A Controversy About Warm-Blooded Animals

This Fossil Friday features the exceptionally well-preserved fossil bird Nahmavis grandei from the Eocene Green River Formation of Wyoming, as an example for a fossil representative of warm-blooded animals. Nowadays, every high-school biology class teaches that mammals and birds, even though both are warm-blooded tetrapods, are not closely related and were derived from different reptilian-like ancestors. Their similar physiology is attributed to so-called convergent evolution, thus is claimed to have had an independent evolutionary origin. However, if generally anatomical, physiological, genetic, and behavioral similarities are mostly explained by common descent, why are all warm-blooded animals not grouped together as descendants of a common warm-blooded ancestor? Indeed, based on much earlier observations of John Ray (1693), Charles Darwin’s famous opponent at the British Museum for Natural History, the paleontologist Richard Owen (1866), who had coined the word dinosaur, had first suggested to group birds and mammals together in a taxon Haematothermia, based on their similar warm-blooded physiology.

This was mostly ignored by other biologists until about 120 years later, when the two maverick biologists Søren Løvtrup (1977, 1985) and Brian Gardiner (1982, 1993) took up the idea and again suggested that all warm-blooded vertebrates form a clade Haemothermia, thus birds and mammals would be most closely related sister groups. They both suggested that the complex physiological similarities are unlikely to be convergences and rather represent deep evolutionary homologies. Nobody less than the famous French vertebrate paleontologist Philippe Janvier (1984) even published a reconstruction drawing how a hypothetical ancestor of Haemothermia might have looked like (reproduced by Sivgin 2020). This went against a growing consensus among evolutionary biologists that mammals were derived from synapsid “mammal-like reptiles” like the Permian pelycosaurs, while birds were diapsids more closely related to dinosaurs and crocodiles as well as other living reptiles. Consequently, their suggestion was immediately met with harsh criticism (Benton 1985, 1991, Kemp 1988, Gauthier et al. 1988a, 1988b) and their “radical hypothesis” (Peters 2014) was ultimately rejected as absurd (Kuhn-Schnyder 2009). The reason were the numerous other similarities from skeleton to genomics (e.g., Janke & Arnason 1997) that rather supported the mainstream view.

Pterosaurs and Dinosaurs

However, it must be noted that Gardiner (1993) explicitly agreed that pterosaurs and dinosaurs are close relatives of birds, and “mammal-like reptiles” were relatives and ancestors of mammals. He simply included those reptile-like groups in Haematothermia, and indeed there has been considerable evidence accumulated in the past decades that they also were warm-blooded. Here is a quote from Gardiner’s (1993) abstract:

An exhaustive parsimony analysis of amniote phylogeny using 97 characters has substantiated the hypothesis that mammals and birds are sister groups. This deduction is further supported by parasitological and molecular evidence. The presumed importance of “synapsid” fossils in amniote phylogeny is questioned and it is concluded that they represent a transformation series which, when broken down into constituent monophyletic groups, does not support the separation of the Mammalia from the remainder of the amniotes. Fossil members of the Haematothermia include pterosaurs and “dinosaurs” (both stem-group birds) and Dinocephalia, Dicynodontia, Gorgonopsida and Therocephalia (all stem-group mammals). The Dromaeosauridae are the most crownward stem-group birds and the Morganucodontidae the most crownward stem-group mammals.

Thus, Gardiner (1993) rather suggested that Synapsida and Archosauria are sister groups, which is a hypothesis that is still endorsed by the highly controversial fringe paleontologist David Peters (2024) in his large reptile tree based on 2323 taxa and 236 characters.

