Parental investment and the control of sexual selection: predicting the direction of sexual competition

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Abstract
We outline a technique for deducing the general direction of sexual competition which does not involve direct measurement of operational sex ratio. In doing so, we identify three key parameters that are responsible for the control of sexual selection; the relative `times out' from reproductive activity for each sex, the adult sex ratio, and the extent of collateral investment. Reproductive events can be complex, involving collateral investments from several individuals, either simultaneously or sequentially. For example, two males may invest in the clutch of a single female. The direction of sexual competition depends on all three components. If the adult sex ratio is unity, it is easy to see that techniques involving comparison of potential rates of reproduction give the same prediction of the direction of sexual competition as times out, basically because potential reproductive rate involves the use of times out. Increased collateral investment by one sex increases the probability of sexual competition among the opposite sex. We stress that it is important to use the population mean values for relative times out and not the maximum possible value for an individual because it is the availability of mates within the population that will determine whether an individual will compete.