Fish literally get brainier when they have to think harder, and less brainy when they don’t. At least, that is the implication of two studies by Frederic Laberge at the University of Guelph in Canada and colleagues that show fish brains grow larger relative to their body size in more challenging environments and shrink in less challenging ones.

Changing relative brain size as needed could help fish save vital resources. “The brain is known to be one of the most energetically expensive tissues to maintain,” says Laberge.

In one study, Laberge and his team studied lake trout (Salvelinus namaycush) across six consecutive seasons in two different lakes in Ontario, Canada. They found that brain size relative to body size increased in autumn and winter and decreased in spring and summer.

Lake trout avoid warm water so they are limited to deeper water in the summer, Laberge says. But during the winter they forage in shallower waters near the shore, which is a more complex environment. This higher cognitive demand appears to boost brain growth.

“That’s the assumption,” Laberge says, though the study can’t show why these changes happened.

Read more: Predators may make prey get smart and grow more brain cells

In a second study, he and colleagues compared the brain sizes of rainbow trout that had escaped from a fish farm in Canada and begun living wild in a lake with those that remained captive. After seven months, the brains of the escaped trout were 15 per cent heavier relative to body size than those of the captive fish.

This increase was specific to the brain, Laberge says. There was no change in the relative size of the heart, for instance.

Previous lab studies by other groups have suggested that fish brains are plastic and change size as needed, Laberge says. Fish in labs have smaller brains compared with the same fish in the wild, and enriching their environment increases brain size. But his team is the first to show this happening in the wild.

In mammals, new neurons develop only in a few specific areas during adult life. But in fish, amphibians and reptiles, this occurs throughout the brain, Laberge says. This could be why the brains of at least some fish can grow when required, though the team hasn’t studied what is happening at a cellular level.

References: Authorea, DOI: 10.22541/au.162312317.76022973/v1 and bioRxiv, DOI: 10.1101/2021.06.17.448828