Sexual Selection, Sperm Competition and Sexual Conflict - An Introduction to Behavioural Ecology - page 185

BOX 7.1 MEASURING THE STRENGTH OF SEXUAL SELECTION
(KLUG ET AL ., 2010)
A common measure of the maximum potential strength of directional sexual
selection (the 'opportunity for sexual selection') is Is , which is a standardized
measure of intrasexual variation in mating success, measured as the square of
the coefficient of variation in mating success for a given sex (Wade, 1979;
Shuster & Wade, 2003). It has often been assumed that Is will be determined by
the sex ratio of individuals capable of mating at any one time (the 'Operational
Sex Ratio', or OSR; Emlen & Oring, 1977). Males usually invest less in gametes
and parental care, so they are more often available in the mating pool,
competing for mates. Thus, the OSR will most often be male-biased.
The simple example in Fig. B7.1.1 shows that it is not possible to make general
predictions about the relationship between the OSR and sexual selection without
also knowing how the OSR influences the ability of males to monopolize mates.
In A, the addition of a new male to the mating pool may lead to an increase in
mate monopolization by the topmost male (B) or it may make mate
monopolization more difficult (C). In D, the OSR changes without changing
absolute density of competitors (a male arrives, a female leaves), again leading
either to increased (E) or decreased mate monopolization (F). In the first diagram
(below), Is behaves as predicted: it increases as mate monopolization increases
(A to B) and decreases as mate monopolization decreases (A to C). In the second
diagram (next page), Is also increases as more males remain unmated (D to E),
but it increases too when mating success is as egalitarian as possible (D to F).
Fig B7.1.1