular, he measured the extent of each species’ binocular vision—how much the images from the left and right eyes overlap.
     Depth perception and motion detection, among other optical feats, are greatly assisted by a large amount of binocular overlap, which enables the brain to judge the relative positions of objects in view. For each dinosaur reconstruction, Stevens mapped the region visible to each eye, then calculated the regions’ overlap. The overlap was determined largely by whether the eyes faced forward or to the side, and whether the snout or its bumps blocked the view.
          Stevens’s analysis shows that Allosaurus and Carcharodontosaurus had only a narrow binocular overlap. To detect prey against a complex background, either the prey or the dinosaurs’ own heads had to move. Hence both genera were probably ambush-predators, like modern crocodiles. Tyrannosaurus, along with Daspletosaurus, Nanotyrannus, Troodon, and Velociraptor, possessed a much wider overlap, conferring excellent depth perception and enabling both a stealthy approach and rapid pursuit. Thanks to that and its large, widely separated eyes, T. rex may even have had the sharpest, most detailed 3-D view of the world any animal has ever experienced, making it perfectly equipped for active hunting. And the answer to the question on every schoolkid’s mind? "Very scary!" (Journal of Vertebrate Paleontology 26:321–30, 2006)

—Nick W. Atkinson

The prevalence of obesity and Type 2 diabetes in certain populations is often attributed to a “thrifty” genotype, selected for by frequent famines throughout those populations’ prehistory. People who express the thrifty genotype are presumably predisposed to accumulate reserves of fat in times of plenty, for later use in times of famine. In today’s constant plenty—so the theory goes—people with that once-useful genotype are prone to such metabolic problems as obesity and diabetes, which are common in groups such as Australian Aborigines, Native Americans, and Polynesians.
     When the late American geneticist James V. Neel first articulated the theory in 1962, he also argued that the thrifty genotype should be more prevalent among hunter-gatherers than among farmers, because––he assumed––the hunter-gatherer lifestyle is the more prone to severe food shortages. But Daniel C. Benyshek, a medical anthropologist at the University of Nevada at Las Vegas, and James T. Watson, a physical anthropologist at Indiana University–Purdue University Indianapolis, contend that Neel’s generalization was too broad.
      With an extensive database on nutrition and food availability in preindustrial societies, which was compiled in the 1950s, Benyshek and Watson compared twenty-eight hunter-gatherer societies with sixty-six agriculturalist societies. They detected no link between lifestyle and amount of available food, or between lifestyle and frequency or duration of food shortages. Feast-or-famine cycles were probably common throughout human prehistory, and they may indeed favor thrifty genotypes, the anthropologists say. But the cycles seem to have been equally likely among foraging and farming economies. (American Journal of Physical Anthropology 131:120–6, 2006)

—Stéphan Reebs

	Berry cluster from a wild female grapevine plant Photo by Rafael Ocete, University of Seville, Spain

The wild Eurasian grape, Vitis vinifera sylvestris, was first domesticated for winemaking in Transcaucasia, the region between the Black Sea and the Caspian, perhaps as long as 8,000 years ago. Early viticulturists selected for large, sweet grapes in a variety of colors, and they learned to propagate the plant vegetatively. The domestic cultivars found their way south, and by 5,000 years ago large-scale vineyards and wineries were well-established in Sumer and Egypt. From there the practice spread west, reaching the Iberian Peninsula by 2,800 years ago. Does that history, which is based on archaeological evidence, imply that all grapevine cultivars around the Mediterranean today are descendants of those wild Transcaucasian ancestors?
     Probably not, say Rosa Arroyo-García and José Miguel Martinez-Zapater, both plant geneticists at the National Center of Biotechnology in Madrid, Spain. With the help of an international team of collaborators, Arroyo-García and Martinez-Zapater discovered molecular evidence against a single ancestral grape population. Their team analyzed the DNA of more than 1,200 cultivars and wild plants from around the Mediterranean. They discovered that more than 70 percent of Portuguese and Spanish cultivars are related to local wild-grape populations—a hint that the wild grape may have been domesticated more than once. Now there’s a ripe fact to impress your fellow wine connoisseurs with next time you sip a rich red from Andalusia. (Molecular Ecology, doi 10.1111/j.1365-94X.2006.03049.x, 2006)

—S.R.

