"As yet, there is probably no evidence that would definitively prove whether or not some dinosaurs were warm-blooded. Scientists have explored numerous lines of evidence to try to answer this question.
"There is a clear difference in bone structure between modern cold-blooded and warm-blooded animals. Warm-blooded animals tend to have highly vascularized bone tissue. Cold-blooded animals, on the other hand, have relatively dense bone, sometimes even showing annual growth rings. Dinosaurs tend to have highly vascularized bone early in life and then develop dense bone with growth rings as they reach maturity. New evidence suggests that the different bone types are more related to growth rates than to warm- or cold-bloodedness.
"In modern animal communities, there are far fewer warm-blooded predators relative to prey than cold-blooded predators. This is because warm-blooded animals tend to eat far more than cold-blooded animals and so a given amount of prey will support far fewer warm-blooded predators. Unfortunately, it is very difficult to determine the actual relative abundance of predator versus prey from the fossils because the fossil record does not always preserve an accurate representation of the original animal community.
"Dinosaurs were able to survive even at high latitudes. For example, dinosaur fossils have been found in Alaska and Canada well above the Arctic Circle. During the time that these animals lived, however, the climate was generally much warmer than today's. It is also possible that these animals migrated into these areas during the warm summer months and retreated to lower latitudes ahead of the cold and long polar nights.
"Modern warm-blooded animals tend to have more erect postures than cold-blooded animals. Most dinosaurs have erect postures and therefore it has been suggested that this indicates that they had high activity levels and were warm-blooded. Along these lines, scientists have looked indirectly at the potential blood pressures of dinosaurs; warm-blooded animals tend to have relatively high blood pressures. Blood pressure can be estimated by looking at the vertical distance between the head and the heart. For some sauropod dinosaurs, this estimated blood pressure is very high indeed. Brachiosaurus would have a blood pressure of about 500 millimeters of mercury. This figure is about five times higher than that of a human. On the other hand, other dinosaurs--such as ceratopsians--would have a very low blood pressure, closer to that of living reptiles, based on this analysis.
"Modern warm-blooded animals have relatively larger brains than living cold-blooded animals. It is thought that large brains are needed to coordinate active, highly energetic animals. Most dinosaurs have very small brains relative to their body size. In fact, their brains closely resemble those of modern reptiles. A few small, predatory dinosaurs have relative brain sizes that are comparable with those of some living birds, such as ostriches, however.
"There is evidence from dinosaur trackways, mass accumulations of certain dinosaur fossils, and nesting sites that dinosaurs were social animals. Some have argued that such sophisticated behavior is suggestive of warm-bloodedness.
"Birds are warm-blooded and probably evolved from a group of meat-eating dinosaurs. Therefore, it has been argued, their dinosaur ancestors were also warm-blooded. But recent study of the bone structure of some of the earliest birds has revealed that it resembles that of modern cold-blooded reptiles, suggesting that the first birds were cold-blooded and that warm-bloodedness developed later.
"More recently, some researchers have looked for respiratory turbinates in the nasal passages of dinosaurs. Respiratory turbinates are fine, scroll-shaped bones found in the noses of most modern warm-blooded animals (mammals and birds). These structures are believed to function as moisture-recovery organs, recapturing water from warm and moist exhaled air. Without these structures, many warm-blooded animals would quickly dehydrate, especially in dry climates. So far, no respiratory turbinate structures have been found in dinosaurs. It has been argued, however, that many dinosaurs lived in warm and moist environments where water conservation may not have been important.
"Dinosaurs were a very long-lived and diverse group of animals. There is as yet no widely accepted definitive answer as to whether or not they were warm-blooded. It is quite possible that dinosaurs had a metabolism that is different from that of living animals. Indeed, the large size attained by many dinosaurs may have led to what has been called 'inertial homeothermy' or 'gigantothermy.' That is, the large bulk of the animals would have allowed them to maintain a fairly constant temperature even without a high metabolism. Certainly, dinosaurs were active and, in some cases, social animals that successfully competed with warm-blooded mammals during their 160-million-year-long reign on the earth."