Gardiner is said to still have embraced the Haematothermia hypothesis until later in his life (Naish 2008, 2012). Nevertheless, the idea of such a clade of warm-blooded animals was quickly buried and forgotten by the scientific community again, so that the work of Løvtrup and Gardiner is not even mentioned anymore in modern treatises on the origin of endothermy in vertebrates (e.g., Koteja 2004, Nespolo et al. 2011, Benton 2021, Grigg et al. 2022, Faure-Brac et al. 2024). After all, isn’t it really silly to just look at a superficial similarity like warm-bloodedness and ignore all the conflicting evidence. Yes, that might have been silly indeed, but it was not at all what Løvtrup and Gardiner did. Indeed they assembled substantial evidence for their Haematothermia hypothesis that went far beyond only a superficial similarity in physiology, but included a cladistic analysis of 28 specific similarities, of which even the most ardent critics recognized at least 8 as valid (Kemp 1988).

Also, other authors often admitted that birds and mammals share many similarities of the “cardiovascular, renal, gastrointestinal, endocrine and nervous systems” (Carvalho & Gonçalves 2011). Even more recently, a paper on the supposed convergences between birds and mammals published by Wu & Wang (2019) in the Proceedings of the Royal Society, confirmed these similarities and suggesting even more:

Extant birds and mammals share a number of highly similar characteristics, including but not limited to, enhanced hearing, vocal communication, endothermy, insulation, shivering, respiratory turbinates, high basal metabolism, grinding, sustained activity, four-chambered heart, high blood pressure, and intensive parental care.

A Very Incomplete List

Here is a very incomplete list of a dozen complex derived similarities shared by birds and mammals, which I stumbled upon during a quick survey of the recent scientific literature I made for this article:

1. Visceral endothermy or warm-bloodedness means that birds and mammals share the ability to maintain a stable internal body temperature, a characteristic crucial for active living in a wide range of environmental conditions (Nespolo et al. 2011). If this endothermy would be homologous in birds and mammals we should expect that they acquired this trait at the same time, which is exactly what we find (Benton 2021), allegedly based on a shared adaptation to nocturnality in their early evolution (Wu & Wang 2019). Of course, the warm-bloodedness correlates with high metabolic rates in birds and mammals, compared to most reptiles, supporting their increased energetic demand for sustained activity and thermoregulation.
2. Mammals and birds possess a four-chambered heart that efficiently separates oxygenated and deoxygenated blood, facilitating high metabolic rates required for endothermy. However it must be noted that a four-chambered heart is also present in crocodylians, who may have secondarily reverted to ectothermy (Grigg et al. 2022). With this knowledge I suppose that Gardiner would have decided to include crocs in his Haematothermia clade of synapsids and archosaurs.
3. Both birds and mammals posses integumental structures (feathers in birds and fur in mammals), made from keratin and originating from placodes that are homologous to reptilian scales, as specialized body coverings for insulation to reduce heat loss, which is of course a crucial adaptation for endothermy (Chernova 2005, Dhouailly 2009, Di-Poï & Milinkovitch 2016).
4. Even though very different in organisation, birds and mammals have the most complex lungs among vertebrates and a highly efficient respiratory systems that support their high metabolic demands (Meyer et al. 1981, Powell & Hopkins 2004, West et al. 2007).
5. Both birds and mammals exhibit relatively large brains compared to body size, particularly in regions associated with higher cognitive function, such as learning, problem-solving, and social behaviors. More specifically, only mammals and birds possess a well-developed neocortex, called dorsal ventricular ridge (DVR) in birds (UChicago Medicine 2012, Kebschull 2020, Stacho et al. 2020, Ball & Balthazart 2021). Apart from the increased relative brain size and highly laminar telencephalic areas, birds and mammals also share a complex cerebellar folding, enhancing motor control and coordination, as well as advanced auditory circuits capable of processing complex sounds (Striedter & Northcutt 2019).
6. Only mammals and birds have episodic-like memory (Rattenborg & Martinez-Gonzalez 2011, 2013).
7. Even though sleep was for decades considered to be exclusive to mammals and birds, it is meanwhile shown to be a widespread phenomenon in the animal kingdom. However, “REM sleep, the major source of dreams, and slow wave sleep are unique to mammals and birds” (AAAS 2015, Hayashi et al. 2015). 
8. Birds and mammals have specialized hearing mechanisms, including middle ear ossicles (ossicular chain) that transmit sound vibrations effectively, allowing for acute auditory sensitivity (Köppl 2011, Anthwal et al. 2012: fig. 1).
9. Both groups possess complex endocrine hormonal systems that regulate growth, metabolism, and reproduction. Actually, “birds produce homologues of the vast majority of mammalian hormones. These can have similar roles in birds and mammals.” (Scanes 2015). For instance, the thyroid and adrenal glands play essential roles in metabolic rate control, and prolactin controls seasonality in birds and mammals (Stewart & Marshall 2022).
10. Both birds and mammals engage in complex social behaviours, including cooperation, communication, intricate mating rituals and significant parental care, including prolonged juvenile periods and provisioning of food, which enhances offspring survival in challenging environments. Play behaviour was long considered to be unique to mammals and birds (Dinets 2023), but it has been meanwhile recorded from a few ectothermic animal species as well, but it is still only widely occurring and well-developed in birds and mammals.
11. A cladistic study of 16 ultrastructure characters of spermatozoa (Jamieson & Healy 1992) strongly supported the bird + mammal clade (Haemothermia) with three uniquely derived similarities, and did not substantiate the traditional grouping. This is significant because it is a totally independent source of evidence unrelated to warm-blooded physiology.
12. In spite of differential rates of transposable element accumulation “the genome size in mammals and birds shows remarkably little interspecific variation compared with other taxa.” The results of a study by Kapusta et al. (2017) imply that “DNA removal in both mammals and birds has proceeded mostly through large segmental deletions”, which has been called an “accordion model” of genome size evolution.

