One of the things that made vertebrate evolution successful is endothermy or the ability to maintain body heat through metabolic means. However, it is still unclear how and when endothermy developed in birds and mammals. In a new report conducted by Enrico L. Rezende and his team of researchers from the Center of Applied Ecology and Sustainability and the Institute of Environmental and Evolutionary Sciences in Chile combined a heat transfer model with the data on theropod body size. The researchers proceeded to reconstruct the evolution of metabolic rates in birds. The results of this experiment show that the reduction of body size was necessary for endothermy to evolve in birds all while maximizing the thermal niche expansion and reducing the costs of elevated energy requirements.
Because of their primary findings, the researchers came up with the hypothesis that there is an increase in metabolism when theropod dinosaurs (ancestors of modern birds) begin to shrink in terms of body size during the Early-Middle Jurassic period approximately 180 to 170 million years ago. This is the reason why there is a gradient of metabolic levels in the phylogeny of these dinosaurs. Although more primitive species of theropods exhibit lower metabolism rates, its non-avian descendants probably have a decent amount of thermoregulating skills and improved metabolism. Another advantage of this study is that it was able to provide a tentative time sequence for the key evolutionary transitions and for the emergence of the smaller feathered dinosaurs capable of flight and endothermy.
According to the researchers, the evolution of endothermy among birds and mammals is an important event in vertebrate evolution as it is pivotal to their widespread geographical and ecological distribution. Endothermy is something unique to birds and mammals and this is the reason behind their greater mobility, stamina, and tolerance in various conditions.
The scientists considered two fundamental questions in understanding the origin and reason for endothermy to occur within the evolution of birds and mammals: what are the costs and benefits of this strategy compared to ectothermy? And what are the specific conditions that triggered the transition to endothermy? The researchers tried to provide answers to these questions by using the Scholander-Irving model of heat transfer. This model is used to study thermoregulation for more than 60 years.
Rezende and his team proceeded to quantify the cost of endothermy when the benefits include greater mobility and efficiency in foraging and avoiding predators. Endotherms are also highly tolerant of changes in the environment through increased growth rates and homeostasis or the steady rate of internal physical and chemical conditions. The team was able to quantify the thermal niche that theropods were able to occupy and its expansion to be able to do an estimate to the net benefit of endothermy. After this methodology, they calculated the costs and benefits of having an endothermic lifestyle referencing an ectothermic ancestor -- or the cold-blooded dinosaurs whose regulation of body temperature is dependent on their environment -- and their endothermic descendants.
By replicating the calculations with the exact body size estimates, the results of the study showed that smaller body sizes in theropod dinosaurs reduced the costs to evolve into endothermy. The team then explored how the Scholander-Irving model and the phylogenies and body size reconstructions can shed light on the evolution of warm-blooded birds and their theropod ancestors. Through fossil records and reconstructed ancestral body size, Rezende and his colleagues were able to get an estimate of the costs of evolving into endothermy in the lineage of birds.
For the team to be able to quantify energy costs within alternative scenarios, they did a simulation on the evolution of body size along the avian lineage and was able to obtain the distribution of cost per degree and explained the reduced costs using two phenomena. The first phenomenon is the expansion in the thermal niche which is based on increasing metabolic rate. This said expansion is disproportionately higher in much larger cold-blooded animals because they can maintain high body temperature. Together with a relatively low mass-independent metabolic rate caused by initial stable thermoregulation (homeothermy), the team noted that the larger the starting size of a cold-blooded ancestor, the cheaper the transition to endothermy.
The second phenomenon is during miniaturization when the animals traded the energy costs of being larger in size to becoming endothermic. The high energy turnover rates evolved regardless of the impact on food and water requirements. The outcome of the experiment qualitatively agreed with other models regarding the evolution of endothermy despite the variations in the availability of resources.
