Scientists have figured out why mammals have large brains

(ORDO NEWS) — In the largest study on the evolution of mammalian brain size over the past 150 million years, scientists compared the brain weights of 1,400 living and extinct species and charted changes in brain and body size. The authors refuted the long accepted belief that the level of intelligence in animals depends on the ratio of brain size to body size, and identified two turning points in the evolution of the mammalian brain.

The article was published in the journal Science Advances.

For a long time, scientists considered the relative size of the brain to be a reflection of cognitive abilities and based on this indicator they built the main theories of the evolution of life.

Researchers from seven countries – biologists, evolutionary statisticians and anthropologists – compared in a new study how mammals have changed brain size relative to body size over a long period of evolution. At the same time, instead of data on the mass of the brain, the authors used data on the endocranial volume of the skulls obtained from the analysis of 107 fossils, among which are the skulls of ancient whales and the oldest monkey skull ever found.

The authors found that the species with the largest relative brain sizes – humans, dolphins, and elephants – went to this by different evolutionary paths. Thus, elephants grew in size over time, and their brains grew even faster than their bodies. Dolphins, on the other hand, became smaller and smaller, and their brains also decreased.

In great apes, evolution followed the path of increasing diversity with a general tendency to increase the size of the brain and body. Interestingly, early hominins of the human line showed a relative decrease in body size and an increase in brain size compared to great apes.

The researchers note that the complex and ambiguous patterns of brain-body co-evolution they have identified require a reevaluation of the deeply ingrained paradigm that brain-to-body ratio is used as a measure of intelligence of any kind.

“Many mammals with large brains, such as elephants, dolphins and great apes, also have large body sizes. But this is not always the case. The California sea lion, for example, has a relatively small brain size in contrast to their outstanding intelligence,” press release from Flinders University of Australia, first author Jeroen Smaers, an evolutionary biologist at Stony Brook University in New York.

Taking evolutionary history into account, current research shows that the California sea lion has achieved a low brain-to-body ratio due to rigorous natural selection for body size in the semi-aquatic species niche. In other words, its low relative brain size is associated with an increase in the body, not a decrease in the brain.

“We have disproved the long-standing dogma that relative brain size is associated with intelligence,” said study leader Kamran Safi, a researcher at the Max Planck Institute for Animal Behavior in Germany. according to a new habitat or mode of transportation – in other words, it has nothing to do with intelligence.”

The study also found that most of the changes in mammalian brain size occurred after two catastrophic events in Earth‘s history: a mass extinction 66 million years ago and climate change 23-33 million years ago.

Following the mass extinction at the end of the Cretaceous, researchers have noted a dramatic shift in brain and body scaling in rodents, bats and predators that have flocked to empty niches left behind by extinct dinosaurs. A cooling in the Late Paleogene led to profound changes in other groups of mammals: seals, bears, whales and primates underwent evolutionary shifts in brain and body size.


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