All these striking similarities would have to be considered as convergent adaptations, which were the result of similar selective pressures in birds and mammals that have led to the independent origin of these complex traits. So, both alternatives, the mainstream view and the Haematothermia hypothesis, imply a lot of convergences, so that many similarities cannot be readily explained with common descent. Ultimately, a bureaucratic counting of which alternative is supported by a few more similarities (see Kemp 1988) decides for evolutionary biologists, which common descent hypothesis is embraced and which is rejected as absurd. If you look at many of the publication dates of the references in my list above, you see that a lot more characters that would support Haematothermia have been discovered since the time of Løvtrup and Gardiner.

To be clear, I do not suggest that the Haematothermia hypothesis is a better alternative to the mainstream view, but rather suggest that the strongly conflicting data point to a deeper problem. In the view of us critics of neo-Darwinian evolution, the large amount of incongruent and conflicting evidence rather questions all alternatives and the very paradigm of common descent itself. Even though common descent may well still be true, either on a universal level or at least for more restricted groups, it cannot be convincingly demonstrated by just pointing to shared similarities. Those similarities would have to be overwhelmingly congruent and mostly point to the same nested hierarchy, if the story of a single tree of life would be true. But they don’t. Incongruent evidence is found abundantly in all groups and all levels of the taxonomic hierarchy. The theory of Darwinism made a prediction of similarities that neatly fall into a nested hierarchy without significant incongruence, but it failed the test by empirical data miserably. Other processes than common descent with modification have to account for the similarities and differences between organisms, and intelligent design definitely is a premier candidate.

Ignoring the Evidence

How do popularizers of Darwinism such as Richard Dawkins react? Unsurprisingly, they just ignore the evidence and boldly tell their gullible fanboys (and girls) that evolution is a proven fact because all data unambiguously suggest a single true tree of life. Is this mere ignorance or deliberate deception? The materialist-naturalist world view critically depends of Darwinian evolution and must defend it at all cost, even if it means that the facts have to be tweaked, fudged, and denied to fit the theory. And all critics must be silenced as dangerous science-deniers and peddlers of pseudoscience and evil religious superstition. More and more people no longer fall for this crude propaganda and rather follow the evidence wherever it leads. Isn’t this what science is all about, or at least should be?

