Beschreibung:
<jats:title>Abstract</jats:title><jats:p>Heterospecific mating frequency is critical to hybrid zone dynamics and can directly impact the strength of reproductive barriers and patterns of introgression. The effectiveness of post‐mating prezygotic (<jats:styled-content style="fixed-case">PMPZ</jats:styled-content>) reproductive barriers, which include reduced fecundity via heterospecific matings and conspecific sperm precedence, may depend on the number, identity and order of mates. Studies of <jats:styled-content style="fixed-case">PMPZ</jats:styled-content> barriers suggest that they may be important in many systems, but whether these barriers are effective at realistic heterospecific mating frequencies has not been tested. Here, we evaluate the strength of cryptic reproductive isolation in two leaf beetles (<jats:italic>Chrysochus auratus</jats:italic> and <jats:italic>C. cobaltinus</jats:italic>) in the context of a range of heterospecific mating frequencies observed in natural populations. We found both species benefited from multiple matings, but the benefits were greater in <jats:italic>C. cobaltinus</jats:italic> and extended to heterospecific matings. We found that <jats:styled-content style="fixed-case">PMPZ</jats:styled-content> barriers greatly limited hybrid production by <jats:italic>C. auratus</jats:italic> females with moderate heterospecific mating frequencies, but that their effectiveness diminished at higher heterospecific mating frequencies. In contrast, there was no evidence for <jats:styled-content style="fixed-case">PMPZ</jats:styled-content> barriers in <jats:italic>C. cobaltinus</jats:italic> females at any heterospecific mating frequency. We show that integrating realistic estimates of cryptic isolation with information on relative abundance and heterospecific mating frequency in the field substantially improves our understanding of the strong directional bias in F1 production previously documented in the <jats:italic>Chrysochus</jats:italic> hybrid zone. Our results demonstrate that heterospecific mating frequency is critical to understanding the impact of cryptic post‐copulatory barriers on hybrid zone structure and dynamics, and that future studies of such barriers should incorporate field‐relevant heterospecific mating frequencies.</jats:p>