This study exposes problems in the current use of terminology regarding the metabolics and thermoregulation of organisms. Endothermy in the Seymour paper is defined as the state in which energy production is high enough to sustain body temperatures within a certain, high temperature zone. Ectothermy is the state in which energy production is so low that thermoregulation depends mainly on external heat. Others have used endothermy to include the presence of a high resting metabolic rate. But the etymology of endothermy only means that most body heat is generated internally, ectothermy merely means that most body heat is absorbed from the environment. If additional attributes are added to these simple terms, then workers are left without terms that define strict ectothermy and endothermy, those terms of course being the ones appropriate for that specific task. The misuse of the term endotherm is particularly problematic. For example, assume a gigantic land animal has a low resting and activity metabolism, but uses its bulk and perhaps other insulation to achieve a stable body temperature in which most of its body heat is generated internally. In that case it is an endotherm. Yet as typically characterized including the Seymour analysis, endotherms are presumed to have an elevated metabolism. It is troubling is how the regular practice of individual researchers to defining these widely used terms in differing manners -- sometimes explicitly, more often not – contributes to considerable confusion. So much so that it is not always clear what a particular worker means when they use these terms. Also dubious is the use of the phrase “good endotherm,” in that doing so can give the unintended impression that some endotherms are “better” than others. An endotherm that is not a “good” endotherm is not a “bad” endotherm.

An effort has therefore been conducted to rationalize the issue via a set of terms that are based on strict etymology [1,2]. It is long been common to refer to organisms with the low basal or resting metabolic rates observed in modern amphibians and reptiles as bradymetabolic, and those with higher resting metabolisms as tachymetabolic. Likewise, an animal with a low aerobic capacity can be labeled bradyaerobic, one that can use oxygen for elevating internal temperatures and/or sustaining exercise levels above the modern reptilian maximum tachyaerobic. An organism that is both bradymetabolic and bradyaerobic is bradyenergetic, one that has both an elevated resting metabolism and oxygen capacity is tachyenergetic. An organism that generates most of its internal heat with high resting and active oxygen consumption is a tachyenergetic endotherm. A giant that manages to use low internal heat production to generate most of its body heat is a bradyenergetic endotherm.

The terms bradyaerobic, bradyenergetic, tachyaerobic and tachyenergetic are much more pertinent to the thesis of this paper than are ectothermic and endothermic that make no direct reference to power production used to sustain activity. Many researchers overestimate the ability of temperature stability to sustain high levels of activity. Seymour understands that power production is the critical factor. He correctly explains how even very large land animals those that are tachyenergetic enjoy a major and critical power and activity advantage over those that are bradyenergetic, even if the later achieve stable body temperatures via large mass. This hypothesis has been explicitly developed into the theory that it is necessary for land animals that weigh more than about a tonne to be at least tachyaerobic (and probably tachymetabolic because the first seems the require the latter in vertebrates) because only high energy animals have the high levels of sustainable aerobic power needed to operate effectively in 1 G [2,3]. The hypothesis has been labeled terramegathermy, which is a superior hypothesis to gigantothermy regarding land giants. Terramegathermy explains why all land tetrapods that are known to have been bradyenergetic have not exceeded a tonne, while a large number of tachyenergetic mammals exceeded that mass in the Cenozoic, and in the Mesozoic multi-tonne dinosaurs exhibit numerous characteristics of being tachyenergetic endotherms.

1. Paul G (2002) Dinosaurs of the Air. Baltimore: The Johns Hopkins University Press.
2. Paul G (2012) Evidence for avian-mammalian aerobic capacity and thermoregulation in Mesozoic dinosaurs. In: M Brett-Surman T Holtz J Farlow. The Complete Dinosaur. Bloomington: Indiana University Press. pp. 819-871.
3. Paul G (1998) Terramegathermy and Cope's Rule in the land of titans. Modern Geology 23: 179-217.