Thermogenesis
[1] Depending on whether or not they are initiated through locomotion and intentional movement of the muscles, thermogenic processes can be classified as one of the following: One method to raise temperature is through shivering.
[10] In this process, substances such as free fatty acids (derived from triacylglycerols) remove purine (ADP, GDP and others) inhibition of thermogenin, which causes an influx of H+ into the matrix of the mitochondrion and bypasses the ATP synthase channel.
[14] The low demands of thermogenesis mean that free fatty acids draw, for the most part, on lipolysis as the method of energy production.
[17] Rather, avian (birds) and eutherian (placental mammalian) lineages developed the ability to perform thermogenesis independently through separate evolutionary processes.
[17] The fact that the same evolutionary character evolved independently in two different lineages after their last known common ancestor means that thermogenic processes are classified as an example of convergent evolution.
The reason that avians and eutherians both developed the capacity to perform thermogenesis is a subject of ongoing study by evolutionary biologists, and two competing explanations have been proposed to explain why this character appears in both lineages.
This high level of care is theorized to give new born or hatched animals the opportunity to mature more rapidly because they have to expend less energy to satisfy their food, shelter, and temperature needs.
The primary difference between the two theories is that the "parental care" model proposes that a specific biological function (childcare) resulted in selective pressure for higher metabolic rates.
Despite both relying on similar explanations for the process by which organisms gained the capacity to perform non-shivering thermogenesis, neither of these explanations has secured a large enough consensus to be considered completely authoritative on convergent evolution of NST in birds and mammals, and scientists continue to conduct studies which support both positions.
BAT NST occurs when Uncoupling Protein 1 (UCP1) performs oxidative phosphorylation in eutherians’ bodies resulting in the generation of heat (Berg et al., 2006, p. 1178).
[22] Skeletal muscle NST might also be used to maintain body temperature in heterothermic mammals during states of torpor or hibernation.
[17] Given that early eutherians and the reptiles which later evolved into avian lineages were either heterothermic or ectothermic, both forms of NST are thought not to have developed fully until after the K-pg extinction roughly 66 million years ago.
This is because early eutherians would not have had the capacity for non-shivering thermogenesis as it currently exists, so they more frequently used torpor and hibernation as means of thermal regulation, relying on systems which, in theory, predate BAT NST.
The proposed model is purely theoretical and relies on the use of light-activated PoXeR pumps integrated into the inner membrane of mitochondria.
[25] However, the method is invasive, relies on gene therapy, and requires several clinical trials as well as hospitalization to integrate the system at the level of white or muscle adipose tissue in the abdominal fat.
