Any organism dies if its body temperature leaves the vital range. Thus, controlling the body temperature (or having a wide vital temperature range) is essential for any organism.
The body temperature of ectothermic (cold-blooded) reptiles (lizards, snakes, turtles, and crocodiles) is mainly influenced by ambient temperature. In reptiles, thermoregulation is largely a behavioral manipulation of heat input from the environment, principally the sun, or its avoidance.
Endothermic (warm-blooded) birds and mammals including humans have an internal metabolic heat production and heat retention. It is high enough to permit constant body temperatures above ambient temperatures. A substantial amount of energy that endotherms gain from food is converted to heat. This energy is not available anymore for other vital functions and “lost”. Thus, in comparison to similar-sized ectothermic reptiles, endothermic birds and mammals have a much higher energy demand.
It is assumed that endothermic birds and mammals evolved from ectothermic reptile-like species. The acquisition of endothermy was one remarkable event in the evolutionary history of mammals and birds. Endothermy had significantly contributed to the evolutionary success of mammals and birds as it allowed them to conquer the complete globe. However, the big question still remains: How could such a costly metabolism evolve?
Our explanation is that endothermy was initiated in an ectothermic reptile-like animal by mutations in genes that controlled body growth. Mutations caused that growth ceases much earlier in life in this proto-endothermic animal than in its ancestor. Accordingly, the animal’s size and appearance resemble that of the juvenile ectothermic ancestor (e.g. the skull, body proportions). However, its appetite and energy intake, and hence metabolic rate, the life history parameters like body mass/size at hatching or birth, age and size at which sexual maturity is reached, and the growth rate during early ontogeny are still retained from its ectothermic ancestor.
Surprisingly, an animal showing all these characteristics already shares them with recent endothermic birds and mammals. An early stop in growth during ontogeny thus could have played a key role in the evolution of endothermy. It generated variability in these characteristics of ancestral ectotherms, which was subject to natural selection in the past and resulted in many adaptations linked to endothermy in today’s birds and mammals.
Two, not mutually exclusive, mechanisms might explain how this change in growth and metabolism could have evolved from an ectothermic metabolism.
First, the descendants could have been retained the ectothermic ancestor’s energy intake and metabolism during growth. Metabolised energy, which the ectothermic ancestor stored in new tissue and thus used for growth, now dissipates by heat. This would result in a higher metabolic rate than expected from its body mass for a similar-sized reptile-like animal. Dissipating heat could be used to reach a higher body temperature. A good analog for that mechanism might be a car running idle instead of using the fuel for driving.
The second mechanism could be that somatic cell size decreased in the proto-endothermic animal but not (so much) cell number. Its body size, in turn, would decrease, whereas its metabolic rate and heat production would be increased in comparison to its body mass. This is because the proto-endothermic animal has more cells and hence (maybe larger) mitochondria (structure within a cell which generates energy for the cell) within a given body volume unit than its ancestor.
Also, its vascularization system of tissues could become more complex per body volume unit. The latter is because somatic cell size decreased in the proto-endothermic animal but not (so much) cell number. A good analog for that mechanism might be a processor of a computer. When further transistors are added to the CPU, it becomes more complex in terms of units and crosslinking (and faster) but also more heat is generated during work.
However, at the moment the considered mechanisms are speculative and have to be tested in further studies. Nevertheless, we think that our hypothesis, and tests on the proposed mechanisms, will contribute to a better understanding of physiological processes within endotherms including humans.
These findings are described in the article entitled Was endothermy in amniotes induced by an early stop in growth during ontogeny, recently published in the journal The Science of Nature. This work was led by Jan Werner and Eva Maria Griebeler from Johannes Gutenberg-Universität Mainz.