Gregory S. Paul, a noted dinosaur researcher and illustrator, adds his perspective:
"It is not yet known whether there will ever be 'definitive proof' of the metabolic status of dinosaurs. At this time, researchers must use multiple lines of evidence to infer dinosaur energetics. Also, the physiological alternatives are themselves complex, and cannot be adequately described by simple terms such as warm or cold blooded. Some or all dinosaurs may have had high aerobic exercise capacity like birds and mammals (tachyaerobic), or low aerobic exercise capacity like reptiles (bradyaerobic). Dinosaurs may have had high resting metabolic rates like birds and mammals, or low resting rates like reptiles. How well dinosaurs could thermoregulate and control body temperature is yet another unanswered question.
"Large hips suggest that most dinosaurs had large, aerobically capable leg muscles like those of big-hipped birds and mammals, rather than small leg muscles like those of bradyaerobic reptiles. Many dinosaur trackways show that dinosaurs walked as fast as do birds and mammals, and much more rapidly than can be sustained by reptiles. Predatory dinosaurs appear to have developed birdlike respiratory systems able to take in large amounts of oxygen. The herbivorous ornithopod dinosaurs may have had a mammallike diaphragm for efficient respiration. The earliest and most primitive dinosaurs, however, had smaller hips and less advanced respiratory systems, suggesting their aerobic capacity was lower than in other dinosaurs, birds and mammals.
"From the fossil evidence, at least some dinosaurs appear to have grown more rapidly than any wild reptile, suggesting the juveniles were very aerobically active and able to seek out large amounts of food.
"The great size and especially the towering height of the sauropod dinosaurs may have required extra-high-pressure, oversized hearts that consumed much more oxygen than is observed in reptiles.
"The evidence for resting metabolic rates is more ambiguous. Some continental birds and mammals, for example, have poorly developed respiratory turbinates or nasal passages no larger than those of dinosaurs of similar size. Some birds and mammals have very high aerobic exercise-to-resting ratios, but no tachyaerobic animal has a reptilian resting metabolic rate. If most dinosaurs were tachyaerobic, then they probably had high resting metabolic rates as well.
"It is even less clear whether dinosaurs thermoregulated as effectively as do birds and mammals. Growing dinosaurs laid down rings inside their bones more often than birds and mammals, and this hints they did not regulate body temperature to the same degree.
"In conclusion, the evidence that most dinosaurs had high aerobic exercise capacity like birds and mammals is very good. It is probable that their resting metabolic rates were higher than in reptiles. It is possible that many dinosaurs did not maintain a constant body temperature. The energetics of dinosaurs almost certainly were not reptilian, but just how closely they approached the avian-mammalian condition is not yet certain."
Philip N. Froelich, director of the School of Earth and Atmospheric Sciences at the Georgia Institute of Technology, offers yet another view:
"One possibility would be to look at oxygen isotopic abundances in the annual growth bands of teeth or bones of high latitude (and hence, presumably, seasonally influenced) terrestrial dinosaurs. The ratio of oxygen isotopes depends on temperature. An absence of seasonal variations in oxygen 18--a heavy version of the common oxygen 16 atom--would strongly suggest that the animals maintained a constant internal temperatures.
"Such a finding would not, however, constitute 'proof' that dinosaurs were warm blooded, as there are external mechanisms that cold-blooded animals employ to regulate body temperatures and thereby influence metabolic rates. Proof would have to come from discovery of intact dinosaur remains in which the soft tissue had not been replaced or altered, and from which the biomolecules responsible for thermoregulation could be extracted, identified and characterized. Such a proof is very, very unlikely, as it would require an almost impossible level of preservation over 65 million years, plus the advent of biotechnology that does not (yet) exist to elucidate the thermoregulation biochemistry of an extinct species.