Avian neurons use three times less glucose than mammalian neurons

Birds have impressive cognitive abilities and display high levels of intelligence. Compared to mammals of about the same size, bird brains also contain many more neurons. Now reports a new study Current Biology on September 8 helps explain how birds can afford to have more brain cells: their neurons get by on less fuel in the form of glucose.

“What surprised us the most is not per se that the neurons use less glucose – one could have expected that given the different sizes of their neurons,” says Kaya von Eugen from the Ruhr University Bochum. “But the magnitude of the difference is so great that the size difference can’t be the only factor. That means there must be something else in the bird brain that allows them to keep costs so low.”

A landmark 2016 study showed that the bird brain contains many more neurons than a similarly sized mammalian brain, the researchers explained. Since brains are generally made up of energetically expensive tissue, this raised a crucial question: how can birds support so many neurons?

To answer this question, von Eugen and colleagues set out to determine the neuronal energy balance of birds based on studies on pigeons. They used imaging techniques to estimate the birds’ glucose metabolism. They also used modeling approaches to calculate the brain’s metabolic rate and glucose consumption.

Their studies revealed that the brains of pigeons use a surprisingly small amount of glucose (27.29 ± 1.57 μmol glucose per 100 g per minute) when the animal is awake. That means a surprisingly low energy budget for the brain, especially compared to mammals.

This means that neurons in the bird brain use, on average, three times less glucose than those in the mammalian brain. In other words, their neurons are less expensive for reasons that are not yet clear.

Von Eugen says it’s possible the differences may be related to the birds’ higher body temperatures or the specific layout of their brains. Bird brains are also smaller, on average, than mammalian brains. But their brains retain impressive abilities, perhaps in part due to their less expensive but more numerous neurons.

“Our result explains how birds are able to support such a high number of neurons without compromising processing power,” says von Eugen. “In the long parallel evolution of birds and mammals, birds evolved smaller brains with high numbers of neurons capable of advanced cognitive performance. A bird brain-specific layout – may have generated a possible advantage in neural information processing with higher efficiency: cheap neurons with advanced processing capacity.

The researchers say they now want to understand more about how birds’ neurons use less glucose. While they have ideas about how it might work, more study and testing is needed to “uncover the precise mechanistic explanation for how birds achieve such higher neural processing efficiency”.