Dispersal and mating in a size-dimorphic ant

Abstract

Alternative reproductive strategies linked to size are often connected with differences in dispersal and mating behaviour. Temporal and spatial mating isolation of different sized individuals can lead to size-assortative mating, which may maintain intraspecific size polymorphism. In the size-dimorphic ant Myrmica ruginodis, newly produced sexuals of the larger macrogyne morph join nuptial flights to mate, but mating behaviour of the smaller microgyna females remains elusive and even less is known about males. In this study, we examined the differences in dispersal strategies of the two size morphs as a possible mechanism causing reproductive isolation between them. We developed a new method for sampling sexuals dispersing from their nests, and sampled also mating couples and random males from a nearby mating site. We studied propensity and timing of dispersal and assortative mating of the size morphs based on their expected and observed participation in the nuptial flight. Our study shows that predominantly, macrogynes participate in the nuptial flight, while microgynes stay close to their natal nest. However, dispersal is not associated with male size, and larger males, preferred by all females, are available for both morphs. Size-assortative mating, mainly caused by macrogynes mating with large males, exists at the nuptial flight and should promote mating isolation of the size morphs, but microgynes counterbalance this to a certain degree by mating with large males. Size-associated alternative dispersal strategies in size polymorphic species provide insight into the mechanisms enhancing genetic divergence, a potential early step in the speciation process.

Significance statement

We studied the association between size and dispersal in the size-dimorphic ant Myrmica ruginodis, to evaluate potential premating isolation between the morphs. To date, there is a shortage of studies on mating and dispersal behaviour of ants in natural settings. We captured dispersing sexuals from their natal nests, separating those dispersing on foot or by flight—a technique never applied to dispersing ants before—and assessed mate selection at the mating swarm. We found that dispersal propensity depends on the size of the females (but not males) so that the larger macrogyne morph joins a nuptial flight and the smaller microgyne morph rarely does. Furthermore, macrogyna females actively choose to mate with large males causing size-assortative mating, creating partial mating isolation between the size morphs and opening a possibility for genetic divergence between the morphs.