Trade-offs between postcopulatory riding and mate location in the blue milkweed beetle

In the blue milkweed beetle, Chrysochus cobaltinus (Coleoptera:Chrysomelidae), males remain stationary on females' backs forprolonged periods after a single, brief copulation. Lone malesoften attack pairs, and takeovers, in which rival males replacethe resident on the female's back, are common. I used removalexperiments to measure the effect of postcopulatory riding onlatency to remating. Females whose males were removed were morelikely to remate in the next 2 h than females that were allowedto remain paired. When females were removed, males were alsomore likely to remate in the 2-h period following removal thanwere males allowed to continue riding. This indicates that malesdelay remating by their mates at the expense of mating opportunitieswith additional females. The predicted reproductive successof guarding and nonguarding males was calculated using a modelbased upon experimentally derived latencies to remating andsimulated levels of last male paternity. Guarders outperformednonguarders when the proportion of offspring sired by the lastmale (P₂) was 0.4 or greater. Data on survival and lifetimemating success were used to evaluate survival trade-offs ofpostcopulatory riding. Although males sacrificed feeding timeto ride on females' backs, there was a positive relationshipbetween the proportion of time males spent riding and theirlongevity in the patch. These results indicate that postcopulatoryriding has no survival costs and indicate that postcopulatoryriding can be an effective paternity assurance mechanism evenwhen takeovers are common.