Effects of social structure on reproductive activity in male fathead minnows (Pimephales promelas)

The selection of alternative reproductive phenotypes is often thought to be the result of physiological state, with small individuals forced energetically to postpone the allocation of resources to reproduction. However, for male fathead minnows (Pimephales promelas), we show that seasonal reproductive activity is modulated by social status. In enclosure and pond experiments, small males advanced their reproductive condition, held nesting territories, and spawned earlier in the reproductive season only when lat ge males were absent or removed from the population. Since differences in the timing of reproduction among small males were not size- or condition-dependent, the common explanation for the selection of alternative reproductive phenotypes, based on state-dependence, is insufficient. In the absence of large, socially dominant individuals, small males produced comparable numbers of offspring as the treatment with large males, although the offspring of these uninhibited small males were smaller at the end of the growing season than the young of large males. Thus, interactions among conspecifics may account for much of the phenotypic diversity observed within and among natural fathead minnow populations, through their direct and indirect effects our growth, recruitment and survival.