Female Bluefin Killfish Judge Fish by Their Color

You know the old adage don't judge a book by its cover? Well, apparently that doesn't apply to female bluefin killfish, who, according to recent findings, prefer males that are yellow in color.

In nature, male bluefin killifish exhibit three distinct color forms: blue, red, and yellow. However, it is difficult to determine whether these various colors impact the males' strategies for attracting a mate, and whether female fish respond differently to each shade.

So, researchers at the New York University Polytechnic School of Engineering conducted an experiment in which they substituted the notoriously erratic live male bluefin killifish with replicas that were controlled by a robotics-based platform. The fake fish used were made to resemble body size, aspect ratio, colors, and motion pattern of adult male killifish.

In order to figure out how female bluefin killfish would react, the researchers controlled the replica's movements using a robotic arm, performing the typical movements associated with male courtship behavior - known as a "mating dance."

"The benefit of using a robotic platform is that we can standardize the stimulus, or in this case, the 'mating dance' of the killifish," Maurizio Porfiri, professor of mechanical and aerospace engineering, said in a press 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."

During each trial, Porfiri and his collaborators would expose the female fish to their designed replicas, in each scenario representing a variation in color naturally present in male killifish. Once looking back at webcam footage, they surprisingly found that females were more likely to swim in the section of the tank containing the yellow-colored replicas, rather than those with the blue and red varieties.

The research team believes that this sunny color may serve as a "superstimulus" for these killfish, resulting in the females' preference for yellow mates.

The findings were published in the journal Bioinspiration & Biomimetics.