Jan. 17, 2006 — Swordtails, colorful, common aquarium fish, size up their swords before fighting, and often a male just has to show his big sword to scare off challengers, according to a new study in the latest Animal Behavior journal.

Yet another study in the same publication concluded that female swordtails prefer male swordtails with big, striped swords.

Researchers even think the female fixation on these bright appendages led to their emergence in the first place, since female fondness for novel traits and bright colors appears to have preceded the appearance of swords on males.

The finding suggests the creation of some secondary sexual characteristics may be influenced, and even controlled, by the opposite sex. For male swordtails, this characteristic is an extension of the caudal fin, or tail, that looks like a pointy sword.

"It is not terribly rigid and cannot be manipulated very efficiently," said Kari Benson, a professor of biology at Lynchburg College. "It is not useful as a weapon. It is only used as a visual signal in a fight."

Benson, who co-authored the first paper with Alexandra Basolo, also a Lynchburg professor, explained to Discovery News, "The sword might signal many things: how strong and vigorous he is, how well he ate while growing up or simply by indicating that he is large."

The researchers staged duels between swordtails, Xiphophorus helleri, by putting two comparably sized males with different-sized swords in a tank and then observing their interactions, which usually involved aggressive behavior, such as chasing and biting.

The researchers then artificially manipulated sword size by affixing fake plastic swords to the fish.

Repeatedly, the male with the bigger sword won. Often all he had to do was to show his sword and then the other male slunk away.

Basolo, associate professor of biological sciences at the University of Nebraska, told Animal Planet News that the loser really looks the part.

"His body darkens, due to a stress response, he puts his head down and he then positions himself at the periphery of the tank," she said, and added that the loser even folds in his fins.

Brian Trainor, a postdoctoral researcher in the College of Social and Behavioral Sciences at Ohio State University, told Animal Planet News, "One part I was a bit amazed with was that in the sword manipulation experiment, somehow the fish were able to determine their own artificial sword sizes," he said.

Trainor added, "I think people usually think that fish are not too bright. Not only were these swordtails able to assess their competitors' apparent sword length, but also they were able to compare that with their own sword length. I thought that was a cool result."

He even believes there is a swordtail-human link, since both species seem to have evolved traits and behaviors that do not offer any apparent survival benefits.

The sword, for example, appears to be useless, save for its visual impact. For humans, he said possible examples include artistic abilities and senses of humor.

Like male fish showing off their swords to females, he suggested that these human abilities might exist, in part, to impress members of the opposite sex. Like the fish swords, they can also weed out competitors who may not be as talented or humorous.

Trainor added, "Our studies in swordtails show that sexually selected traits can be complex and used in multiple behavioral contexts, much like many aspects of human behavior."