Females Judge Males by Their Colors: Killifish Prefer Yellow Fins on Robots

Don't judge a book by its cover--unless you're a killifish. Scientists have found that female killifish prefer males with yellow fins, despite the various color morphs that male bluefin killifish display.

In the wild, male bluefin killifish have three distinct color morphs: blue, red and yellow. Yet it's difficult to determine whether color impacts the males' strategies for attracting a mate. It's even more challenging to test whether female fish respond differently to the various color morphs. That's why scientists decided to take it to the lab.

The researchers created replicas of male killifish with various color morphs in order to see whether color impacts the males' strategies for attracting a mate. Then, they exposed fertile female killifish to these robotic replicas in a divided tank. They moved the replicas with a robotic arm in order to replicate the typical motions associated with male courtship.

"The benefit of using a robotic platform is that we can standardize the stimulus, or in this case, the 'mating dance' of the killifish," said Maurizio Porfiri, one of the researchers, in a news release. "We eliminate the other variables that you would normally see when dealing with live animals, allowing us to focus just on the male's color and whether it has an impact on the female's preference."

In the end, the researchers found that female killifish seemed to prefer yellow-colored replicas.

"The only trials in which the female spent long time periods swimming near the replica was when the mate was yellow," said Porfiri. "In the presence of a yellow male, the females were actively engaged, swimming parallel to the replica and darting back and forth between the mate and the nest."

The findings actually overturn previous studies in which females showed a slight preference for blue or red fins. Instead, it seems that females prefer yellow fins; in other species, notably mosquitofish, females have also shown a preference for yellow pigmentation.

The findings are published in the journal Bioinspiration and Biomimetics.