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A mesotherm () is a type of animal with a thermoregulation intermediate to cold-blooded and warm-blooded .
The first trait distinguishes mesotherms from , the second from endotherms. For instance, endotherms, when cold, will generally resort to shivering or metabolizing brown fat to maintain a constant body temperature, leading to higher . A mesotherm, however, will experience lower body temperatures and lower metabolic rates as ambient temperature drops. In addition, mesotherm body temperatures tend to rise as body size increases (a phenomenon known as gigantothermy), unlike endotherms. This reflects the lower surface area to volume ratio in large , which reduces rates of heat loss.
While extant taxon mesotherms are relatively rare, good examples include tuna, (e.g., the great white shark), the leatherback sea turtle, some species of bee, naked mole rats, , and .
Historically, the same word was used by de Candolle to describe that require a moderate degree of heat for successful growth. In his scheme, a mesotherm plant grew in regions where the warmest month had a mean temperature greater than and the coldest month had a mean temperature of at least .
This viewpoint was challenged by D'Emic, who argued that because growth rates are sensitive to seasonal variation in resources, dinosaur maximum growth rates were underestimated by Grady et al. Adjusting dinosaur rates upwards by a factor of two, D'Emic found dinosaurs to grow similarly to , and thus were likely endothermic. However, sensitivity to seasonal variation in resources should be true for all vertebrates. If all vertebrate taxa were similarly adjusted, the relative differences in rates does not change. Dinosaurs remain intermediate growers and good candidates for mesothermy.
Nonetheless, the dinosaur mesothermy hypothesis requires further support to be confirmed. Fossil , which can reveal an organism's body temperature, should be particularly informative. Recently, a study of theropod and sauropod isotopes offered some support for dinosaur mesothermy. Feathered theropods are probably the best candidates for dinosaur endothermy, yet the examined theropods had relatively low body temperatures . Large sauropods had higher body temperatures , which may be reflective of mesothermic gigantothermy. Future isotopic analysis of small, juvenile dinosaurs will better resolve this question.
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