“As clever as a guppy” is not a huge compliment. But intelligence does matter to these tropical fish: big-brained guppies are more likely to outwit predators and live longer than their dim-witted peers.
Alexander Kotrschal at Stockholm University, Sweden, and his colleagues bred guppies (Poecilia reticulata) to have brains that were bigger or smaller than average. His team previously showed that bigger brains meant smarter fish.
When put in an experimental stream with predators, big-brained females were eaten about 13 per cent less often than small-brained ones. There was no such link in males, and the researchers suspect that their bright colours may counter any benefits of higher intelligence. They did find, Kotrschal says , that large-brained males were faster swimmers and better at learning and remembering the location of a female.
“This is exciting because it confirms a critical mechanism for brain size evolution,” says Kotrschal. It shows, he adds, that interactions between predator and prey can affect brain size.
It might seem obvious that bigger brains would help survival. Yet previous research simply found a correlation between the two, leaving the possibility open that some third factor may have been driving the effect. Now, direct brain size manipulation allowed Kotrschal’s team to pin it down as a cause of better survival.
“This is the first time anyone has tested whether a larger brain confers a survival benefit,” says Kotrschal. “The fact that large-brained females survived better in a naturalistic setting is the first experimental proof that a larger brain is beneficial for the fitness of its bearer. This is like watching evolution happen and shows how brain size evolves.”
Size is a critical trait underlying cognitive abilities, but it is not the only trait, and there is enormous variation in brain size of vertebrates – both relatively and absolutely.
“Our study shows for the first time how differences in vertebrate brain size can evolve and helps explain why brain size is so variable,” says Kotrschal.
“This study shows us that large brains can improve the odds of survival in a dangerous world,” agrees David Reznick from the University of California in Riverside.
Given that brains are energetically costly this means there is an optimal brain size that balances these costs and benefits, he says.
“We know that brain size varies among species and, in particular, that humans have large brains,” says Reznick. “These differences among species means that brain size must evolve.”
But we have never seen the process in action before, he says, meaning we had to guess the reasons for a change in brain size or its consequences.