REPRODUCTIVE RATES, OPERATIONAL SEX-RATIOS AND MATE CHOICE IN ST PETERS FISH

Operational sex ratio (OSR) theory predicts that sexual differences in potential reproductive rates (PRRs) create biases in the OSR and thus determine the relative strength of sexual selection (competition and choice) operating on each sex. Although this theory is well accepted, empirical studies that quantify it are still lacking. This paper prese nts such a study. I measured the natural OSR of Galilee St. Peter's fi sh (Sarotherodon galilaeus) in the held (Lake Kinneret) and examined t he direction of mate choice in the laboratory. The OSR in Lake Kinnere t was male biased. Both a male-biased sex ratio and higher male reprod uctive rates (twice as fast as females) contributed to the skew in the OSR, but the sexual differences in PRR were shown to be the main fact or causing variation in the OSR. Females, the sex with the lower PRR, were more selective for mates. The faster male reproductive rate may e xplain why females are more selective for mates despite varying less i n quality.