Nest defense by deceit in the fathead minnow, Pimephales promelas

Summary

Individual male fathead minnows spawn and guard eggs for a period of several weeks during a much longer total breeding season. Solitary males remain pale and lose both wet weight and dry weight throughout the nesting period, whereas competitive males, who defend nests adjacent to territorial rivals, turn dark and do not lose wet weight despite losing dry weight. A solitary male must contend only with territorial encroachment by females (potential mates and egg predators), but a competitive male must defend the nest against both males and females. A competitive male always attacks female intruders, whereas he initially displays to intruding males. Territorial rivals, though not egg predators, may try to usurp his nest. Successful competitive nest holders may appear more robust than their true condition and thereby prevent eviction by newly reproductive male intruders. Such newcomers preferentially attack residents in relatively poor condition and pale coloration.