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Source: Flinders University

Scientists have pieced together a timeline of how brain and body size evolved in mammals over the last 150 million years.

The international team of 22 scientists, including biologists, evolutionary statisticians, and anthropologists, compared the brain mass of 1400 living and extinct mammals.

For the 107 fossils examined—among them ancient whales and the oldest Old-World monkey skull ever found—they used endocranial volume data from skulls instead of brain mass data.

The brain measurements were then analyzed along with body size to compare the scale of brain size to body size over deep evolutionary time.

The findings, published in Science Advances, showed that brain size relative to body size—long considered an indicator of animal intelligence—has not followed a stable scale over evolutionary time.

Famous “big-brained” humans, dolphins, and elephants, for example, attained their proportions in different ways.

Elephants increased in body size, but surprisingly, even more in brain size. Dolphins, on the other hand, generally decreased their body size while increasing brain size. Great apes showed a wide variety of body sizes, with a general trend towards increases in brain and body size. In comparison, ancestral hominins, which represent the human line, showed a relative decrease in body size and increase in brain size compared to great apes.

The authors say that these complex patterns urge a re-evaluation of the deeply rooted paradigm that comparing brain size to body size for any species provides a measure of the species’ intelligence.

“At first sight, the importance of taking the evolutionary trajectory of body size into account may seem unimportant,” says Jeroen Smaers, an evolutionary biologist at Stony Brook University and first author on the study.

“After all, many of the big-brained mammals such as elephants, dolphins, and great apes also have a high brain-to-body size. But this is not always the case. The California sea lion, for example, has a low relative brain size, which lies in contrast to their remarkable intelligence.”

Associate Professor Vera Weisbecker, who participated in the research, said that the paper also proved an important point regarding marsupial mammals, such as koalas, kangaroos, or possums.

“Scientists are often prejudiced against marsupials. They are considered primitive and small-brained because they are born at tiny sizes and their brain mostly develops after birth. However, this study shows that marsupial brains have a similar relationship with body size as other mammals, such as bats, some rodents, and shrews”

By taking into account evolutionary history, the current study reveals that the California sea lion attained a low brain-to-body size because of the strong selective pressures on body size, most likely because aquatic carnivorans diversified into a semi-aquatic niche.

In other words, they have a low relative brain size because of selection on increased body size, not because of selection on decreased brain size.

“We’ve overturned a long-standing dogma that relative brain size can be equivocated with intelligence,” says Kamran Safi, a research scientist at the Max Planck Institute of Animal Behaviour and senior author on the study.