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Vertebrates have extremely different brain sizes: even with the same body size, brain size can vary a hundredfold. As a rule, mammals and birds have the largest brains in relation to their body size, followed by sharks and reptiles. Amphibians and most fish, on the other hand, have the smallest brains of all vertebrates.

Why is this the case? In some animal groups, species that live in groups have larger brains than solitary species. They have to cope with rapidly changing social situations and therefore need a more powerful brain.

In addition, mammals and birds, which generate their own body heat and therefore have a higher and more stable body temperature, have larger brains than most other vertebrates, whose body temperature is determined by the ambient temperature. But so far we do not have a solid explanation for this difference. Moreover, even within these groups, there are still major differences.

Brain tissue requires a constant amount of energy. Unlike other organs, the brain cannot simply shut down during sleep or periods of hunger. So when the brain grows larger, the organism must find the energy to supply it. According to the "Expensive Brain Hypothesis," the brain can only grow if it produces the additional energy itself or if it improves the organism's chances of survival so much that it can afford to grow and reproduce more slowly.

This explains, for example, why monkey species that do not have to endure periods of hunger and thus energy loss throughout the year have larger brains, and why the brains of sedentary birds are larger than those of migratory birds.

Researchers at the Max Planck Institute for Animal Behavior in Konstanz have investigated whether these correlations apply to all vertebrates. In their study in the journal Proceedings of the National Academy of Sciences, they found that in all vertebrate groups, body temperature has a significant influence on brain size.

Species that can keep their bodies constantly warm can usually afford larger brains, as these are more efficient in warm environments. This also holds for so-called cold-blooded species that live in warm waters or specifically select such places.

In addition, according to the researchers, the size of the offspring also limits brain size in adulthood. Since the costs of a large brain in relation to weight are particularly high for young animals, it pays to keep the value low at first. Those lineages that manage to both keep their bodies warm and produce large young have the biggest brains for a given body size.

"We humans were lucky to be warm-blooded. In addition, our babies are large and fed for years. This allowed the evolution of one of the largest brains of all vertebrates in relation to weight," says Professor Carel von Schaik, head of a fellow group at the Max Planck Institute of Animal Behavior.

A constantly high body temperature was therefore a prerequisite for evolution to produce larger brains. However, this ability originally developed for other reasons—presumably, so that mammals could remain active at night and birds could fly longer distances. Only then was the door open for brain growth. In evolution, innovations can therefore have unexpected consequences and open up completely new possibilities.