The ones with the biggest brains could indeed be the fittest to survive in this world as researchers have found that in times of crisis, even fish with larger brain sizes have an advantage over their peers.
The researchers examined why investing in a larger brain might provide an evolutionary advantage to compensate for the fact that brain mass is very expensive to develop and maintain.
"We have provided the first experimental proof that a large brain offers an evolutionary advantage," explained first author Alexander Kotrschal from the Stockholm University in Sweden.
They conducted an artificial selection experiment and successfully generated large- and small-brained guppies.
The researchers released 4,800 guppies from these selection lines into large semi-natural streams, which also contained a natural predator.
About half a year later, significantly more guppies with large brains had survived.
The researchers suggested that large-brained fish have an advantage that allows them to better evade predation.
"Our findings support the hypothesis that large brains provide a survival benefit under predation pressure," co-author Dustin Penn from the University of Veterinary Medicine Vienna, Austria.
The findings appeared in the journal Ecology Letters.