Behavioral Ecology Vol. 11 No. 6: 670-675
© 2000 International Society for Behavioral Ecology
Competitor-to-resource ratio, a general formulation of operational sex ratio, as a predictor of competitive aggression in Japanese medaka (Pisces: Oryziidae)
Department of Biology, Concordia University, 1455 de Maisonneuve Blvd. West, Montréal, Québec, H3G 1M8, Canada
Address correspondence to J. W. A. Grant. E-mail: grant{at}vax2.concordia.ca .
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 and sexual selection. We introduce a general version of OSR, competitor-to-resource ratio (CRR, the number of potential competitors divided by the number of resource units), as a potential way of predicting the intensity of competition for any resource. We manipulated CRR over a broad range (0.5-8) by varying both the number of competing male Japanese medaka fish (Oryzias latipes) and the number of resources, either females or food items. We tested whether the rate of male—male aggression differed depending on resource type and whether it increased monotonically or followed a dome-shaped relationship with increasing CRR. The patterns of competitive aggression in relation to CRR did not differ significantly between resource types. In addition, the per capita rate of aggression followed a dome-shaped curve; it was low when CRR was less than one, initially increased as CRR increased, was highest at a CRR of about two, and then decreased when CRR was greater than two. However, competitor number, independent of CRR, had a significant and negative effect on rate of aggression. We suggest that CRR is a valuable predictor 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.
Key words: aggression, competitor-to-resource ratio, Japanese medaka, mating systems, operational sex ratio, Oryzias latipes, resource competition, territoriality.
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