Mammals bulking up instead of growing bigger brains helped them survive extinction

Prehistoric mammals prioritised bulking up over brain size to boost their chances of survival after the extinction of the dinosaurs, a new study has claimed.

The research, published on Thursday in the journal Science, said mammals boosted their body size for the first 10 million years after dinosaurs died out. This helped them adapt to radical shifts in the make-up of Earth’s animal kingdom.

Scientists, including those from the University of Edinburgh in the UK, said the size of mammals’ brains decreased compared with their body weight following the catastrophic asteroid impact 66 million years ago that ended the reign of dinosaurs.

Until now, researchers said it was unclear how prehistoric mammals developed in the first few million years following the mass extinction.

To shed light on this mystery, they performed CT X-ray scans on newly discovered fossils from the 10-million-year period after the extinction, called the Paleocene.

Findings revealed that the relative brain sizes of mammals at first decreased because their body size increased at a much faster rate.

You can see here the Paleocene Arctocyon on the left, and a later Eocene crown-group placental mammal on the right. Look at just how much smaller that Arctocyon brain is!! It almost looks like a reptile.

The study said prehistoric animals relied heavily on their sense of smell, while their vision and other senses were less well developed – indicating it may have been initially more beneficial to be big than highly intelligent in order to survive in the post-dinosaur era.

Then about 10 million years later, early members of modern mammal groups, such as primates, began to develop larger brains and a more complex range of senses and motor skills.

Such traits at a time when competition for resources was far greater would have improved their survival chances, scientists said.

Based on the findings, they said big brains may not always be better to invade new environments or survive extinctions.

“Large brains are expensive to maintain and, if not necessary to acquire resources, would have probably been detrimental for the survival of early placental mammals in the chaos and upheaval after the asteroid impact,” study lead researcher Ornella Bertrand, of the University of Edinburgh’s School of GeoSciences, said in a statement.

“The mammals that usurped the dinosaurs were fairly dim-witted, and only millions of years later did many types of mammals develop bigger brains as they were competing with each other to form new ecosystems,” said study senior author Steve Brusatte, also based at the University of Edinburgh.

The findings, according to the researchers, can lead to a better understanding of these bizarre prehistoric animals and the evolution of the mammalian brain.