Mesothermy

The question of whether dinosaurs were cold-blooded or warm-blooded has been one of the longest-running debates in paleontology, dating back to the 19th century when Richard Owen first coined the term "Dinosauria." During the "Dinosaur Renaissance" of the 1960s and 1970s, Robert Bakker and John Ostrom championed the view that dinosaurs were active, warm-blooded animals more akin to mammals and birds than to modern reptiles. However, the fossil record provided ambiguous signals: rapid bone growth suggested high metabolic rates, while other evidence was less conclusive. Prior to the formal proposal of mesothermy, the debate was largely framed as a binary—either dinosaurs were endotherms or they were ectotherms. The 2014 study by Grady et al. offered a third possibility.

Published in Science on June 13, 2014 (DOI: 10.1126/science.1253143), this study compiled ontogenetic growth data for 381 extant and fossil vertebrate species, including 21 dinosaur taxa representing all major clades. The researchers employed a metabolic scaling approach to investigate the relationship between growth rate, metabolic rate, and body temperature.

The key findings were as follows. First, the study demonstrated that animals with faster growth rates have higher metabolic rates and higher body temperatures, with mammals growing approximately ten times faster than reptiles and exhibiting metabolic rates ten times higher. Second, after correcting for effects of body size and temperature, dinosaur metabolic rates were found to be intermediate between those of modern endotherms and ectotherms, clustering closest to extant mesotherms such as tuna, lamnid sharks, and leatherback turtles. Third, feathered dinosaurs and primitive birds such as Archaeopteryx grew distinctly slower than modern birds: Archaeopteryx took approximately two years to reach adult size, compared to about six weeks for a similarly sized modern red-tailed hawk. Fourth, the study concluded that the modern endotherm–ectotherm dichotomy is "overly simplistic" and proposed mesothermy as a distinct thermoregulatory category.

The research was conducted at the University of New Mexico, supervised by Professor Felisa A. Smith and Professor Brian J. Enquist of the University of Arizona. Lead author John M. Grady compiled an extensive database of over 30,000 rows of growth and energy-use data.

Tunas (Thunnus spp.): Tunas conserve metabolic heat generated by their swimming muscles through vascular countercurrent heat exchangers (retia mirabilia), maintaining body temperatures up to 20°C above the surrounding water. However, when diving into deep, cold water, their metabolic rates can drop substantially, demonstrating that their thermoregulation is not as tightly controlled as in true endotherms. Cruising speed in fishes with regional endothermy is approximately 2.4 times faster than in ectothermic fishes of comparable size and temperature.

Lamnid sharks (Lamnidae): Great white sharks (Carcharodon carcharias), shortfin mako sharks, and other lamnids exhibit regional endothermy, maintaining elevated temperatures in their swimming muscles, brain, and eyes through countercurrent heat exchange systems. This allows them to remain active predators in cold temperate and even subpolar waters. A 2023 study published in PNAS (Griffiths et al.) demonstrated that the extinct megatooth shark Otodus megalodon likely also exhibited endothermic or mesothermic physiology.

Leatherback sea turtles (Dermochelys coriacea): The largest living sea turtle uses a combination of large body size, thick subcutaneous fat layers, and heat generated from vigorous swimming to maintain body temperatures substantially above ambient water temperatures. A 2021 study published in Ecosphere (Dodge et al.) showed that mesothermy allows leatherbacks to forage in cool-temperate waters but requires behavioral thermoregulation through altered dive patterns as sea surface temperatures change.

Echidnas (Tachyglossus aculeatus): These monotremes maintain a relatively low body temperature of approximately 30.7°C—well below the 37°C typical of placental mammals—and exhibit wide fluctuations in body temperature, dropping substantially during torpor and hibernation. Their metabolic profile is often described as intermediate between that of "higher" placental mammals and "lower" ectothermic vertebrates, making them a frequently cited example of mesothermic physiology among mammals.

Grady and colleagues argued that mesothermy may have been a key factor in the ecological dominance of dinosaurs over 130 million years during the Mesozoic Era. The medium-powered metabolic strategy would have provided a performance advantage over contemporary ectothermic reptiles—enabling faster locomotion, more sustained activity, and improved predator avoidance—while avoiding the enormous caloric demands of full endothermy. As Professor Smith noted, a mammal the size of Tyrannosaurus rex would face extreme difficulty finding sufficient food, whereas a mesothermic dinosaur of the same size could sustain itself on substantially less energy. This energetic efficiency may also have facilitated the evolution of gigantic body sizes seen in sauropods and large theropods.

The mesothermy hypothesis has been subject to significant scholarly debate since its publication.

