An Aug. 12 article on the growth rate of Tyrannosaurus rex misspelled a researcher's name. The correct spelling is Peter J. Makovicky. (Published 8/13/04)

If you think today's teenagers are difficult to live with, consider Tyrannosaurus rex, which at age 15 was gaining about 4.6 pounds per day -- 1,679 pounds per year -- and not from gobbling peanut butter sandwiches.

Life was almost certainly a picnic -- all the time: "Yes, that would certainly be one corollary of this study," said paleontologist Peter J. Mackovicky of Chicago's Field Museum. "To maintain a high growth rate, these animals would have to eat a lot."

Mackovicky is part of a six-member team that used seasonal rings in T. rex bones to determine the legendary carnivore's rate of growth. The results, reported in today's issue of the journal Nature, were billed by the Field Museum as "the ultimate teenage growth spurt."

The team, led by Gregory M. Erickson of Florida State University, examined bones from seven of the world's 30 known T. rex specimens, including the Field Museum's own Sue, to determine how fast body mass increased at various ages.

They found that T. rex weighed about a ton at 13 but about a year later embarked on a four-year growth binge, to reach about 4.5 tons by age 18, after which it began to level off to an adult weight of around about six tons by 20.

Mackovicky estimated T. rex's life span at 30 years, based on the fact that Sue, named for the researcher who discovered it, is the oldest known specimen and was suffering from bone lesions and arthritis when it died at 28.

"What we wanted to do was figure out how dinosaurs got so big," Mackovicky said in a telephone interview. "Let's face it. That's one of the things that fascinate all of us."

Mackovicky said researchers have known for years that dinosaurs, like modern reptiles, grew seasonally, and that dinosaur bones showed the seasons in rings much like tree rings.

Evidence from modern animals suggests that growth rate has little or nothing to do with the reptiles' cold-bloodedness, which makes them less active in cold weather, Mackovicky said. Instead, "it's a deep-seated physiological link." They grow the way they grow because that's the way they are.

For decades, researchers had difficulty assessing dinosaur growth rates because weight-bearing bones, which allowed them to estimate body mass, develop increasingly large marrow cavities in their centers. The cavities erode the interior growth rings, making it hard to estimate the animal's age even as new outer rings are laid down.

Mackovicky credited Erickson for finding that non-weight-bearing dinosaur bones -- ribs, pubic bones, belly bones and some leg bones -- did not have marrow cavities and thus displayed the entire growth record. Also, it was easier to obtain the smaller bones for the invasive cross-cutting needed to expose the ring history.

The team examined 60 bones from 20 skeletons, including seven T. rex and three smaller, related species with slower growth rates. Each of the specimens also had to have available a weight-bearing thighbone so the team could correlate age to body mass.

What the team discovered was that T. rex grew very large, very quickly, to become a fearsome predator. The adult T. rex was 40 feet long and 13 feet tall, with a five-foot-long skull and 50 teeth, each about eight inches long. T. rex became extinct about 65 million years ago.

Besides the spectacular growth rate, the new research may offer fresh insights on what kind of hunter T. rex was. The "conundrum," Mackovicky said, is that the growth spurt began just after its weight had reached 2,000 pounds, changing T. rex from a fleet chaser into more of a muscle-bound lumberer.

"Maybe they go after bigger, slower animals," Mackovicky suggested. "But it's interesting because their mass-to-muscle ratio is leading them to this biomechanical boundary zone just when they're going into super-puberty," with their growth rate taking off.

Or maybe they hunted in packs, with younger, faster dinosaurs chasing prey toward the stronger adults.

Or maybe larger T. rex abandoned the chase altogether and spent lunchtime stealing kills from smaller predators or scavenging carcasses.

"You have to keep in mind, though, that there are two kinds of speed," said evolutionary biomechanist John R. Hutchinson, lead author of a 2002 study that exposed T. rex as a relative plodder. "You run fast relative to your body size, or you run fast in absolute terms."

Young T. rex, Hutchinson suggested, had a fast relative speed -- body-lengths per second -- but began to lose that at 2,000 pounds. Absolute speed, about 25 mph maximum, might stay the same for a while, because the animal could compensate for loss of quickness with longer strides. And although an adult T. rex might slow to a maximum of 10 or 12 mph, only elite human runners can do better.

"We can't achieve the kind of precision that says T. rex got to a certain age and got slower," Hutchinson said in a telephone interview from his University of London office. "We can say running ability decreased with size, and did so quickly, because growth occurred quickly. I wouldn't go much further than that."

An adult T. rex would have had a five-foot-long skull and 50 teeth.