Sexual dimorphism in ichneumonine parasitic wasps (Hymenoptera: Ichneumonidae: Ichneumoninae) and the neglected influence of the ecological niche

Abstract

Sexual dimorphism is a ubiquitous phenomenon, but its ecological role and evolutionary significance are still poorly understood in many hyperdiverse lineages. We used geometric morphometrics to characterize and quantify sexual dimorphism in the head and mesosoma of 24 species in the wasp subfamily Ichneumoninae (Hymenoptera: Ichneumonidae). Our results show that sexual dimorphism varies substantially among species but is generally more pronounced on the head, with males occupying a noticeably smaller region of the morphospace than females. Phenotypic trajectory analyses showed a trend towards conserved trajectories in the difference between males and females, indicating that dimorphism seems to manifest as predictable shape changes across species. Species parasitizing pupae, particularly in concealed substrates, were found to have higher sexual dimorphism for the head than species parasitizing more exposed hosts such as larvae. For the mesosoma, this difference was not significant, with the degree of sexual dimorphism more influenced by phylogenetic history. We hypothesize that female head shape has adapted to the demands related to finding hosts in concealed habitats, whereas species that attack exposed or weakly concealed hosts have kept a generalized head shape in both sexes. These results highlight the importance of investigating intersex ecological differences as drivers of sexual dimorphism.