New research on swordtail fish suggests male courtships displays aren't always just for appealing to the ladies, but for showing off and scaring away other males too.

Behavioral ecologist Heidi Fisher was investigating northern swordtails she collected in Mexico.

Male swordtails court by swimming close alongside females, raising their typically large sail-like dorsal fins and quivering briefly.

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To see what female swordtails found attractive, Fisher tested them against video animations of males, which they varied by body size, coloration, the movement of the male, "and of course dorsal fin size," Fisher told LiveScience. "I assumed they would prefer the large dorsal fin, and was surprised when they didn't."

Instead, female swordtails preferred males with small fins more than large ones.

To see why male swordtails might flaunt an easily concealed large dorsal fin during courtship if females avoided males with large fins, Fisher and her colleague Gil Rosenthal tested males against video animations of other males.

Male swordtails were less aggressive toward video animations of males with large dorsal fins.

When paired male and female swordtails were confronted with another swordtail, the scientists also discovered that males raised their dorsal fins more frequently when courting in the presence of other males.

Fin raising, a key aspect of the male courtship display, "is not even intended for females," Fisher said.

"Males must balance what females are attracted to with what will deter rival males — the two conflict, so it's a balancing act for a male to obtain a mate," she added.

The scientists will report their findings in print in the journal Biology Letters.

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