Evolution of Warm Bloodedness Helped Mammals in Enlarged and Better Functioning Brains

Enlarged brains are correlated with advanced brain functioning and more complex behaviour. Mammalian brain enlargement was an evolutionarily crucial event. However, what are the driving forces and key factors that underlie the development of large brain to body size ratios in mammals still remain intriguing. It was during the Triassic period that the evolution of enlarged brains in mammals and birds began, and it coincided with the emergence of endothermic organisms, i.e., warm-blooded organisms. During this period, mammals evolved from the therapsids—the mammal-like reptiles.

Enlarged brains have continued in primates and humans that have the largest brain-to-body ratio till date. An analysis done by Snell in 1891 revealed one striking observation, that is mammals and birds have developed larger brains which are approximately 5-50 times heavier than the lower level vertebrates with similar body mass. The analysis had revealed what seemed a surprising fact at that time, that a larger brain would demand more energy. The basic of brain functioning of any kind is the electrical firing (electrical activity) of individual neurons. This electrical process is an energy demanding one — much more demanding than other cell types in the body. Enlarged brains means more neurons and more electrical activity, and hence, more energy demand.

The research done by Martin revealed that the total amount of energy available from the body to be supplied to the brain may have had constraining effects on its size. It was also revealed that organisms having enlarged brains do not have higher basal metabolic rates in comparison to the organisms with smaller brains. Another finding was that there is no significant difference in the percentage of energy supplied by the body to the brain between the cold-blooded and the warm-blooded animals. The question then arises is: given the similar metabolic processes and similar ratios of energy supply to the brain from the body, how do mammals manage the energy demanded by enlarged brains? There have been many studies looking for answers to this critical question and have revealed many aspects of the phenomenon. One of the important aspects that came out as a logical conclusion from all these efforts is that temperature plays an important role in brain enlargement.

Increased Body Temperature and Brain Function

The basic tenet of the warm-blooded organisms is that they have an increased body temperature than the cold-blooded organisms. This increased temperature gave the endothermic organisms an edge in brain functioning. In reality, this increased temperature made the neurons more energy efficient in producing electrical signals, the basis of brain functioning. To have an insight at how this is made possible, let’s have a look at the way a neuron generates and propagates electrical signals.

The basic unit of neuronal electrical activity is known as a spike or an action potential. When a spike is generated in a neuron, it influences other neurons in the vicinity and excites them, and in this process, a network among neurons shape up. A spike from a neuron is sent to a nearby neuron through axons—the lengthy part that can be observed in a neuron. Modern neuroscience has come to the understanding that the neural networks in the brain are responsible for myriads of cognitive processes that a brain can perform. Neural imaging studies have established that in primate brains, different brain regions are recruited for giving rise to different functions. For example, the brain region that is used for vocal and speech is different from the brain region used for visual cognition. Now, spike generation is a process that requires intense energy, so does the propagation of spikes among neurons forming a network.

The warm-blooded organisms could generate spike through a much more energy efficient way than the cold-blooded ones. But how do they do it? McCormick and his colleagues have shown that generation of spikes is a temperature dependent process. With an increase in temperature, more spikes are generated increasing the excitability of a neuron and also facilitating the process of network formation. As the warm-blooded organisms have increased body temperature, this process of generating spikes becomes easier. So, with the same amount of energy supplied from the body to the brain, the warm-blooded animals can generate more neuronal excitation than the cold-blooded ones.

One study also argues that during the terrible mass extinction in the course of the Permian period, which was some 252 million years back, warm bloodedness of some mammalian ancestors gave them the edge to survive. This has also opened up the possibility of origin of mammal endothermy older than previously thought.