References

• AAAS 2015. Neurons that regulate sleep stages revealed. EurekAlert! October 22, 2015. https://www.eurekalert.org/news-releases/746676
• Anthwal N, Joshi L & Tucker AS 2012. Evolution of the mammalian middle ear and jaw: adaptations and novel structures. Journal of Anatomy 222(1), 147–160. DOI: https://doi.org/10.1111/j.1469-7580.2012.01526.x
• Ball GF & Balthazart J 2021. Evolutionary neuroscience: Are the brains of birds and mammals really so different? Current Biology 31(13), R840–R842. DOI: https://doi.org/10.1016/j.cub.2021.05.004
• Benton MJ 1985. Classification and phylogeny of the diapsid reptiles. Zoological Journal of the Linnean Society 84(2), 97–164. DOI: https://doi.org/10.1111/j.1096-3642.1985.tb01796.x
• Benton MJ 1991. Amniote phylogeny. pp. 317–330 in: Schultze H-P & Trueb L (eds). Origins of the Higher Groups of Tetrapods: Controversy and Consensus. Cornell University Press, Ithaca (NY) and London (UK), 576 pp. https://www.jstor.org/stable/10.7591/j.ctv75d32t.13
• Benton MJ 2021. The origin of endothermy in synapsids and archosaurs and arms races in the Triassic. Gondwana Research 100, 261–289. DOI: https://doi.org/10.1016/j.gr.2020.08.003
• Carvalho O & Gonçalves C 2011. Comparative Physiology of the Respiratory System in the Animal Kingdom. The Open Biology Journal 4, 35–46. DOI: https://doi.org/10.2174/1874196701104010035
• Chernova OF 2005. One More Example of Morphological Convergence: Similarity between the Architectonics of Feather and Hair. Doklady Biological Sciences 405(3), 446–450. DOI: https://doi.org/10.1007/s10630-005-0161-3
• Dinets V 2023. Play behavior in ectothermic vertebrates. Neuroscience & Biobehavioral Reviews 155: 105428, 1–10. DOI: https://doi.org/10.1016/j.neubiorev.2023.105428
• Di-Poï  N & Milinkovitch MC 2016. The anatomical placode in reptile scale morphogenesis indicates shared ancestry among skin appendages in amniotes. Science Advances 2(6): e1600708, 1–8. DOI: https://doi.org/10.1126/sciadv.1600708
• Dhouailly D 2009. A new scenario for the evolutionary origin of hair, feather, and avian scales. Journal of Anatomy 214(4), 587–606. DOI: https://doi.org/10.1111/j.1469-7580.2008.01041.x
• Faure-Brac MG, Woodward HN, Aubier P & Cubo J 2024. On the origins of endothermy in amniotes. iScience 27(4): 109375, 1–12. DOI: https://doi.org/10.1016/j.isci.2024.109375
• Gardiner BG 1982. Tetrapod classification. Zoological Journal of the Linnean Society 74(3), 207–232. DOI: https://doi.org/10.1111/j.1096-3642.1982.tb01148.x
• Gardiner BG 1993. Haematothermia: Warm-Blooded Amniotes. Cladistics 9(4), 369–395. DOI: https://doi.org/10.1111/j.1096-0031.1993.tb00232.x
• Gauthier JA, Kluge AG & Rowe T 1988a. Amniote phylogeny and the importance of fossils. Cladistics 4(2), 105–209. DOI: https://doi.org/10.1111/j.1096-0031.1988.tb00514.x
• Gauthier JA, Kluge AG & Rowe T 1988b. The early evolution of the Amniota. pp. 103–155 in: Benton MJ (ed.). The Phylogeny and Classification of the Tetrapods. Volume 1: Amphibians, Reptiles, Birds. Oxford University Press, Oxford (UK), 373 pp.
