Competitor-to-resource ratio, a general formulation of operational sex ratio, as a predictor of competitive aggression in Japanese medaka (Pisces : Oryziidae)

Operational sex ratio (OSR), the number of potentially mating males divided by the number of fertilizable females, plays a central role in the theory of mating systems by predicting the intensity of intra-sexual competition a nd sexual selection. We introduce a general version of OSR, competitor-to-r esource ratio (CRR, the number of potential competitors divided by the numb er of resource units), as a potential way of predicting the intensity of co mpetition for any resource. We manipulated CRR over a broad range (0.5-8) b y varying both the number of competing male Japanese medaka fish (Oryzias l atipes) and the number of resources, either females or food items. We teste d whether the rate of male-male aggression differed depending on resource t ype and whether it increased monotonically or followed a dome-shaped relati onship with increasing CRR. The patterns of competitive aggression in relat ion to CRR did not differ significantly between resource types. In addition , the per capita rate of aggression followed a dome-shaped curve; it was lo w when CRR was less than one, initially increased as CRR increased, was hig hest at a CRR of about two, and then decreased when CRR was greater than tw o. However, competitor number, independent of CRR, had a significant and ne gative effect on rate of aggression. We suggest that CRR is a valuable pred ictor of the rate of competitive aggression and may be a useful concept for synthesizing ideas about resource competition and monopolization that are currently dispersed in the separate bodies of literature on mating systems, social foraging and territoriality.