The trouble with old bones is that they can’t tell you a whole lot about metabolism and the temperature of blood. We used to think that dinosaur metabolic rates were slow, that they just lumbered across the landscape, making giant thuds as they went. But then we saw them chasing Jeeps... so which is it? According to a new study, their metabolic rates weren’t slow like salamanders, and they weren’t fast like finches -- they were somewhere in between.

Endotherms like us and birds are so-called warm-blooded animals; we keep our body temperatures constant. If we lose excess heat, metabolism increases to make up the loss. Ectothermic animals (often imprecisely labeled as “cold-blooded”) need external sources to regulate their body temperature: On a hot road, a snake can strike fast, but throw it on ice, and it’ll just chill out.

But it doesn’t have to be dichotomy. There’s a middle path, and dinosaurs may be the earliest group to display this intermediate energetic profile -- called mesothermy, or maybe “lukewarm-bloodedness.” Some mesotherms we see today include tuna, leatherback sea turtles, spiky egg-laying echidnas, and lamind sharks (which include great whites and some of the fastest-swimming sharks in the ocean).

A team led by John Grady from the University of New Mexico devised a new way to analyze the metabolism of extinct animals by looking not only at their growth rate, but also at estimates of metabolic rates based on changes in body size as the animal develops from a baby to an adult. They used a comparative dataset with a wide spectrum of extinct and living vertebrates -- 381 animals, 21 of which are dinosaurs -- to analyze the relationship between growth and metabolic rates.

To determine growth rate for extinct animals, they looked at the yearly growth rings in the fossils; like tree rings, but in bone. (Below is a diagram of comparative growth rates in vertebrates.) Then they demonstrated that animals who grow faster require more energy and have higher body temperatures. Based on growth estimates, the team was able to calculate dinosaur metabolic rates. ​

According to their findings, dinosaur metabolism is closest to existing mesotherms who rely on internally-generated metabolic heat to maintain their body temperatures in various weather -- but only to a point. They aren’t entirely endothermic and don’t regulate their body heat at a constant temperature. Echidnas, for example, rely on their metabolism to reach about 31 degrees Celsius, but they can vary plus or minus 10 degrees. Meanwhile tuna stay up to 20 degrees Celsius warmer than their surrounding water, but when they dive deep, their metabolic rate also plunges.

Interestingly, feathered dinosaurs and primitive birds grew slower than their descendants, modern birds. The first bird, Archaeopteryx, took two years to reach maturity, whereas a red-tailed hawk about the same size takes just 6 weeks. While dinosaurs didn’t grow as fast as modern birds or mammals, they did grow significantly faster than modern reptiles. “This higher energy use probably increased speed and performance,” Grady says in a news release. “Mesothermic dinosaurs were likely faster predators or better able to flee from danger than the large reptiles found earlier in the Mesozoic.”

Mesothermy in dinosaurs may have helped them become ecologically dominant and probably helped them become so massive. “A lion the size of a T-Rex,” study coauthor Felisa Smith of UNM says, “while a frightening thought, would quickly starve to death because it would be so hard to find enough food.” A medium-powered energetic strategy likely conferred an advantage over slow-moving reptiles, Grady explains, but without the high overhead costs of modern birds and mammals.

The work was published in Science this week.

[University of New Mexico via Nature, National Geographic]

Images: John Grady