The reduction of nuptial gifts in sclerosomatid Opiliones coincides with increased behavioural sexual conflict

Abstract

Nuptial gifts serve to increase donor fitness through a variety of pathways independent of their effect on the recipient. Some nuptial gifts deliver benefits similar to those of antagonistic male behaviours: functioning to secure additional copulations, increase sperm transfer or storage, or increase paternity share. These commonalities may result in evolutionary transitions between solicitous and coercive strategies, wherein behavioural sexual conflict could function to secure mates in lieu of nuptial gifts. In temperate leiobunine harvesters (Arachnida: Opiliones), nuptial gifts have been repeatedly lost, resulting in two primary mating syndromes: an ancestral, sacculate state in which males endogenously produce high-investment nuptial gifts and females lack pregenital barriers, and a derived, nonsacculate state in which females have pregenital barriers and males produce significantly reduced, low-investment nuptial gifts. In this study, we investigated whether behavioural sexual conflict compensates for reduced nuptial gifts in nonsacculate harvesters by comparing the intensity of pre-, peri-, and postcopulatory behavioural sexual conflict between the nonsacculate speciesLeiobunum vittatumandL. euserratipalpeand the sacculate speciesL. aldrichiandL. bracchiolum. We additionally sought to establish an automated behavioural analysis pipeline by developing analogues for metrics traditionally scored manually. Our results revealed significantly higher sexual conflict behaviour in nonsacculate species, indicating that the loss and reduction of pre- and pericopulatory nuptial gifts may contribute to increased behavioural antagonism. Sexual conflict behaviour also differed significantly betweenL. vittatumandL. euserratipalpe, indicating there are multiple suites of antagonistic behaviours. Together, these results suggest that multiple behavioural strategies may be effective substitutes for nuptial gifts in leiobunine Opiliones, although the mechanisms through which male fitness is increased requires further research.HighlightsTemperate Opiliones have repeatedly evolved reduced nuptial gift investmentReduced-gift (nonsacculate) species possess morphology indicative of antagonismWe tested whether sexual conflict is higher in nonsacculate versus sacculate speciesNonsacculates displayed significantly higher behavioural antagonism than sacculatesNonsacculate species differed in their suites of antagonistic behaviours