Author: Sugar Beast
1
The evolution of endothermy, which can use their metabolism to regulate and maintain body temperature, represents a major shift in vertebrate history. However, exactly how and why birds and mammals evolved intrinsic warmth remains a topic of major controversy.
In a new report published in Science Advances, the researchers combined a heat transfer model with body size data from theropod dinosaurs, which reconstructed the evolution of metabolic rate along the main lineage of birds. The results show that the reduction of body size into the emergence of constant temperature forms the least resistant evolutionary pathway, which reduces the high energy demand while maximizing the expansion of the thermal niche.
2
The evolution of thermogenesis in birds and mammals is an important shift in the evolutionary process of vertebrates, and it is an excellent example of evolutionary convergence between different species. The emergence of internal temperature is critical to the wide geographical distribution and ecological success of these species. Although there are a few types of invertebrates that can also raise their body temperature to a higher temperature than their surroundings, relying on endogenous heat generated by themselves to maintain a high constant body temperature during rest is a property unique to birds and mammals.
The internal temperature properties give these animals greater flexibility, endurance and tolerance. However, from an energy perspective, this attribute is a costly survival strategy. From the existing fossil record, scientists have not found any substantial traces of internal temperature. Therefore, the evolutionary process and pattern of internal thermogenesis has always been the most controversial issue in vertebrate evolution.
In order to understand when and how internal warmness began in the evolution of birds and mammals, there are two basic questions that need to be discussed:
1. What are the costs and benefits of internal temperature compared to external temperature (ectothermy, i.e., animals absorb heat from the environment and rely on behavioral regulation to adapt to temperature changes in the environment)?
2. Under what conditions would this shift towards internal temperature be more advantageous?
3
In the new study, the researchers used a heat transfer model used to study thermostat regulation in thermostatic animals to answer these questions. Previously, this model was rarely used to study extrathermal properties. But because all living organisms can generate endogenous heat, this model still works in a thermally stable state. With this important hypothesis, the researchers avoided using complex models often used to study extrathermality, making the analysis much simpler.
Using this model, they quantified the costs and benefits of internal temperature based on parameters such as animal mobility, foraging efficiency, ability to avoid predators, tolerance to the environment and ability to settle, and growth rate. Using accurate body size data, they calculated the cost-effectiveness of adopting a thermostatic lifestyle by comparing them with their ancestors of extratrophobeatlimates and offspring of internal thermogenesis.
They combined heat transfer models with phylogenetics and body reconstructions derived from the fossil record to reveal the evolution of the internal temperature of the ancestors of birds and theropod dinosaurs. Using body size data reconstructed from the fossil record, body sizes ranging from 10 to 100,000 kilograms were simulated, and the cost of evolution along bird lineages to promote internal temperature was calculated.
The results showed that after the animals became smaller and smaller, the energy consumption was significantly reduced. That is, the miniaturization of body type has evolved to reduce the energy consumption required for internal temperature.
4
So why is the reduction in body size accompanied by a reduction in energy costs? They explained the possible causes with two phenomena.
First, after the increase in metabolic rate, the expansion of the thermo-ecological niche is more likely to occur in large extratemperaminate animals. This is because these large animals have a greater ability to maintain higher body temperatures because of their great temperature. The researchers noted that the larger the original size of the extratemperaminate ancestors, the lower the cost of the transition to introthermia.
The shrinking body size is consistent with the evolution from ancestral theropod dinosaurs to basal birds, who used the energy costs that would have been used to increase to maintain internal temperature | [1]
Furthermore, in the process of miniaturization, these animals spent their energy consumption on internal constancy, which helps to understand how they evolved such a high energy conversion rate. Birds need 15 to 20 times more food than reptiles compared to reptiles of the same size.
5
From the new results, the researchers found that the increase in metabolic rate spanned much of the early to mid-Jurassic period (about 170 million to 180 million years ago). They speculate that it is precisely because metabolism increases with shrinking body size that there are gradient changes in metabolic levels in the phylogenetics of theropod dinosaurs.
The basic theropod dinosaurs may have shown lower metabolic rates, and more recent non-avian lineages with better metabolisms appear to regulate temperature better. These analyses provide a key preliminary evolutionary chronology for the emergence of small, thermostatic, and feathered dinosaurs. Their study shows that thermostatic animals existed before they evolved in flight.
References:
[1] https://advances.sciencemag.org/content/6/1/eaaw4486
This article is authorized to be reproduced from the public account "Principle", if you need to reprint, please contact the original account.