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

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Abstract
In the blue milkweed beetle, Chrysochus cobaltinus (Coleoptera: Chrysomelidae), males remain stationary on females' backs for prolonged periods after a single, brief copulation. Lone males often attack pairs, and takeovers, in which rival males replace the resident on the female's back, are common. I used removal experiments to measure the effect of postcopulatory riding on latency to remating. Females whose males were removed were more likely to remate in the next 2 h than females that were allowed to remain paired. When females were removed, males were also more likely to remate in the 2-h period following removal than were males allowed to continue riding. This indicates that males delay remating by their mates at the expense of mating opportunities with additional females. The predicted reproductive success of guarding and nonguarding males was calculated using a model based upon experimentally derived latencies to remating and simulated levels of last male paternity. Guarders outperformed nonguarders when the proportion of offspring sired by the last male (P2) was 0.4 or greater. Data on survival and lifetime mating success were used to evaluate survival trade-offs of postcopulatory riding. Although males sacrificed feeding time to ride on females' backs, there was a positive relationship between the proportion of time males spent riding and their longevity in the patch. These results indicate that postcopulatory riding has no survival costs and indicate that postcopulatory riding can be an effective paternity assurance mechanism even when takeovers are common.

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