• Grigg G, Nowack J, Eduardo Pereira Wilken Bicudo J, Chandra Bal N, Woodward HN & Seymour RS 2022. Whole-body endothermy: ancient, homologous and widespread among the ancestors of mammals, birds and crocodylians. Biological Reviews 97, 766–801. DOI: https://doi.org/10.1111/brv.12822
• Hayashi Y, Kashiwagi M, Yasuda K, Ando R, Kanuka M, Sakai K & Itohara S 2015. Cells of a common developmental origin regulate REM/non-REM sleep and wakefulness in mice. Science 350(6263), 957–961. DOI: https://doi.org/10.1126/science.aad1023
• Jamieson BGM & Healy JM 1992. The Phylogenetic Position of the Tuatara, Sphenodon (Sphenodontida, Amniota), as Indicated by Cladistic Analysis of the Ultrastructure of Spermatozoa. Philosophical Transactions: Biological Sciences 335(1274), 207–219. https://www.jstor.org/stable/55582
• Janke A & Arnason U 1997. The complete mitochondrial genome of Alligator mississippiensis and the separation between recent Archosauria (birds and crocodiles). Molecular Biology and Evolution 14(12), 1266–1272. DOI: https://doi.org/10.1093/oxfordjournals.molbev.a025736
• Janvier P 1984. El divorcio del ave y del cocodrilo. Mundo Cientifico 32, 14–16.
• Kapusta A, Suh A & Feschotte C 2017. Dynamics of genome size evolution in birds and mammals. PNAS 114(8), E1460–E1469. DOI: https://doi.org/10.1073/pnas.1616702114
• Kebschull J 2020. Bird brains: They are more like ours than we thought. Stanford Luo Lab — Principles of Neurobiology Blog November 17, 2020. https://luolab.stanford.edu/news/bird-brains-they-are-more-ours-we-thought
• Kemp TS 1988. Haemothermia or Archosauria? The interrelationships of mammals, birds and crocodiles. Zoological Journal of the Linnean Society 92(1), 67–104. DOI: https://doi.org/10.1111/j.1096-3642.1988.tb01527.x
• Köppl C 2011. Birds – same thing, but different? Convergent evolution in the avian and mammalian auditory systems provides informative comparative models. Hearing Research 273(1-2), 65–71. DOI: https://doi.org/10.1016/j.heares.2010.03.095
• Koteja P 2004. The Evolution of Concepts on the Evolution of Endothermy in Birds and Mammals. Physiological and Biochemical Zoology 77(6), 1043–1050: DOI: https://doi.org/10.1086/423741
• Kuhn-Schnyder E 2009. Bemerkungen eines Paläontologen zur Klassifikation der Amniota von B. G. Gardiner (1982) und S. Løvtrup (1985). Journal of Zoological Systematics and Evolutionary Research 25(1), 1–8. DOI: https://doi.org/10.1111/j.1439-0469.1987.tb00909.x
• Løvtrup S 1977. The Phylogeny of Vertebrata. John Wiley, London (UK).
• Løvtrup S 1985. On the classification of the taxon Tetrapoda. Systematic Zoology 34(4), 463–470. DOI: https://doi.org/10.1093/sysbio/34.4.463
• Meyer M, Scheid P & Piiper J 1981. Comparison Birds/Mammals: Structure and Function of the Gas Exchange Apparatus. Proceedings of the 28th International Congress of Physiological Sciences, Budapest, 1980. Advances in Physiological Sciences Vol. 10 Respiration, 135–143. DOI: https://doi.org/10.1016/b978-0-08-026823-1.50021-1
• Naish D 2008. Goodbye from the stem-haematotherm, goodbye from me. Tetrapod Zoology March 6, 2008. https://scienceblogs.com/tetrapodzoology/2008/03/06/the-stem-haematotherm
