Based on body thermoregulation, we generally divide animals into two categories. Those who can generate and regulate their body heat themselves are called endotherms, or warm-blooded, because the heat generated by their metabolism is distributed around the body through blood flow. Animals who lack this ability are called ectotherms, and have to rely on external sources of heat. This is why lizards and snakes are often seen sunbathing. Fish, amphibians and reptiles are all ectotherms, whereas birds and mammals are endothermic, which is often considered as a key part of their ecological success.
Endothermy comes at considerable energetic costs – endotherms need to consume, on average, ten times as much food as ectotherms of the same size – but it also provides a competitive advantage against the evolutionarily older ectotherms. Maintaining a stable temperature allows warm-blooded animals to stay alert and respond to potential threats or hunting opportunities. It means they can be active during cold nights, occupy both warm and cold climates and move for a long time without running out of energy.
A new study led by Professor Benton at Bristol University suggests that warm-bloodedness evolved in response to the Permian-Triassic Mass Extinction (PTME) as a result of an ecological arms race between the ancestors of today’s birds and mammals.
The PTME took place at the Permian-Triassic boundary some 250 million years ago. It is the largest known extinction; repeated events of extreme climate warming and ocean acidification crises wiped out around 95 per cent of all existing species, and nearly destroyed all life on Earth.
Two major tetrapod clades survived: archosaurs and synapsids, the ancestors of modern birds and mammals respectively. They quickly became top predators of the Triassic, and competed for dominance in the new environments emerging across the globe. Through this rivalry, they evolved a number of new traits to aid survival, which could include endothermy.
The evolution of posture could be key to understanding how warm bloodedness came about.
In tetrapod physiology, the acquisition of an erect posture was a very important change. Originally, most archosaurs and synapsids were sprawlers, with their legs sticking out from their bodies similar to today’s lizards. This impedes their ability to breathe while running, and therefore limits sprawlers to short distances. This can be observed in any modern lizard – they move in a sort of stop motion manner, instead of continuously running like a dog would, for example. Birds and mammals, who have their legs positioned under their bodies, don’t face these limitations.
Until recently it was thought that tetrapods gained an erect gait gradually throughout the Triassic, from early sprawler tetrapods in the Permian to fully erect dinosaurs in the Late Triassic. This theory was upturned when a 2009 study by Kubo and Benton revealed “an instant switch from sprawling to parasagittal [erect] locomotion across the Permian-Triassic Boundary”. Posture can be easily determined by looking at the tracks an animal leaves behind. The more spread out the imprints are, the more sprawler-like the posture. Benton and Kubo compared 461 fossil tetrapod tracks with the tracks of modern animals and found that all medium and larger sized archosaurs and synapsids “switched posture overnight”. Bear in mind that overnight in geological time really means several hundreds or thousands of years, but relative to most evolved features, this is remarkably fast.
Erect gait would allow these animals to run faster, further and with lower energy expenditure. Moreover, a later 2013 study by Kubo and Kubo shows further increase in stride length and speed in archosaurs throughout the Triassic. This points to increasing activity levels in the animals and corresponds with the idea of an ecological arms race. It is easy to see the advantages that endothermy and a fast metabolism would present in a world that was becoming increasingly fast paced. The question asked by Benton is: was this the case?
There is a large number of physiological traits associated with endothermy that were studied in Permian and Triassic fossils. Some examples include evidence of whiskers, differentiated teeth, and presence of a diaphragm or feathers, all of which are strong markers of endothermy.
Benton’s study reviewed research from recent years and gathered evidence for these traits in early synapsids and archosaurs. Further evidence came from bone histology – bone tissue indicative of fast growth, typical for endotherms, was found in both clades. Phylogenetic analysis of both archosaurs and synapsids also confirmed that many of these markers originated in the Early Triassic.
The 2020 study brings together exciting new information about the origin of warm-bloodedness, but recognises a need for further research to confirm or reject the proposal. While endothermy probably originated during the PTME, there is still much to be learned about it’s evolution. Benton concludes “It is likely that the ‘origin of endothermy’ in archosaurs and synapsids was not a single event in each clade, but rather several, and perhaps stepping through a dozen or more adaptations over tens of millions of years.”
Written by Adéla Pafková and edited by Ailie McWhinnie.
Adéla is a second year ecological and environmental science student.