Effects of male apparent length on female preference for absolute body size in Xiphophorus helleri

Abstract

Female green swordtails (Xiphophorus helleri) prefer males of longer body and sword lengths, which may reflect perceptual constraints. Larger males project a larger image onto the female’s retina at a given viewing distance, which may elicit greater stimulation of the visual system and a stronger behavioral response. This study investigates the effects of male apparent size, as affected by proximity and sword length, on female preference for absolute body size in X. helleri. Females were presented pairs of dummy males in ten treatments organized into six categories: (1) the Actual Length Control where identically sized males were held at different viewing distances from the female; (2) the Sword Compensation Treatment in which viewing distance and apparent length (body + sword) were held constant, pairing sworded with swordless dummies of equal total length; (3) the Body Length Preference Treatment where male distance from the female was held constant, while varying body size; (4) the Apparent (Sword) Length Preference Treatments where male distance from the female was held constant, while varying sword length; (5) the Apparent Length Controls where body length, sword length, and viewing distances varied while holding apparent length constant; and (6) the Experimental Treatment in which the smaller-sworded male of the pair appeared larger. In each of the Actual Length Control, Body, and Sword Length Treatments, females preferred the closer, larger-bodied, and longer-sworded males, respectively. However, females exhibited preferences in neither any of the Apparent Size Control treatments nor any of the Sword Compensation Treatment. Additionally, females in the Experimental Treatment preferred the apparently longer-sworded dummy to the male whose sword was actually longer. These results suggest that female preference for male body and sword length is based on the male’s apparent rather than actual size.