• Naish D 2012. The Haematothermia hypothesis. Tetrapod Zoology October 3, 2012. https://www.scientificamerican.com/blog/tetrapod-zoology/the-haematothermia-hypothesis/
• Nespolo RF, Bacigalupe LD, Figueroa CC, Koteja P & Opazo JC 2011. Using new tools to solve an old problem: the evolution of endothermy in vertebrates. Trends in Ecology & Evolution 26(8), 414–423. DOI: https://doi.org/10.1016/j.tree.2011.04.004
• Owen R 1866. On the Anatomy of Vertebrates. Volume 2. Longmans Green & Co., London (UK).
• Peters D 2014. Gardiner 1982 – a radical hypothesis on reptile relationships. Pterosaur Heresies March 17, 2014. https://pterosaurheresies.wordpress.com/2014/03/17/gardiner-1982-a-radical-hypothesis-on-reptile-relationships/
• Peters D 2024. Reptile Family Tree. http://www.reptileevolution.com/images/tetrapods/LargeReptileFamilyTree3.pdf
• Powell FL & Hopkins SR 2004. Comparative Physiology of Lung Complexity: Implications for Gas Exchange. Physiology 19(2), 55–60. DOI: https://doi.org/10.1152/nips.01469.2003
• Rattenborg NC & Martinez-Gonzalez D 2011. A bird-brain view of episodic memory. Behavioural Brain Research 222(1), 236–245. DOI: https://doi.org/10.1016/j.bbr.2011.03.030
• Rattenborg NC & Martinez-Gonzalez D 2013. Episodic-like memory and divergent brain systems in mammals and birds. PNAS 110(40), E3741–E3741. DOI: https://doi.org/10.1073/pnas.1312035110
• Ray J 1693. Synopsis Methodica Animalium Quadrupedum et Serpentini Generis. Robert Southwell, London (UK).
• Scanes CG 2015. Avian Endocrine System. Chapter 22, pp. 489–496 in: Sturkie’s Avian Physiology. 6th Ed. Academic Press / Elsevier, Amsterdam (NL). DOI: https://doi.org/10.1016/b978-0-12-407160-5.00022-1
• Sivgin TK 2020. Unorthodox Ideas about Bird Origins. Manospondylus August 26, 2020. https://www.manospondylus.com/2020/08/unorthodox-ideas-about-bird-origins.html
• Stacho M, Herold C, Rook N, Wagner H, Axer M, Amunts K & Güntürkün O 2020. A cortex-like canonical circuit in the avian forebrain. Science 369(6511): eabc5534, 1–12. DOI: https://doi.org/10.1126/science.abc5534
• Stewart C & Marshall CJ 2022. Seasonality of prolactin in birds and mammals. JEZ-A Ecological and Integrative Physiology 337(9-10), 919–938. DOI: https://doi.org/10.1002/jez.2634
• Striedter GF & Northcutt RG 2019. The Rise of Endothermy: Mammals, but also Birds. Chapter 6, pp. 337–422 in: Brains Through Time: A Natural History of Vertebrates. Oxford University Press, Oxford (UK). DOI: https://doi.org/10.1093/oso/9780195125689.003.0006
• UChicago Medicine 2012. Homolog of Mammalian Neocortex Found in Bird Brain. UChicago Medicine News September 30, 2012. https://www.uchicagomedicine.org/forefront/news/homolog-of-mammalian-neocortex-found-in-bird-brain
• West JB, Watson RR & Fu Z 2007. Major differences in the pulmonary circulation between birds and mammals. Respiratory Physiology & Neurobiology 157(2-3), 382–390. DOI: https://doi.org/10.1016/j.resp.2006.12.005
• Wu Y & Wang H 2019. Convergent evolution of bird-mammal shared characteristics for adapting to nocturnality. Proceedings of the Royal Society B 286(1897): 20182185, 1–10. DOI: https://doi.org/10.1098/rspb.2018.2185