Covariation in life-history traits, demographics and behaviour in ischnuran damselflies: the evolution of monandry

Abstract The apparent erratic variation in life history traits, coloration patterns, and behaviours that exists among species within the damselfly genus Ischnura is shown to be interpretable when the species are partitioned into three groups. One group consists of species whose males are missing a pair of stout basal spines on the penultimate segment of their accessory penes. These are the only ischnurans in which males, by tandem guarding females, prevent sperm displacement. The other two groups can be recognized by the relative frequency with which mating occurs: monandrous species mate infrequently, polyandrous species more often. Compared to polyandrous species, monandrous species contain smaller size individuals, have greater sexual size dimorphism, have shorter duration copulations, do not have male biased operational sex ratios at aquatic sites, and are more likely to contain monochromatic females. Females belonging to the monandrous species tend to develop a characteristic form of pruinescence at maturity that obscures their underlying colour, and mature at a younger age. We propose that copulation serves only for sperm addition in monandrous species, for both sperm addition and displacement in polyandrous tandem guarding ischnurans, and for contact guarding as well as sperm addition and displacement in polyandrous species that do not tandem guard.