Smart is sexy – new study on fish doing puzzles hints intelligence partly evolved via sexual selection

We humans often underestimate the intelligence of other animals. You’ve probably seen videos of monkeys, ravens or parrots solving puzzles.

But fish also possess impressive problem-solving skills, despite the notorious slander that goldfish have a three-second memory.

The intelligence of animals can be a useful tool when testing various ideas in biology. For example, could intelligence have evolved in part thanks to sexual selection, rather than as a means of survival?

In a new study published in Nature Ecology & Evolution, we used distinct tests to measure cognitive abilities of male mosquitofish – a thumb-sized fish endemic to central America but now a major pest in many parts of the world, including Australia.

We then tracked how many offspring each male produced when competing for mates in small ponds. Our study showed that smarter males had more offspring than their less intelligent brethren.

Our findings imply that the evolution of cognitive abilities may have been driven by sexual selection, with smarter males gaining more mating opportunities.

To be smart is to survive

Cognitive abilities, such as learning and problem solving, likely arose because they helped animals gather food, find shelter and avoid predators.

Individuals that were better at these tasks lived longer and passed on genes to their offspring that improved the offspring’s performance. Natural selection favoured smarter survivors who had more descendants than the average individual.

As a result, populations became smarter over time.

But there is another explanation for the evolution of intelligence: smarter is sexy. A better brain might help an animal find more mates, have more sex, and eventually have more babies.

If this is the case, intelligence partly evolved through sexual selection, where traits that boost mating and fertilisation success become more common over generations.

We did our study on male fish – sexual selection is usually stronger on males than females, because in most species there are more males seeking mates than females ready to mate and breed.

Measuring animal IQ

Even in humans, intelligence can be difficult to pin down: maths skills, creativity, street smarts, and standardised IQ tests all capture different aspects of human braininess.

For animals, this challenge is tougher still. But biologists broadly agree that cognition is the ability to acquire, store, process, and act on information; and that distinct cognitive abilities are governed by different brain regions.

We designed four special underwater tests to tap into these distinct cognitive abilities of our male mosquitofish.

First, we measured their spatial learning by placing fish in a maze with a single correct route that led them to a shoal of their compatriots. Mosquitofish are highly motivated to swim with other fish, so reaching this shoal acts as a reward for solving the maze.

Second, we measured their self-control (formally called “inhibitory control”) by placing a transparent barrier between the fish and a reward. We then documented how quickly a male learned not to swim into the barrier but to detour around it.

Then, we measured associative learning by presenting a fish with two coloured corridors once a day. One colour (for example, green) led to a dead end, while the other (for example, red) to a reward.

The number of days it took a male to consistently choose the correct corridor – the one with a reward – indicated how quickly they learned the association.

Lastly, we reversed the colour cues to measure reversal learning. If green, for example, was previously the dead end, it now became the reward corridor, while red became the dead end. This tested how quickly the fish could “overwrite” his previously learned association to learn the new one.

A winning edge in mating

After these tests, we moved the males to ponds where they competed for mates. Two months later, the females gave birth, and genetic paternity tests revealed who fathered each offspring.

Males that scored highly on self-control and spatial learning had significantly more children. But why?

Something about these males seemingly gave them an edge in securing mating opportunities. Perhaps females recognised and preferred smarter males? Maybe smarter males were better at chasing the females and forcing them to mate (a common, if unpleasant, practice in mosquitofish).

Future research is needed to observe the males’ mating behaviours more closely and see if smarter and dumber males differ in how they court mates.

Our research sheds light on the evolution of our most prized possession – the brain. It seems that sophisticated intelligence isn’t only driven by our need to find food or avoid danger to survive, but also by the complex challenges of finding love.