Longhorn bee (Eucera nigrescens) visits a red clover blossom. Bees like this one, which specialize in a limited number of flower species, are among the pollinators that have most declined in recent years. This species has declined substantially in the Netherlands and gone extinct in the United Kingdom. Whether it is flower losses that lead to pollinator declines, or pollinator losses that lead to plant declines is not yet known. Photo by Nico Vereecken

There’s less buzz in the fields of Europe these days. A new study confirms what many observers have suspected: that the number of bee species in the United Kingdom and the Netherlands has declined markedly in recent decades. Strikingly, so have plant species that depend on them to reproduce. A team of biologists led by Jacobus C. Biesmeijer and William E. Kunin of the University of Leeds in England made the discoveries. For the two countries, the team studied records of change in plant populations, and compiled nearly a million observations of pollinating bees and hoverflies that were made since the late nineteenth century. In more than 60 percent of the sites surveyed, there were fewer species of bees after 1980 than there were before.
     Biesmeijer and Kunin can’t tell which decline came first, the plants’ or their pollinators’, nor whether one decline is causing the other. The biologists are also uncertain about the cause; the likely culprits are habitat loss, climate change, pesticide use, and pathogens. But whatever the reason, the loss of bee species is particularly worrisome to farmers who grow any number of crops that rely on the winged pollinators. Those of us outside Europe may also soon feel the sting: domestic honey bees in the United States are in trouble, increasing farmers’ reliance on wild pollinators. And the investigators think wild pollinators around the world may be declining, too. (Science 313:351–4, 2006)

—S.R.

Survival in the jungle takes more than dumb luck. A recent study of predators and their prey shows that relatively small-brained species are more likely to become a predator’s meal than bigger-brained ones.
     Susanne Shultz and Robin I. M. Dunbar, both evolutionary biologists at the University of Liverpool in England, studied the diets of chimpanzees and five cat species (golden cat, jaguar, leopard, ocelot, and puma) in five forests across Africa and South America. The investigators found that prey species such as antelopes, whose brains are small relative to their body size, were overrepresented in the predators’ diets compared with their numbers in the local environment. By contrast, prey species with relatively large brains, such as monkeys, were underrepresented in the predators’ diets.
     Shultz and Dunbar can’t say whether predators avoid hunting big-brained prey species, or big-brained species are simply better at outwitting their pursuers. Either way, the evolutionary outcome is the same: predation may be a factor in selecting for bigger brains. Ultimately, however, behavior plays a large role in making an animal easy or difficult for a predator to catch. Bigger brains confer greater learning ability and behavioral flexibility, so using one’s head may well enable one to keep it. (Biology Letters, doi:10.1098/ rsbl.2006.0519, 2006)

—N.W.A.

Three views of the left lower jaw of Teilhardina belgica, showing the last two premolars and the three molars. The fossil, from the early Eocene of Belgium, represents the oldest known European primate. This species is intermediate in the lineage starting with the primitive T. asiatica from China and continuing with the more evolved T. brandti and T. americana from Wyoming. Photo Thierry Smith, Royal Belgian Institute of Natural Sciences

The earliest known primate fossils occur in 55-million-year-old geological deposits ranging across Asia, Europe, and North America. That almost simultaneous appearance has long posed a riddle to paleontologists: where did primates originate, and how did they subsequently disperse? Shortly before those early primate fossils were laid down, the Earth began a 100,000-year period of global warming. A new study now shows that the timing of the primates’ rise may have been no coincidence: environmental changes caused by the warming probably enabled them to disperse fast enough to account for their rapid emergence on three continents.
     Like today’s global warming, the ancient warming was caused by massive re-leases of carbon-bearing greenhouse gases. Those releases altered the ratios of various carbon isotopes in the atmosphere, notably decreasing the fraction of carbon-13. Rock layers that trapped air during that period now help paleontologists calibrate the dating of geologic formations worldwide.
     The relative dip in carbon-13 enabled Thierry Smith, a paleontologist at the Royal Belgian Institute of Natural Sciences in Brussels, and two colleagues to date early primate fossils more precisely than ever before. With their new dates, the investigators determined that a small, big-eyed tree-dweller of the genus Teilhardina, and possibly other early primates, originated in Asia, spread to Europe, and then continued on to North America. The entire journey took less than 25,000 years. Smith and his team theorize that the warm temperatures enabled the primates to cross the Atlantic at high latitudes, over a land bridge then linking the two continents via Greenland. Because Teilhardina was strictly arboreal, the investigators think evergreen forest must have covered a wide swath of the north. (PNAS 103:11223–7)

