Small, but mighty: Researchers have gained new insights into the basics of the often high performance of bird brains. Apparently, they can afford a high density of nerve cells, since their neurons get by with three times less energy than those of mammals. This emerges from an analysis of glucose consumption in the brains of pigeons. Exactly what this high energy efficiency is based on is now an exciting question in brain research.
"You sparrow brain!" Little mass means little performance - according to this simple formula, birds were long considered to be rather "weak-headed". However, numerous studies in recent years have clearly refuted this: the cognitive abilities often correspond to those of mammals or even surpass them, and the bird world also has some very clever people to offer: the intelligence of some corvids and parrots can even be compared to that of primates. This raised the question of how the rather small brains of birds could produce such minds. For example, the brains of crows and parrots, which weigh about 10 to 20 grams, can easily compete with the 400 gram brains of chimpanzees. The ratio of brain and body mass plays a decisive role here – but this tends to be significantly lower in birds than in mammals.
How can birds afford so much brains?
Basically, the secret of the performance of the rather small bird brains is apparently based on a high nerve density in comparatively small neurons, as an earlier study has already shown. The bottom line is that there is even a plus: Some bird species have two to three times as many nerves in their brains as many mammals of the same size. This, however, gave rise to a new question: How can birds afford this in terms of energy? Nerve tissue is extremely expensive. The human being represents the extreme: Our brain makes up only about two percent of our body weight, but consumes about 20 to 25 percent of the entire body energy. "The brain is by far the most expensive organ in our body in terms of energy and we could only afford it over the course of evolution if we were able to learn to successfully supply ourselves with a lot of energy," says Onur Güntürkün from the Ruhr University Bochum.
This also applies to other animals: energetic costs limited the increase in the number of neurons. But why is this limit apparently wider in birds than in mammals? To answer this question, Güntürkün and his colleagues took a close look at the energy consumption of pigeon brains. It should be emphasized that these often underestimated birds also have considerable cognitive abilities. The study used the imaging method of positron emission tomography: Using a special contrast agent, the scientists recorded how much "fuel" the nerve cells in the brain of the pigeons consumed in the form of glucose when they were awake or anesthetized.
Amazingly low energy requirement
Their evaluations and modeling of energy consumption showed that when awake, the pigeon's brain only needs 27 micromoles of glucose per 100 grams of neuronal tissue. "Breaked down to the individual neurons, this results in a rate three times lower than that of an average mammalian neuron," the researchers write. "What surprised us is not that the nerve cells consume less glucose at all - that was to be expected given their smaller size," says Güntürkün. "But the fact that the difference is so large means that birds have additional mechanisms that reduce the energy consumption of nerve cells," explains the scientist. According to the team, the results in the pigeons can probably be transferred to other bird species. "Our study fits into a growing number of studies showing that birds have developed their own and very successful way of developing intelligent brains during evolution," says Güntürkün.
The first author of the study, Kaya von Eugen Kaya from the Ruhr University Bochum, continues: "In the long parallel evolution of birds and mammals, birds have developed smaller brains with a high number of neurons, which lead to advanced cognitive performance in the location. And it seems that the combined action of bird-specific elements has led to a potential advantage in neuronal processing of information with higher efficiency: energy-conserving neurons with advanced processing capacity," the researcher said.
So far, however, it remains unclear what exactly is behind the thriftiness of bird neurons. "This could be partly related to the higher body temperature of birds, but probably also to additional factors that are currently completely unknown," says Güntürkün. Therefore, the scientists now want to conduct further investigations to "find the precise mechanistic explanation for how birds achieve their increased efficiency in neuronal processing."
Source: Ruhr University Bochum, specialist article: Current Biology, doi: 10.1016/j.cub.2022.07.070