Short Essay: The Evolution of Endothermy

Since I haven’t uploaded anything in a while, I thought I’d share my latest essay with you. Unfortunately, it is limited to 1000 words, and so some of the more interesting details are left out. However, you can investigate the subject yourselves! It’s quite exciting.

The Evolution of Endothermy

This essay attempts to gather evidence, from several sources, for the reasons behind the appearence of endothermy, the necessary changes in the heart and circulatory system to accomodate endothermy, and during which time periods endothermy is likely to have arisen. It is proposed that endothermy has evolved multiple times: during the evolution of early mammals, and during the divergence of Triassic Theropods. The advantages and disadvantages to both endothermy and the contrasting ectothermy are suggested, and it is proposed that endothermy is not an advanced state, but simply an adaptation to a specific niche in a given ecosystem. To support this, the stem crocodilian Poposaurus gracilis is referenced as an example of an endothermic species that shares a strong evolutionary link with todays modern ectothermic crocodilians, seemingly adapting to an ectothermic lifestyle from a previously endothermic lifestyle. The formation of the heart during embryonic development is briefly considered, and a study is referenced that observed a gene, Tbx5, that has a role in forming the ventricular septation required to accomodate endothermy in an organism. Separation of blood in this manner allows for a multiple pressure system, vital to endothermy.

Endothermy is the ability of an organism to maintain it’s internal temperature through it’s metabolism. Endothermic organisms are able tolive in a greater range of habitats and temperatures as they can produce their own body heat and, unlike ectotherms, do not require a constant environmental temperature depedence. The usual source of internal heat comes from digestion of food and muscle contraction.

Endothermy has been found in both mammals and birds. Since both the mammal and avian classes share reptilian ancestors, it has been proposed that some reptiles were in fact endothermic, and that endothermy evolved during the Mesozoic on more than one occasion. Todays evidence suggests that endothermy was present in Mesozoic archosaurs, the ancestors of modern birds, though it is still in discussion. It could, however, be suggested that the evolution of endothermy was a single event, which would likely put its appearance in the Permian period, when the synapsids and the sauropsids appear to have formed distinct groups. This theory would suggest, however, that some sauropods would have reverted back to an ectothermic lifestyle.

Advantages and Disadvantages of Endothermy as opposed to Ectothermy
Both endothermy and ectothermy have their advantages, dependant on the organisms lifestyle. Endotherms, namely mammals and birds, tend to be more active than species of amphibians or reptiles. An endotherm can sustain a consistent body temperature in a large range of environments through internal heating. The heat is produced through digestion of food and muscle contraction, e.g. Shivering. The increase in energy would allow an organism to more readily escape predation, and would also increase stamina. However, such advantages do come at a cost. Endotherms require a lot more energy than endotherms, estimated to be about 4 to 10 times more.[1][2] Whereas an ectotherm can go a substantial period of time without eating, an endotherm needs to eat on a regular basis in order to sustain homeostasis.
Unlike endothermic species, ectothermic species regulate their body temperature by use of the environmental conditions. By moving into warmer or cooler areas, ectotherms can control their body temperature whilst using a lot less energy from metabolism. Some ectotherms can go for months without eating, whereas many endotherm species may show drastic declines during food shortages.
Both endothermy and ectothermy provide advantages in different niches. An ambush predator may be better suited to an ectothermic lifestyle, especially if it lives in a tropical region. Predators which are more active when hunting, like the Cheetah (Acinonyx jubatus), require more energy and a sustained body temperature, and would benefit more from being endothermic.

The Origin of Endothermy
As previously noted, the idea that endothermy has evolved on multiple occasions appears most likely. Mammals, including amniotes, marsupials and the monotremes, are all endothermic, as are the vast majority of extinct species. Similarly, all modern birds are endothermic, and it is widely accepted that theropods and closely-related archosaurs were also endothermic, seemingly before the appearance of Avian and Crocodilian ancestors (considering the Crocodilians apparant endothermic ancestry).
The change to endothermy requires some morphological changes, most noticeably the change from a 3-chambered heart (Seen in Amphibians and non-Crocodilian Reptiles) to a 4-chambered heart (Mammals, Birds and Crocodilians).[3] Hearts with distinct left/right ventricles are almost exclusive to modern endotherms, the exception being modern day Crocodilians, which have a complete septation of the ventricle, yet exhibit an ectothermic lifestyle. It is suggested by Seymour, R. S. et al, among others, that the ancestors to modern Crocodilians were endothermic. Such would explain the heart structure that is unique among vertebrates.[4] This is also suggested by the findings of Crocodilian ancestors such as Poposaurus gracilis, a bipedal archosaur.[5] (Were the Crocodiles to be descended from endothermic ancestors, as is most likely, it shows to me that the evolution of endothermy is not an advancement as suggested by others, but simply an adaptation to a different niche that requires an alternate lifestyle).
It is without doubt that the formation of the heart is a likely indicator of whether a species is ectothermic or endothermic, but why the need for this change, and how did it arise?
The evolution of new means of collecting, storing and using energy opened up new niches for endotherms. Were it not for endothermy, the long-sustained flight of birds would not be possible, nor would the hunting habits of many predatory mammals. Such a change would have presumably been followed by a large radiation of endothermic species into new niches and habitats. As is seen from todays wildlife, only endotherms can survive habitats such as Antarctica or the Arctic Circle. Being endothermic allowed for a greater distribution, which at the time would have been a great benefit.
A change from an ectothermic to an endothermic lifestyle required a different circulatory system.[3] Ventricular septation allowed for differing blood pressure around the body, and separated oxygenated and de-oxygenated blood.[4] Such features, it can be argued, are necessary for an endothermy lifestyle. The origin of the 4-chambered heart could be synchronous to the origin of endothermy.
A study by Koshiba-Takeuchi, K et al (2009) tracked the movement of transcription factor gene Tbx5 in the development of a mouse, turtle and anole heart. The results showed that Tbx5 was essential in ventricular septation during development.[3] This eventual change in heart formation allowed for the separation of blood required for endothermy.

Since their origin endotherms have diversified greatly, and can now be said to be the dominant group in a majority of habitats. The ability to sustain ones own internal temperature opened up a great number of niches for exploitation. However, as shown by the Crocodilian lineage, endothermy is not necessarily an advancement, but simply an adaptation to a new role.
The evidence is beginning to show that endothermy has evolved on mulitple occasions, most obviously during the evolution of mammals, and again somewhere within the close ancestry of both birds and crocodilians. As yet there are no clear dates, but further studying of genes such as Tbx5, the development of the heart in embryos, fossil finds and DNA sequences may make clear the origins of endothermy.

[1] – Lecture 14: Evolution of Endothermy. The University of Manchester. 24-11-2011 18:13
[2] – The Evolution of Endothermy. University of California, Berkeley. 24-11-2011 18:16
[3] – Koshiba-Takeuchi, K. et al. Reptilian heart development and the molecular basis of cardiac chamber behaviour. Nature 461 95-98 (2009)
[4] – Seymour, R. S. et al. Evidence for endothermic ancestors of crocodiles at the stem of Archosaur evolution. Physiol Biochem Zool 77 (6) 1051-1067 (2004)
[5] – Gauthier, J. A. et al. The bipedal stem crocodilian Poposaurus gracilis: Inferring function in fossils and innovation in archosaur locomotion. Bulletin of the Peabody Museum of Natural History 52 107-126 (2011)