Gigantothermy (sometimes called ectothermic homeothermy or inertial homeothermy) is a phenomenon with significance in
biology
Biology is the scientific study of life and living organisms. It is a broad natural science that encompasses a wide range of fields and unifying principles that explain the structure, function, growth, History of life, origin, evolution, and ...
and
paleontology
Paleontology, also spelled as palaeontology or palæontology, is the scientific study of the life of the past, mainly but not exclusively through the study of fossils. Paleontologists use fossils as a means to classify organisms, measure ge ...
, whereby large, bulky
ectotherm
An ectotherm (), more commonly referred to as a "cold-blooded animal", is an animal in which internal physiological sources of heat, such as blood, are of relatively small or of quite negligible importance in controlling body temperature.Dav ...
ic animals are more easily able to maintain a constant, relatively high
body temperature
Thermoregulation is the ability of an organism to keep its body temperature within certain boundaries, even when the surrounding temperature is very different. A thermoconforming organism, by contrast, simply adopts the surrounding temperature ...
than smaller animals by virtue of their smaller
surface-area-to-volume ratio
The surface-area-to-volume ratio or surface-to-volume ratio (denoted as SA:V, SA/V, or sa/vol) is the ratio between surface area and volume of an object or collection of objects.
SA:V is an important concept in science and engineering. It is use ...
. A bigger animal has proportionately less of its body close to the outside environment than a smaller animal of otherwise similar shape, and so it gains heat from, or loses heat to, the environment much more slowly.
The phenomenon is important in the biology of ectothermic
megafauna
In zoology, megafauna (from Ancient Greek, Greek μέγας ''megas'' "large" and Neo-Latin ''fauna'' "animal life") are large animals. The precise definition of the term varies widely, though a common threshold is approximately , this lower en ...
, such as large
turtle
Turtles are reptiles of the order (biology), order Testudines, characterized by a special turtle shell, shell developed mainly from their ribs. Modern turtles are divided into two major groups, the Pleurodira (side necked turtles) and Crypt ...
s, and aquatic reptiles like
ichthyosaur
Ichthyosauria is an order of large extinct marine reptiles sometimes referred to as "ichthyosaurs", although the term is also used for wider clades in which the order resides.
Ichthyosaurians thrived during much of the Mesozoic era; based on fo ...
s and
mosasaur
Mosasaurs (from Latin ''Mosa'' meaning the 'Meuse', and Ancient Greek, Greek ' meaning 'lizard') are an extinct group of large aquatic reptiles within the family Mosasauridae that lived during the Late Cretaceous. Their first fossil remains wer ...
s. Gigantotherms, though almost always ectothermic, generally have a body temperature similar to that of
endotherm
An endotherm (from Greek ἔνδον ''endon'' "within" and θέρμη ''thermē'' "heat") is an organism that maintains its body at a metabolically favorable temperature, largely by the use of heat released by its internal bodily functions inst ...
s. It has been suggested that the larger
dinosaurs
Dinosaurs are a diverse group of reptiles of the clade Dinosauria. They first appeared during the Triassic Geological period, period, between 243 and 233.23 million years ago (mya), although the exact origin and timing of the #Evolutio ...
would have been gigantothermic, rendering them virtually
homeothermic
Homeothermy, homothermy, or homoiothermy () is thermoregulation that maintains a stable internal body temperature regardless of external influence. This internal body temperature is often, though not necessarily, higher than the immediate envir ...
.
Disadvantages
Gigantothermy allows animals to maintain body temperature, but is most likely detrimental to endurance and muscle power as compared with endotherms due to decreased anaerobic efficiency.
Mammals' bodies have roughly four times as much surface area occupied by
mitochondria
A mitochondrion () is an organelle found in the cells of most eukaryotes, such as animals, plants and fungi. Mitochondria have a double membrane structure and use aerobic respiration to generate adenosine triphosphate (ATP), which is us ...
as reptiles, necessitating larger energy demands, and consequently producing more heat to use in thermoregulation. An ectotherm the same size of an endotherm would not be able to remain as active as the endotherm, as heat is modulated behaviorally rather than biochemically. More time is dedicated to basking than eating.
Advantages
Large ectotherms displaying the same body size as large endotherms have the advantage of a slow metabolic rate, meaning that it takes reptiles longer to digest their food. Consequently gigantothermic ectotherms would not have to eat as often as large endotherms that need to maintain a constant influx of food to meet energy demands. Although lions are much smaller than crocodiles, the lions must eat more often than crocodiles because of the higher metabolic output necessary to maintain the lion's heat and energy. The crocodile needs only to lie in the sun to digest more quickly and synthesize
ATP.
See also
*
Allen's rule
Allen's rule is an ecogeographical rule formulated by Joel Asaph Allen in 1877, broadly stating that animals adapted to cold climates have shorter and thicker limbs and bodily Appendage, appendages than animals adapted to warm climates. More speci ...
*
Bergmann's rule
Bergmann's rule is an ecogeographical rule that states that, within a broadly distributed taxonomic clade, populations and species of larger size are found in colder environments, while populations and species of smaller size are found in warmer ...
*
Bradyaerobic
Bradyaerobic is a term used in biology that describes an animal that has low levels of oxygen consumption.
By necessity a bradyaerobic animal can engage in short low or high low-level aerobic exercise
Aerobic exercise, also known as cardio, is ...
*
Bradymetabolism
*
Physiology of dinosaurs
The physiology of dinosaurs has historically been a controversial subject, particularly their thermoregulation. Recently, many new lines of evidence have been brought to bear on dinosaur physiology generally, including not only metabolic systems ...
*
Tachyaerobic Tachyaerobic is a term used in biology to describe the muscles of large animals and birds that are able to maintain high levels or physical activity because their hearts make up at least 0.5-0.6 percent of their body mass and maintain high blood pre ...
References
{{reflist
External links
Gigantothermyat
Davidson
Animal size
Thermoregulation