Leopards can’t change their spots … but fish can change their stripes

Posted on December 28, 2013

The Green Swordtail (Xiphophorus helleri) has been a popular model species in research on sexual selection since being mentioned in Darwin’s Descent of Man in 1871.  This species has a prominent black or red stripe running along each side of the body.  Until recently it was thought that stripe colour was a fixed, reliable badge of status indicating different reproductive strategies, where black-striped males mature earlier and at a smaller size than red-striped males, but where red-striped males are dominant and preferred by females.  However, when biologists at the University of Oklahoma examined the sexual behaviour of green swordtails from Rio Actopan, Mexico they discovered that males could change their stripe colour depending on the social context.  When a black-striped male was paired with a smaller male its stripe changed to red, and when a red-striped male was paired with a larger male its stripe changed to black.  Fish were able to change their stripe colour repeatedly, the change being completed in 4-5 minutes on average but in some cases in as little as 1 minute.  These findings suggest that male swordtails  may use a red stripe when trying to attract females or signal their dominance over other males, but a black stripe when trying to avoid male aggression and costly fights.  However, in parallel experiments swordtails from another location, Rio Sarabia, didn’t vary their stripe colour, so more work needs to be done to establish why colour change varies between populations.

Reference:  Rhodes, S.B. & Schlupp, I.  2012.  Rapid and socially induced change of a badge of status.  Journal of Fish Biology 80, 722–727.  http://onlinelibrary.wiley.com/doi/10.1111/j.1095-8649.2011.03212.x/pdf.