Even the powerful tyrannosaurs seem to have encountered a midlife crisis.
Once they made it to about age 2 they could take on just about any other predator and had very little mortality until they reached sexual maturity in their teens, researchers report in Friday's issue of the journal Science.
"Survivorship stabilized at between two and four percent per year until midlife at which point they went through an honest-to-God midlife crisis," Gregory M. Erickson, who teaches comparative anatomy at Florida State University, said in a telephone interview.
His team studied the remains of several species of North American tyrannosaur, including Albertosaurus, Tyrannosaurus, Gorgosaurus and Daspletosaurus. They used growth rings in bones to calculate the mortality rates for the animals over a lifetime.
They concluded that mortality could be high for youngsters, both because some didn't have much resistance to disease and because of predators. "There were a lot of nasty creatures" around then, Erickson said.
But after about age 2, some 70 percent survived to reach sexual maturity between 13 and 16, when mortality increased to 23 percent a year. Their potential lifespan reached to the late 20s and early 30s.
"I think love was a dangerous game for tyrannosaurs," said Erickson. "As they came into sexual maturity there was a good chance they started having antagonistic encounters with each other, fighting over mates, fighting over nesting areas," he said. Females were also under stress to lay eggs and some may have brooded them, meaning they had to stay at the nest and wouldn't have been able to hunt prey, he said.
It's a life pattern -- high mortality young and then stability until midlife -- that is also seen in some modern birds and mammals, he added.
"One implication of these findings is that the previously mysterious rarity of sub-adult tyrannosaur specimens is due to their exceptionally low mortality rates," co-author Philip J. Currie of the University of Alberta, Canada said in a statement.
The conclusion was a surprise to the researchers, who had thought one large collection of fossils found together was an indication of some sort of catastrophic event that killed them all at the same time.
But it appears the deaths may have occurred over some weeks, Erickson said. He speculated they may have been drawn to a source of water in a drought.
The research was supported by the National Science Foundation.