Male birch catkin bugs vary copula duration to invest more in matings with novel females

Recent developments in the study of mating behaviour have emphasized the importance of strategic investment of limited reproductive resources. However, in many cases it can be difficult to interpret traits such as copula duration, because they are interacting phenotypes that ultimately depend upon both the male and female mating partner, and the sexes may frequently disagree over the optimal outcome. Here we report the results of experiments designed to establish which sex controls copula duration in the birch catkin bug, Kleidocerys resedae, and to test for strategic investment by the controlling sex. First, we found that matings of field-caught individuals were relatively short, but that copula duration increased following a period of sexual isolation, reaching a maximum after 2 days. However, copula duration was again shorter in re-pairings of the same individuals 1 h after their first mating. Because these results could be interpreted as a response to sexual isolation by either sex, we next investigated whether copula duration is under male or female control in this species. Experimental pairings between males and females isolated for 1 h or 48 h in all four possible combinations revealed that copula duration depended strongly on the period of male but not of female sexual isolation, implying that this trait is under male control. Finally, if males mated once were re-paired after 1 h with either the same or a novel (but still recently mated) female, we found that they mated for significantly longer with the latter. Collectively, our results imply that male birch catkin bugs in nature are frequently time-, sperm- or seminal fluid-limited, and that, as predicted by theory, they strategically allocate more of their mating effort and ejaculate reserves to novel females, a form of (cryptic) male mate choice.