D'Emic (2015) critique: Michael D'Emic of Adelphi University (later Stony Brook University) argued in a 2015 Science comment that Grady et al. underestimated dinosaur growth rates by assuming year-round continuous growth. Most dinosaurs lived in highly seasonal environments, and lines of arrested growth (LAGs) in dinosaur bones indicate periodic pauses in growth, likely during dry or cold seasons. D'Emic argued that dinosaurs may have grown during only three to nine months of each year, meaning that the bone tissue between LAGs represents only a fraction of a full year. Doubling the growth rates to account for seasonal growth pauses and running them through the same regression framework yielded values clustering around modern placental mammals, suggesting endothermic physiology.

Myhrvold (2015) critique: Nathan Myhrvold independently criticized the statistical methodology used by Grady et al. to estimate growth rates, arguing that deviations from accepted statistical practice produced an artificial intermediate signal.

Grady et al. (2015) response: The original authors acknowledged minor data errors but maintained that incorporating Myhrvold's statistical recommendations had "virtually no effect" on their conclusions. Regarding D'Emic's critique, they argued that basing maximum growth rates on seasonal growth spurts without mathematical corrections would introduce systematic bias. They reaffirmed that dinosaurs of various sizes and phylogenetic positions fell within the mesothermic range.

A landmark 2020 study by Robin R. Dawson and colleagues, published in Science Advances, employed clumped isotope paleothermometry (Δ47) on well-preserved dinosaur eggshells to directly estimate body temperatures across all three major dinosaurian lineages. The results showed that Troodon formosus (a paravian theropod closely related to birds) exhibited body temperatures ranging from 27°C to 38°C—a ~10°C range suggestive of heterothermic or mesothermic thermoregulation. In contrast, the hadrosaurid Maiasaura peeblesorum yielded a body temperature of approximately 44°C, well within the range of modern endothermic birds. A Romanian eggshell tentatively attributed to a dwarf titanosaur sauropod (~900 kg) yielded a temperature of 36°C, similar to estimates for giant sauropods (~35–38°C) despite an at least tenfold difference in body mass. The study concluded that metabolically controlled thermoregulation was likely the ancestral condition for Dinosauria, while acknowledging that different lineages may have exhibited distinct thermoregulatory strategies along the ectotherm–endotherm spectrum.

Gigantothermy (also called inertial homeothermy) is the phenomenon whereby very large ectothermic animals maintain relatively stable and elevated body temperatures through their low surface-area-to-volume ratio, which slows heat loss. This mechanism has been proposed for giant sauropods, whose body masses of 10,000 to 100,000 kg would have resulted in thermal inertia sufficient to buffer body temperature against environmental fluctuations. However, the Dawson et al. (2020) finding that dwarf Romanian titanosaurs may have maintained body temperatures similar to their colossal relatives—despite being an order of magnitude smaller—challenges the gigantothermy model. This suggests that metabolic heat production, rather than passive thermal inertia alone, played a fundamental role in dinosaurian thermoregulation. The distinction between mesothermy and gigantothermy is therefore important: mesothermy implies active metabolic heat generation, whereas gigantothermy is a passive physical consequence of large body size.

Dinosaurs ranged from pigeon-sized feathered coelurosaurs to 30-meter-long sauropods and persisted for over 180 million years across a vast range of environments. It would be surprising if all dinosaurs shared the same physiological profile. Current evidence suggests a range of strategies: small, feathered maniraptorans close to the bird lineage may have been closer to full endothermy; massive sauropods may have benefited from gigantothermy as a supplementary mechanism; and intermediate-sized dinosaurs may have exemplified classic mesothermy. The evolution of insulating structures such as feathers and filamentous integument—now documented in theropods and some ornithischians—may have been driven by selection for heat retention in smaller-bodied taxa before being co-opted for display or flight. As paleontologist Brian Switek (now Riley Black) noted in a 2015 National Geographic article, "it would actually be surprising if all dinosaurs...all shared the same physiology."

As of the mid-2020s, no full consensus has been reached on dinosaur thermoregulation. Mesothermy remains one of several competing hypotheses, alongside full endothermy (supported by D'Emic and others) and variable thermoregulation models. There is broad agreement that non-avian dinosaurs were not fully ectothermic like modern lizards and crocodilians—they grew too fast, were too active, and occupied too many ecological niches for a purely ectothermic physiology. The concept of mesothermy has had a lasting impact by moving the field beyond the endotherm–ectotherm binary and toward a view of thermoregulation as a continuum. Future research using clumped isotope analysis of additional dinosaur taxa, comparative physiology of extant mesotherms, and refined growth-rate estimation methods is expected to further clarify where different dinosaur lineages fell along this spectrum.