Baleen whales, such as this humpback, followed the standard ancestral mammal relationship between body and brain: as their body grew bigger, so did their brain © Earth theatre/

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Study shows how mammals evolved to have bigger brains

New research has demonstrated that, contrary to popular belief, relative brain size in mammals is not solely linked to intelligence but is driven by various evolutionary pressures on body size, including adaptations caused by mass extinction and changes in climate.

The international study – the largest of its kind ever carried out – involved a team of 22 scientists, who investigated 1,400 living and extinct mammal species. In the UK, this involved researchers from the Natural History Museum, the University of Salford, University College London (UCL), and Anglia Ruskin University (ARU).

This groundbreaking work has pieced together a unique timeline of how brain and body size has evolved over the last 150 million years, showing how “big-brained” species have attained their extreme proportions in different ways.

Great apes have a wide range of body sizes but have demonstrated a general trend towards increases in both brain and body size. In comparison, ancestral hominins, which represent the human line, have shown a relative decrease in body size and increase in brain size compared to great apes.

Prof Anjali Goswami, Research Leader at London’s Natural History Museum, said, ‘To me, this study exemplifies the dangers of our own biases in interpreting the natural world. As big-brained humans, we have long assumed that differences in brain size across species was due to selection for big brains. With this work, we now understand that what appear to be differences in relative brain size across species are often because of selection on body size and not due to any great push towards increased cognition or anything to do with brains at all.’

 ‘The most surprising results to me are those for whales. It is incredible that some of the most unusual and largest mammals, such as the blue whale, don’t deviate at all from the ancestral relationship between brain size and body size for mammals, despite being released from the many constraints of life on land. Through one of the most extreme evolutionary transitions that can occur, and thousand to million-fold increases in body size, and yet, they maintain the same allometry all the way through.’

‘Equally surprising was the result that actually dolphins evolved their relatively large brains not by increasing brain size, but rather through decreasing both brain and body size, but just decreasing their brain size less than body size.  The complexity of these patterns has forced us to completely shift our thinking on what drives the evolution of brain size and shows that there are lots of different ways that species end up with big brains.’

The study found that most changes in brain size occurred after two cataclysmic events in Earth’s history. After the mass extinction event at the end of the Cretaceous period 66 million years ago, the researchers noticed a dramatic shift in relative brain-body size in lineages such as rodents, bats and carnivorans, as these animals filled the empty niches left by extinct dinosaurs. 

Roughly 30 million years later, a cooling climate in the Late Paleogene period led to more profound changes, with seals, bears, whales, and primates all undergoing evolutionary shifts in their brain and body size. Elephants and great apes evolved their extreme proportions after this climate change event.

As well as demonstrating that brain size relative to body size has not followed a stable evolutionary trajectory, the study also shows that relatively big brains, long considered an indicator of animal intelligence, can be the result of a gradual decrease in body size to suit a new habitat, climate, or way of moving. In other words, relative brain size may have nothing to do with intelligence at all.

The 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 these aquatic carnivorans diversified into a semi-aquatic niche and their body increased in size at a faster rate than their brain as they adapted to also living on land.

Co-author Dr Jacob Dunn, Associate Professor in Evolutionary Biology at ARU, said: ‘These results make us completely change the way we think about brain evolution in mammals.’

‘For decades, biologists have assumed that relative brain size is a reliable indicator of intelligence among species. This has been especially true for research on primates, including our extinct hominin ancestors. Our study shows that we need to be more cautious when making assumptions about a species’ cognitive ability from its relative brain size, as the relationship between brain size and body size is complex.’

‘Our work highlights how variation in body size is a key driver of shifts in relative brain size, and opens up new opportunities for thinking about the complex ways that brains evolve.’

Dr Dorien de Vries, a post-doctoral research associate at The University of Salford said, ‘Our research shows that the clade containing the New World monkeys such as squirrel monkeys, tamarins, and howler monkeys underwent several evolutionary shifts in their body size and brain size.  This group of monkeys originated from a common ancestor that migrated from Africa to South America, and one could easily speculate that these evolutionary shifts may have been a response to the different ecological pressures and opportunities the monkeys encountered in South America.’

‘However, a parallel mammalian clade, the New World hystricognath rodents, such as capybaras, guinea pigs, and chinchillas, that share the same exceptional migratory history from Africa to South America at roughly the same time, underwent no evolutionary shifts in their brain and body size relationship.  It is fascinating that, despite the shared migratory histories of these two mammalian clades, the evolution of their brain and body size differs so considerably.’

The study The evolution of mammalian brain size is published in the journal Science Advances.

Notes to editors

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