#ALSO OBLIGATORY DISCLAIMER IM NOT AN EXPERT NONE OF THIS IS MY REASEARCH IM JUST A NERD PLEASE DONT YELL AT ME

[libelelle]

hi libelelle! since ur the bird expert, is there a reason why despite being closely related to reptiles most dinosaurs (including birds) are endothermic (warm-blooded) while most reptiles are ectothermic (cold-blooded)? ik dinosaurs being endothermic mostly came from their sheer body mass requiring it (and also why they had feathers), but how did it become so split from reptiles which have primarily remained ectothermic for millions of years?

WOWW this is a really good question and im flattered you'd cme to me to ask it : ] im not technically an expert (YET.) but i can give you an answer!! or at least some interesting info on the topic

first, dinosaurs weren't ALL endothermic, as you know. there is evidence that suggests that many of them were mesothermic (as in, capable of regulating their temperature but not consistently and only to a certain extent) [x] In fact a early birds are believed to have had low metabolic rates as well, being a sort of transistional state between thermal types which allowed the subsequent evolution of flight [x].

the reason for WHY birds are endothermic is debated. Its pretty costly as a strategy, because it relies on burning precious energy to keep the organism heated. As with much of science, there are multiple theories floating around regarding the purpose of endothermy as an adaptation. From what i have gathered from my admittedly hurried and ultimately limited research there are a couple theories that stand out in the debate: one that claims endothermy evolved to expand the organisms thermal niche (basically the temperature range that an organism can tolerate/exists in. apparently not a well defined term [x] if you want to read about that.) and the other claims that it was evolved for increased aerobic capacity (which is basically maximum efficient oxygen usage; this can be a measure of an organism ability to maintian strenuous activity (like. yknow. flying)

A study done on endothermic fish [x] used the repeated evolution of endothermy in Tuna ans their migration into colder waters as evidence for the former, that endothermy evolved as a way to expand an organisms thermal niche. If you ask me, in terms of birds, if we're using this model then likely endothermy evolved in dinosaur ancestors in order to expand from warmer regions into colder region. Dinosaurs existed over a very long period of time, evolving something like 230 mya, and during the time they were alive there was a lot of things happening climate wise (pangaea splitting for example) so theres no doubt that there would have been opportunities for organisms to expand their niche into these new conditions. This is just me spitballing though, im no expert.

On the other side of the coin, one study compared 176 species to study the evolution of metabolic rates[x], which came up with some evidence supporting the aerobic model. Admittedly a lot of this went over my head (i am still learning) but essentially this study provided evidence to the mechanism of the evolution of endothermy under this model. definitely saving this article so that i cant read it thouroughly when i don't have school lighting a fire under my ass. This study aside, in the context of birds, having a high metabolism/aerobic capacity is major for birds, because flying is a very energy intensive skill. Flying requires that they both use oxygen extremely efficiently (look up the respiration system of birds some time. theyre olympic gold medalists in breathing) and that they can use energy efficiently as well. The aerobic model explains the evolution of endothermy in the context of being able to maintain such intense activity so often. The study that proposed this WOULD be really interesting to read but i'm having troubles getting to it thanks to paywalls. growl. heres the link anyways, if you want to look at it [x]. Also a short review that talks about it [x].

ALSO an interesting theory I saw suggests that the advantage of endothermy arose as an adaptation for caring for ones offspring [x], something that being active and warm gives enormous benefits to. Basically, because endothermy allows a bird to control the incubation temps of their eggs and allows for an organism to be more active, endothermy guarentees higher sucess hatching and feeding their chicks. and since reproductive sucess is one of if not the most important thing when it comes to evolution, endothermy evolves in order to facilitate this.

anywasy thats about as much time i can spend on this. theyre making me research wild mustard (BORING BORING BORING) right now

LITERATURE CITED (im practicing....):

Grady JM, Enquist BJ, Dettweiler-Robinson E, Wright NA, Smith FA. 2014. Evidence for mesothermy in dinosaurs. Science. 344(6189): 1268-1272. https://doi.org/10.1126/science.1253143

Erickson GM, et al. 2009. Was Dinosaurian Physiology Inherited by Birds? Reconciling Slow Growth in Archaeopteryx. PLOS one. 4(10): e7390. https://doi.org/10.1371/journal.pone.0007390

Gvoždík L. 2018. Just what is the thermal niche? Oikos. 127(12): 1701-1710. https://doi.org/10.1111/oik.05563

Block BA, Finnerty JR, Stewart AFR, Kidd J. 1993. Evolution of Endothermy in Fish: Mapping Physiological Traits on a Molecular Phylogeny. Science. 260(5105): 210-214. https://doi.org/10.1126/science.8469974

Nespolo RF, Solano-Iguaran JJ, Bozinovic F, Williams TD, Bronstein JL. 2017. Phylogenetic Analysis Supports the Aerobic-Capacity Model for the Evolution of Endothermy. Am Nat. 189(1): 13-27. https://doi.org/10.1086/689598

Bennett AF, Reuben JA. 1979. Endothermy and Activity in Vertebrates. Nature. 206(4419): 649-654. https://doi.org/10.1126/science.493968

Hedrick MS, Hillman SS. 2016. What drove the evolution of endothermy? J Exp Bio. 219 (3): 300–301. https://doi.org/10.1242/jeb.128009

Farmer CG, Garland Jr T, Joseph Travis. 2000. Parental Care: The Key to Understanding Endothermy and Other Convergent Features in Birds and Mammals. Am Nat. 155(3): 326-334. https://doi.org/10.1086/303323