Mesotherm
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A mesotherm () is a type of animal with a thermoregulatory strategy intermediate to cold-blooded
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 ...
s and warm-blooded
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.


Definition

Mesotherms have two basic characteristics: # Elevation of body temperature via metabolic production of
heat In thermodynamics, heat is energy in transfer between a thermodynamic system and its surroundings by such mechanisms as thermal conduction, electromagnetic radiation, and friction, which are microscopic in nature, involving sub-atomic, ato ...
. # Weak or absent metabolic control of a particular
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 ...
. The first trait distinguishes mesotherms from
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 ...
s, the second from endotherms. For instance, endotherms, when cold, will generally resort to
shivering Shivering (also called shuddering) is a bodily function in response to cold and extreme fear in warm-blooded animals. When the core body temperature drops, the shivering reflex is triggered to maintain homeostasis. Skeletal muscles begin to sha ...
or metabolizing brown fat to maintain a constant body temperature, leading to higher metabolic rates. 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 A phenomenon ( phenomena), sometimes spelled phaenomenon, is an observable Event (philosophy), event. The term came into its modern Philosophy, philosophical usage through Immanuel Kant, who contrasted it with the noumenon, which ''cannot'' be ...
known as gigantothermy), unlike endotherms. This reflects the lower surface area to volume ratio in large
animal Animals are multicellular, eukaryotic organisms in the Biology, biological Kingdom (biology), kingdom Animalia (). With few exceptions, animals heterotroph, consume organic material, Cellular respiration#Aerobic respiration, breathe oxygen, ...
s, which reduces rates of heat loss. While
extant Extant or Least-concern species, least concern is the opposite of the word extinct. It may refer to: * Extant hereditary titles * Extant literature, surviving literature, such as ''Beowulf'', the oldest extant manuscript written in English * Exta ...
mesotherms are relatively rare, good examples include
tuna A tuna (: tunas or tuna) is a saltwater fish that belongs to the tribe Thunnini, a subgrouping of the Scombridae ( mackerel) family. The Thunnini comprise 15 species across five genera, the sizes of which vary greatly, ranging from the bul ...
, lamnid sharks (e.g., the
great white shark The great white shark (''Carcharodon carcharias''), also known as the white shark, white pointer, or simply great white, is a species of large Lamniformes, mackerel shark which can be found in the coastal surface waters of all the major ocea ...
), the leatherback sea turtle, some species of bee, naked mole rats, hyraxes, and
echidna Echidnas (), sometimes known as spiny anteaters, are quill-covered monotremes (egg-laying mammals) belonging to the Family (biology), family Tachyglossidae , living in Australia and New Guinea. The four Extant taxon, extant species of echidnas ...
s. Historically, the same word was used by de Candolle to describe
plant Plants are the eukaryotes that form the Kingdom (biology), kingdom Plantae; they are predominantly Photosynthesis, photosynthetic. This means that they obtain their energy from sunlight, using chloroplasts derived from endosymbiosis with c ...
s 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 .


Dinosaur thermoregulation

The thermoregulatory status of
dinosaur 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 ...
s has long been debated, and is still an active area of research. The term 'mesothermy' was originally coined to advocate for an intermediate status of
non-avian dinosaur Dinosaurs are a diverse group of reptiles of the clade Dinosauria. They first appeared during the Triassic period, between 243 and 233.23 million years ago (mya), although the exact origin and timing of the evolution of dinosaurs is ...
thermoregulation, between endotherms and ectotherms. A more technical definition was provided by Grady ''et al'', who argued for dinosaur mesothermy on the basis of their intermediate growth rates, and the empirical relationship between growth, metabolism and thermoregulation in extant
vertebrate Vertebrates () are animals with a vertebral column (backbone or spine), and a cranium, or skull. The vertebral column surrounds and protects the spinal cord, while the cranium protects the brain. The vertebrates make up the subphylum Vertebra ...
s. 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
mammal A mammal () is a vertebrate animal of the Class (biology), class Mammalia (). Mammals are characterised by the presence of milk-producing mammary glands for feeding their young, a broad neocortex region of the brain, fur or hair, and three ...
s, and thus were likely endothermic. However, sensitivity to seasonal variation in resources should be true for all vertebrates. If all vertebrate
taxa In biology, a taxon (back-formation from ''taxonomy''; : taxa) is a group of one or more populations of an organism or organisms seen by taxonomists to form a unit. Although neither is required, a taxon is usually known by a particular name and ...
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 A fossil (from Classical Latin , ) is any preserved remains, impression, or trace of any once-living thing from a past geological age. Examples include bones, shells, exoskeletons, stone imprints of animals or microbes, objects preserve ...
oxygen isotopes, which can reveal an organism's body temperature, should be particularly informative. Recently, a study of
theropod Theropoda (; from ancient Greek , (''therion'') "wild beast"; , (''pous, podos'') "foot"">wiktionary:ποδός"> (''pous, podos'') "foot" is one of the three major groups (clades) of dinosaurs, alongside Ornithischia and Sauropodom ...
and
sauropod Sauropoda (), whose members are known as sauropods (; from '' sauro-'' + '' -pod'', 'lizard-footed'), is a clade of saurischian ('lizard-hipped') dinosaurs. Sauropods had very long necks, long tails, small heads (relative to the rest of their b ...
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.


See also

* Heterothermy * Gigantothermy *
Poikilotherm A poikilotherm () is an animal (Greek ''poikilos'' – 'various', 'spotted', and ''therme'' – 'heat') whose internal temperature varies considerably. Poikilotherms have to survive and adapt to environmental stress. One of the most important s ...
*


References

{{Reflist, 2 Thermoregulation