Fish have "designed" their perfect partners in a new experiment – revealing simple male preferences and more complex female tastes.
University of Exeter scientists put green swordtail fish in tanks with a screen at either end displaying different virtual mates.
Each time a fish chose a favourite (measured by time spent near each option), the winning virtual fish would "survive" and be paired with a winner from another trial to produce a new "generation" of offspring that were tested in the next round of trials.
This allowed fish to hone in on their ideal mate over the course of nine generations.
Males consistently chose larger females, while females showed much more individual variation – favouring, for example, longer tails or brighter colours.
"Studying mate choices in animals is difficult because experiments can only present animals with a limited range of potential mates," said Kathryn Bullough, from the Centre for Ecology and Conservation at Exeter's Penryn Campus in Cornwall.
"Our new approach allowed fish to 'design' their ideal partner, with genetic algorithms creating new sets of options for fish to choose from, allowing them to refine their preferences."
In the experiments, 11 females and 12 males were tested individually, with a moving image of a possible mate displayed at either side of their tank.
The green swordtails were free to swim for three minutes, and their movement was observed in three "zones" – the neutral centre, and a zone near each virtual mate.
If a fish spent longer near one virtual mate than the other, that mate "won" and two more virtual fish were created based on its characteristics and those of another random winning mate.
This mimics sexual selection, the idea that certain attractive traits make an individual more likely to be chosen and reproduce, so these traits reappear and become stronger down the generations.
Due to males' preference for larger females, the final generation of virtual females were more than 24% larger on average than the first generation. This preference probably reflects the fact that larger females can produce more offspring.
One female chose mates that looked increasingly like females. The reasons for this are unclear, but it's possible that this female avoided threatening male characteristics and moved away from them – and therefore towards more female traits.
Native to Central America, green swordtails get their name from the males' distinctive sword-like tail fin.
Dr Laura Kelley said: "Green swordtail fish are good test subjects for this experiment as they respond to videos of other fish, and they provide no parental care for their young. This means their mate choice is relatively straightforward, as fish are primarily looking for good genes to pass onto their offspring.
"A study like this using real fish would take years, and our new method provides a powerful tool to understand how wild animals choose their mates."
The study, funded by the Royal Society and published in the journal Royal Society Open Science, is entitled: "Exploring male and female mate preferences using genetic algorithms and digital phenotypes."
