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

Nuptial gifts serve to increase donor fitness through a variety of pathways independent of their effect on the recipient. Some nuptial gifts deliver similar benefits to antagonistic male behaviors: 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 behavioral 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 behavioral sexual conflict compensates for reduced nuptial gifts in nonsacculate harvesters by comparing the intensity of pre-, peri-, and postcopulatory behavioral sexual conflict between the nonsacculate species Leiobunum vittatum and L. euserratipalpe and the sacculate species L. aldrichi and L. bracchiolum. We additionally sought to establish an automated behavioural analysis pipeline by developing analogs 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 behavioral antagonism. Sexual conflict behavior also differed significantly between L. vittatum and L. euserratipalpe, indicating there are multiple suites of antagonistic behaviors. Together, these results suggest that multiple behavioral strategies may be effective substitutes for nuptial gifts in leiobunine Opiliones, although the mechanisms through which male fitness is increased requires further research.