Giant Dinosaurs Stuck Their Necks Out, Not Up?

Matt Kaplan
for National Geographic News
May 14, 2009

Long-necked dinosaurs didn't graze treetops, according to new research that suggests the prehistoric animals were better off holding their necks horizontal, not upright.

Lifting long necks at steep angles would have put intense pressure on sauropod hearts, requiring dramatic expenditures of energy to keep blood pumping to the brain, a new study of dinosaur circulation says.

Sauropods were giant, long-necked, long-tailed, four-legged plant-eaters that lived about 200 to 66 million years ago (prehistoric time line).

Since long-necked modern animals, such as giraffes, tend to browse on leaves in tall trees, paleontologists have assumed that sauropods—whose necks could be as long as 30 feet [9 meters]—must have done the same.

But Roger Seymour, of the University of Adelaide in Australia, found that sauropods would have spent as much as 75 percent of their bodies' energy to keep their heads held high.

Most mammals use about 10 percent of their energy to circulate blood through their bodies. Giraffes use about 18 percent of their energy to keep blood moving through their long, upright necks.

"Would the increased availability of food in tall trees be worth the cost? This seems doubtful," Seymour said. "It would probably make more energetic sense for [sauropods] to feed with their necks close to horizontal."

By moving their necks side-to-side horizontally, sauropods would have been able to feed on a very large area of plant material without having to move their bodies.

That may not seem like a much of an energy-saving tactic. But in animals that may have weighed 30 to 40 tons, the energetic difference between taking a few steps and not taking a few steps may have been as huge as the animals themselves.

Expensive Treats

Still, some scientists not involved with Seymour's research argue that, in extreme cases, it may have been worth it for saurpods to spend the extra energy to lift their necks.

The bones and joints in some of these animals show that they could lift their necks between 30 and 60 degrees above horizontal, paleontologist Martin Sander, of the University of Bonn in Germany, said.

When food availability at low and medium heights became scarce, the cost of raising the head to get valuable resources may have been worth it, Sander said.

Richard Cowen, of the University of California, Davis, noted that other animals sometimes expend enormous amounts of energy on food.

Cheetahs, for example, sprint after prey, even though the big cats only make the catch one time in four. Likewise, whales use massive amounts of energy to dive deep into cold water, and migratory birds burn heaps of energy flying thousands of miles.

All of these behaviors could be viewed as incredible energy sinks, but we know they are not because the animals gain something significant in return that makes the energy expenditure worth it, Cowen explained.

"It would be reasonable for sauropods to browse occasionally with heads high, as long as the payoff was also high," Cowen said.

Findings to be published in the June 23, 2009, edition of the journal Biology Letters.

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