Discussion
A number of recent studies have found little difference between
the opportunity for selection (standardized variance in male reproductive
success) calculated from apparent male reproductive
success (number of social young produced, Ta) or from actual
male reproductive success (number of young sired, T; see Webster
et al. 2001; Freeman-Gallant et al. 2005; Whittingham and
Dunn 2005). Because standardized variance in reproductive success
measures the opportunity for selection (Arnold and Wade
1984), similarity of variances for Ta and T is typically interpreted
to mean that EPP does not increase the strength of sexual selection
(e.g., references above).
Simple comparisons of apparent to actual reproductive success,
however, can be misleading because the former is not based
on actual parentage, and in this study several results demonstrate
that reproductive promiscuity contributed strongly to the opportunity
for sexual selection. In particular, partitioning the opportunity
for selection into component parts demonstrated that much of the