Operational sex ratio skew measures the intensity of sexual selection

Evidence

It is defined as the ratio of reproductively available males to reproductively available females. The OSR can skew in the direction of females, as it does in jacanas, but it is typically skewed toward an excess of available males. The extent of skew is a good metric for the intensity of sexual selection likely to be acting in the population. — Emlen, Douglas J, Animal Weapons_ The Evolution of Battle, 688

Sexual selection arises whenever individuals of one sex differ in their reproductive success. Some males sire three offspring while others sire four; the result is selection for traits that help an individual sire four. But selection in this case will be relatively weak, since the difference between winners and losers is small. When differences in reproductive success get larger, the intensity of selection increases. In extreme populations sexual selection is so severe that it eclipses all other forms of selection acting on these animals. Nothing else matters—not feeding, physiology, or immunity to parasites or disease—and males sacrifice everything for a chance to win the reproduction race. — Emlen, Douglas J, Animal Weapons_ The Evolution of Battle, 187

Categorization

• DEFINE: Operational sex ratio (OSR) defined clearly as a metric distinct from raw sex ratio.
• XREF: Explains why jacanas show reverse parental care roles — linking to the weapon/sex role asymmetry theme — connects to: Sexual selection is uniquely directional because the social environment itself evolves with the weapons; Extreme skew in mating success drives arms-race escalation in weaponry; Social competition environments evolve in tandem with weapons, unlike fluctuating physical environments — Emlen, Douglas J, Animal Weapons_ The Evolution of Battle, 182
• corroborated: 3 independent passages support this claim — Emlen, Douglas J, Animal Weapons_ The Evolution of Battle, 688; Emlen, Douglas J, Animal Weapons_ The Evolution of Battle, 187

Related

• Sexual selection is uniquely directional because the social environment itself evolves with the weapons (similarity 0.80)
• Male parental care can invert the direction of sexual selection and weaponry (similarity 0.78)

Broader Connections

• Extreme skew in mating success drives arms-race escalation in weaponry #expands (0.55)
• Sexual selection is uniquely directional because the social environment itself evolves with the weapons #complicates (0.45)
• Social competition environments evolve in tandem with weapons, unlike fluctuating physical environments #complicates (0.40) — Emlen, Douglas J, Animal Weapons_ The Evolution of Battle, 182