Male dimorphism and alternative reproductive tactics in harvestmen (Arachnida: Opiliones)

• Male dimorphism has evolved several times independently in the order Opiliones.
• Proximate mechanisms of this dimorphism are unknown, but data suggest a polyphenism.
• Alternative reproductive tactics are reported for some well-studied species.
• Interactions within and between sexes are complex and can bring insights on sexual selection.

Strong sexual selection may lead small males or males in poor condition to adopt alternative reproductive tactics (ARTs) as a way to avoid the risk of being completely excluded from the mating pool. ARTs, sometimes accompanying morphological dimorphism among males, are taxonomically widespread, especially common in arthropods. Here we review the current knowledge on ARTs and male dimorphism in a diverse but relatively overlooked group of arachnids, the order Opiliones, popularly known as harvestmen or daddy long-legs. We begin with a summary of harvestman mating systems, followed by a review of the two lines of evidence for the presence of ARTs in the group: (1) morphological data from natural populations and museum collections; and (2) behavioral information from field studies. Despite receiving less attention than spiders, scorpions and insects, our review shows that harvestmen are an exciting group of organisms that are potentially great models for sexual selection studies focused on ARTs. We also suggest that investigating the proximate mechanisms underlying male dimorphism in the order would be especially important. New research on ARTs and male dimorphism will have implications for our understanding of the evolution of mating systems, sperm competition, and polyandry.This article is part of a Special Issue entitled: Neotropical Behaviour.