In other words: a fish’s brain grows larger when you have to think more. It is in any case the formula It was used by biologist Frédéric Laberge, of the University of Guelph in Ontario, to summarize two of his team’s research, the moon On Trout Lake in Lakes Ontario and other on rainbow trout, both published in June.

Here we are talking about “relative size”, that is, the proportion of the size of the animal occupied by the brain. The study of rainbow trout is unique as it relates to fish that have escaped from a farm. After seven months of swimming in the nearby lake, the trout developed a relative size of 15% larger than those remaining on the fish farm.

In the case of lake trout in their natural environment, brainstem increased during autumn and winter, and decreased during winter and summer.

And this kind of comparison is not the first: the same team published a similar study in 2018 in the brain pumpkin seeds (or sun perch), a freshwater fish. A study estimated the difference in size between the brains of fish that swam close to shore – a “spatially more complex environment” – and those that swam in open water at 8%.

It is clear that swimming in a challenging environment requires more “brain resources” to survive. But so far there have been only a few examples of such an evolutionary mechanism at work in such a short period of time. And the opposite is also true: a smaller brain can also be an advantage, because it requires less energy, and thus can be used to swim faster or hunt for food faster.

Photo: Truite grise. Count Gordon / Yukon Government