When two male fringeheads meet over a contested burrow or shell, they gape their jaws to full extension and press mouths together, gape to gape, shoving in a ritualized contest that can last several minutes. Neither fish typically bites or injures the other; instead the male with the smaller gape usually backs off and abandons the shelter.
Adult sarcastic fringeheads rarely grow beyond about 30 centimetres, yet their mouths can open wider than their own head, and in some individuals the gape approaches the width of the whole body. This disproportion is thought to have been shaped mainly by these contests rather than by feeding needs, since the fish mostly eats small crustaceans and fish that do not require such an extreme gape.
The wrestling behaviour has been documented in observational studies since the 1970s and is well accepted among researchers. Less settled is how consistently mouth size, rather than body size or which fish already occupies the shelter, predicts the winner across different populations; popular retellings also sometimes overstate the mouth-versus-body comparison as a fixed rule rather than a pattern seen in some individuals.