—Ciara Curtin

A trace of life persists in prehistoric fossils. Amphibian bone marrow, discovered in 10-million-year-old fossils of frogs and salamanders from a sulfurous lake in Spain, is so exquisitely preserved that its red and yellow layers of tissue are still visible. The marrow may yield biologically important molecules, such as hemoglobin or even DNA. Maria E. McNamara and Patrick J. Orr, paleobiologists at University College Dublin in Ireland, and four colleagues made the discovery.
     Most fossils form when minerals replace hard tissue, such as bone. Soft tissue decays too rapidly to mineralize, so traces of it are rare in the fossil record. Rarer still is soft tissue preserved organically, without decaying or mineralizing, the way the amphibian bone marrow was. In fact, there is only one other example of organically preserved soft tissue: still-stretchy blood vessels that were discovered last year inside a 70-million-year-old Tyrannosaurus rex femur.
     After the dead amphibians settled on the muddy lake bottom, McNamara’s team postulates, bacteria consumed their skin and muscle, but couldn’t fit through minute pores in their bone to degrade the marrow. Instead, even smaller sulfur molecules seeped in and chemically fixed the marrow in much the same way formaldehyde would.
     Sulfur-rich mud is fairly common in the fossil record, so why is organically preserved soft tissue so extremely rare? Most paleontologists probably never bother to look for it, McNamara says. Her own break came—quite literally—when she was studying fossils that had been cracked, revealing the marrow inside. Many more such finds may be in the offing. (Geology 34:641–4, 2006)

—Edyta Zielinska

As if there weren’t enough ominous consequences of global warming, here’s a little bitty consequence that’s just plain nasty: bigger, badder poison ivy. Already the irksome vine is responsible for more than 350,000 cases of dermatitis annually in the United States alone—and those are just the reported cases. A new study shows that as atmospheric carbon dioxide (CO₂, a major cause of global warming) becomes more abundant, poison ivy proliferates and makes a more toxic form of urushiol, the substance that triggers the rashes.
     From 1996 until 2004 investigators pumped CO₂ into a pine plantation at Duke University in Durham, North Carolina. The idea was to simulate the 54 percent increase in atmospheric CO₂ expected by midcentury from the continued burning of fossil fuels. Jacqueline E. Mohan, a biologist at Duke, and six colleagues report that in the last five years of the study the poison ivy in the plantation grew nearly three times as fast as the plants normally do. And the toxicity of its urushiol shot up by some 33 percent.
      Extra CO₂ boosts photosynthesis in all plants, but vines such as poison ivy invest relatively little of the extra energy they gain in growing more wood for support. Instead, vines can channel most of their extra energy into making even more photosynthetic greenery, a positive feedback loop that is bad news for trees: when vines are on the increase, as they are in many parts of the world, they often interfere with tree growth, and they even kill trees outright by shading or choking them.
     Oh, and did I mention the part about more itching and scratching? (PNAS 103:9086–9, 2006)

—S.R.

	Pumping blood to a giraffe’s brain: is it all heart? Photo courtesy of Il Venerdì, La Repubblica

Rise too quickly from a prone position, and you might see stars, the twinkling signs of your heart’s struggle to send blood up to your suddenly elevated brain. So pity the heart of the giraffe. Its job is to push blood up the carotid artery to a brain towering six feet above. That column of blood is a heavy load. As a result, the base of the giraffe’s carotid artery is the locus of some of the highest blood pressures in the world: twice the pressure in the carotid arteries of people.
     Some physiologists have suggested that the giraffe’s heart may get some help from a siphon effect created by the blood travelling back down the neck veins: perhaps its weight helps pull the blood behind it up the arteries to the head. To test that theory, Graham Mitchell, an animal physiologist at the University of Wyoming in Laramie, and three colleagues built a life-size model of the circulatory system of the giraffe’s neck and head out of PVC pipes, rubber tubing, and a pump.
     A siphon mechanism could indeed assist the “heart,” they discovered, but only when all the “veins” and “arteries” were made of rigid PVC pipe. When the “veins” were made of rubber tubing—which, like a real vein, is collapsible—siphoning was impossible. (The carotid artery is naturally rigid because of the high blood pressure inside.) So it’s just the good old heart that does all the work of moving the giraffe’s blood along, after all. (Journal of Experimental Biology 209:2515–24, 2006)

—S.R.

Copyright © Natural History Magazine, Inc., 2006