The reduction of nuptial gifts in sclerosomatid Opiliones coincides with an increase in sexual conflict-like behaviour

Nuptial gifts serve to increase donor fitness through a variety of mechanisms, including securing additional copulations, increasing sperm transfer or storage, or increasing paternity share. Coercive mating behaviour can provide similar male benefits, potentially allowing for evolutionary transitions between solicitous and coercive strategies, wherein male behavioural antagonism 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 is elevated in nonsacculate harvesters by comparing pre-, peri- and postcopulatory mating behaviour between the nonsacculate species Leiobunum vittatum and Leiobunum euserratipalpe and the sacculate species Leiobunum aldrichi and Leiobunum bracchiolum. We additionally sought to establish an automated behavioural analysis pipeline by developing analogues for metrics traditionally scored manually. Our results revealed significantly different, potentially coercive, behaviour in nonsacculate species, indicating that the loss and reduction of pre- and pericopulatory nuptial gifts may contribute to increased behavioural antagonism. Mating behaviour also differed significantly between L. vittatum and L. euserratipalpe, indicating there are multiple suites of potentially 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.