Are smarter fish sexier?

Is the evolution of intelligence partly driven by love? A new study involving tiny fish swimming in mazes suggests that smarter fish have a better chance of mating and reproducing.

“Evolution of animal intelligence has long been thought to have been driven by natural selection,” says lead author Ivan Vinogradov, a biologist at the Australian National University (ANU.)

“Animals that were better at problem-solving were more adept at gathering food, finding shelter, and avoiding predators and hence lived longer. They then passed these genes onto their offspring, helping future generations become smarter over time.”

But could there be another explanation? For example, could intelligent animals simply have a better chance of mating?

“A better brain might help an animal find more mates, have more sex, and eventually have more babies,” Vinogradov posits.

To investigate this, Vinogradov and team put a species of mosquitofish (Gambusia holbrooki) through its paces.

The fish is no bigger than a matchstick, but is now found globally due to its appetite for mosquito larvae. It was introduced to Australia in 1929 to control mosquitoes and is now classified as a pest.

The team designed a series of underwater problem-solving tests for the male fish, with the aim of measuring four aspects of their intelligence:

1. inhibitory control (the ability to suppress the impulse to swim through a transparent barrier to reach females and instead swim around it)
2. spatial learning (the ability to navigate through a maze to reach females)
3. associative learning (the ability to learn which visual cues in a maze would lead them to females)
4. reversal learning (the ability to recognise when clues had been switched and learn new associations)

Then, the team tracked the fish over the next two months to record how many offspring each male produced – performing 2430 paternity tests in the process.

The data shows that the males which successfully passed the underwater tests mated with more females and produced more offspring.

But not all cognitive abilities gave the fish sexual advantages. Specifically, males that tested for greater inhibitory control and better spatial learning abilities had more offspring. However, test scores for associative and reversal learning didn’t predict how many offspring a male would have.

“This suggests that intelligence in mosquitofish partly evolved through sexual selection, where traits that boost mating and fertilisation success become more common over generations,” Vinogradov says.

“Our study suggests that intelligence in mosquitofish isn’t only driven by their need to find food or avoid predators, but also by the complex challenges of finding love.”

So what exactly gives the cleverer males an edge?

“Perhaps females recognised and preferred smarter males, or maybe smarter males were better at chasing the females and forcing them to mate, a common, if unpleasant, practice in mosquitofish,” says study co-author Michael Jennions, also from ANU.

“Future research is needed to observe the males more closely and see how the mating behaviour of smarter and dumber males differs.”

The research appears in Nature Ecology and Evolution.