Counterstrategies by female frogs to sexual coercion by heterospecifics

In mating systems with intense male–male competition for mates, males may coerce females into matings that impose fitness costs. Females able to alleviate negative fitness effects of sexual coercion by enforcing their mating preferences should have a selective advantage. What behaviours females of externally fertilizing species use to reduce costs of coercion, and how effective these behaviours are, is largely unknown. We experimentally evaluated mechanisms of mate choice in a system where indiscriminate and genetically nonmatching heterospecific males coerce females into matings and females are apparently passive participants of the mating game. We performed experiments using two frog species (Rana dalmatina and R. temporaria) which are sympatric and are often observed in heterospecific matings but do not produce viable offspring. We paired R. dalmatina females with a conspecific or a heterospecific male, placed pairs together with unmated R. dalmatina males and monitored female behaviour. Females paired with heterospecific males did not try to attract the attention of conspecific males, but they delayed egg laying. Females exerted cryptic female choice by laying fewer eggs when paired with heterospecific males. Finally, some females laid a small clutch of eggs, apparently to increase the likelihood of being released by their heterospecific mate and subsequently mate with a conspecific male. Female R. dalmatina thus have subtle but effective means to avoid the complete loss of a year's reproductive effort. In a broader context, females may be able to enforce their mating preferences even in externally fertilizing species where direct female choice is overrun by male–male competition.