First evidence that wild mammals benefit from bigger brains

Red deer on Rum in Scotland

blickwinkel/Alamy Stock Photo

By Claire Asher

We pride ourselves on our big brains, but when it comes to figuring out whether people or other animals with particularly big brains do better than others, the evidence has been lacking.

Now, for the first time, a study in red deer is showing that bigger brained mammals tend to be more successful in the wild, and that brain size is a heritable trait that they can pass on to their offspring.

Corina Logan from the University of Cambridge and her team have looked at the skulls of 1314 red deer (Cervus elaphus) from the Isle of Rum. The complete __life histories of the deer are well known thanks to the Isle of Rum Red Deer Project, which has been collecting data on the island for more than 40 years, spanning seven deer generations.

“This kind of study has not been conducted before because it requires long-term data from a large number of individuals,” says Logan.

Heritable heads

The team found that the ratio of skull volume to body size was highly heritable, explaining 63 per cent of variation between individuals.

Female deer with larger skulls lived significantly longer and raised more offspring to adulthood, though it’s not clear yet why bigger brains are advantageous to females.

It might be that they allow them to switch to alternative resources in times of environmental stress, or that they are simply correlated with longer __life cycles, rather than causing the longer lifespans. But no one knows yet.

Alas poor Bambi, I knew him. And Doris. And Cassius. And Daisy. And Lucius…

Corina Logan

Surprisingly, larger-brained male deer showed no such benefits.

This suggests there may be sex differences in the costs and benefits related to larger brains, says Logan. Any benefits of brain size for male lifespan and reproduction may be masked by other factors.

“In species where males have to fight with other males to get access to females, like deer, the fitness of males may have more to do with being strong enough to successfully compete for females than with other attributes,” says Daniel Sol Rueda from the Centre for Ecological Research and Forestry in Barcelona, Spain.

Slow developers

Larger brains may offer adaptability, ingenuity and a better chance of survival, but they also take longer to develop and require more energy to maintain. Previous laboratory experiments with guppies, for example, found that larger brains were associated with improved learning but fewer offspring.

However, the new study – the first one to look at the issue in the wild – found no evidence of downsides of big brains, such as breeding more slowly or taking longer to reach sexual maturity.

This means natural selection may favour increases in brain size in female deer for their beneficial effects on survival and reproduction.

Logan says it’s possible that large brains aren’t as costly as people think, or there may be other ways that the deer make up for the energetic investment, such as weaker immunity.

Journal reference: Royal Society Open Science, DOI: 10.1098/rsos.160622

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