The plant-eating sauropod dinosaurs, such as Brachiosaurus and Apatosaurus (formerly called Brontosaurus), were the largest animals ever to walk on Earth, weighing up to 80 metric tons. Many also sported very long necks—though researchers have debated their purpose. Now, using some fancy mathematics and an analogy with vacuum cleaners, two scientists in the United Kingdom may finally have the answer.
Researchers generally agree about why dinosaurs got so big . In the dino-eat-dino world of the Mesozoic era, stretching from about 230 million to 65 million years ago, large size not only protected against predators but also gave sauropods the huge guts they needed to digest tons of vegetable matter. Some species of Brachiosaurus, for example, were nearly 30 meters long—nearly twice as long as a semitrailer truck.
There's less agreement on why these dinos had such long necks. Brachiosaurus's neck, for example, stretched 9 meters, about five times longer than that of a giraffe. And just like giraffes, some scientists think Brachiosaurus raised up its neck to browse for vegetation in tall trees . Yet other researchers, notably physiologist Roger Seymour of the University of Adelaide in Australia, have argued that the energy costs of pumping blood to a raised 9-meter neck would have been too high and that it was more likely the neck was used to browse for vegetation close to the ground .
Evolutionary ecologists Graeme Ruxton of the University of Glasgow and David Wilkinson of Liverpool John Moores University, both in the United Kingdom, set out to calculate just how advantageous a long neck would have been to Brachiosaurus. Using a simplified mathematical model and varying parameters such as the weight of the body, the length of the neck, and the height of the neck's attachment to the body, the duo found that a longer neck would have increased the energy efficiency of foraging for plants close to the ground because it would reduce how much the dinosaur had to move its huge body. Thus its 9-meter neck gave Brachiosaurus an 80% energy savings  in foraging over what it would have spent had its neck been only 6 meters long, the pair reports online today in Biology Letters.
The authors say that the energy savings might have even been enough to allow Brachiosaurus to raise its head and browse treetops. Nevertheless, they found that neck length would not have increased indefinitely, because there would have been sharply less energy savings from increasing neck length beyond 9 meters.
Brachiosaurus was something like the kind of clunky cylinder vacuum cleaners manufactured from the 1950s to the 1970s, the researchers suggest, with a heavy body that stayed in one place while a long tube was used to vacuum a room. The mammals that replaced dinosaurs, on the other hand, were more like the lighter, portable vacuum cleaners of today that can be moved about more readily, they say.
"The study is a valuable contribution for understanding the evolution of the long necks of sauropods," says biologist Andreas Christian of the University of Flensburg in Germany. The paper adds to growing evidence that the evolution of large body size and long necks was the result of "a complex interplay" of factors, including the fact that many sauropods did not chew their food but more or less vacuumed it up, adds paleontologist Martin Sander of the University of Bonn in Germany. Had they heavy jaws and teeth, he says, they would not have been able to support the weight of their diminutive heads, which were very small for their large size.
Seymour, for his part, agrees that the authors use a "valid mathematical argument to show that a long neck would have been energetically useful." But he "strongly" disagrees that those necks could have been raised high, arguing that the animal would have needed 50% of its entire energy stores to pump blood to a raised head. "The present paper shows energetically that low browsing with a long neck is advantageous," Seymour contends. "Let's just leave it there and not ask the sauropods to raise their necks."