Some dinosaurs had the capacity to raise their body temperature using heat sources in the environment, such as the sun, and may have been surprisingly good sprinters, say scientists, including one of Indian-origin.
Scientists have long debated the nature of dinosaurs' body temperatures and how those temperatures influenced their activity levels.
Researchers from the University of California - Los Angeles (UCLA) have found that some dinosaurs, at least, had the capacity to elevate their body temperature using heat sources in the environment, such as the sun.
The animals were probably more active than modern-day alligators and crocodiles, which can be active and energetic, but only for brief spurts.
The researchers also found evidence that other dinosaurs they studied had lower body temperatures than modern birds, their only living relatives, and were probably less active.
Led by Robert Eagle, a researcher in the department of earth, planetary and space sciences in the UCLA College, the scientists examined fossilised dinosaur eggshells from Argentina and Mongolia.
Analysing the shells' chemistry allowed them to determine the temperature at which the eggshells formed.
"This technique tells you about the internal body temperature of the female dinosaur when she was ovulating," said Aradhna Tripati, a co-author of the study and a UCLA assistant professor of geology, geochemistry and geobiology.
The Argentine eggshells, which are approximately 80 million years old, are from large, long-necked titanosaur sauropods. The shells from Mongolia's Gobi desert, 71 million to 75 million years old, are from oviraptorid theropods.
Sauropods' body temperatures were warm - over 37 degrees Celsius, according to the study. The smaller dinosaurs had lower temperatures, probably below 32 degrees Celsius.
Warm-blooded animals, or endotherms, produce heat internally and typically maintain their body temperature, regardless of the temperature of their environment; they do so mainly through metabolism.
Cold-blooded animals, or ectotherms, including alligators, crocodiles and lizards, rely on external environmental heat sources to regulate their body temperature.
"The temperatures we measured suggest that at least some dinosaurs were not fully endotherms like modern birds," Eagle said.
"They may have been intermediate - somewhere between modern alligators and crocodiles and modern birds; certainly that's the implication for the oviraptorid theropods," he said.
"This could mean that they produced some heat internally and elevated their body temperatures above that of the environment but didn't maintain as high temperatures or as controlled temperatures as modern birds," he added.
"If dinosaurs were at least endothermic to a degree, they had more capacity to run around searching for food than an alligator would," he said.