Patterns of split sex ratio in ants have multiple evolutionary causes based on different within-colony conflicts

Abstract

Split sex ratio-a pattern where colonies within a population specialize in either male or queen production-is a widespread phenomenon in ants and other social Hymenoptera. It has often been attributed to variation in colony kin structure, which affects the degree of queen-worker conflict over optimal sex allocation. However, recent findings suggest that split sex ratio is a more diverse phenomenon, which can evolve for multiple reasons. Here, we provide an overview of the main conditions favouring split sex ratio. We show that each split sex-ratio type arises due to a different combination of factors determining colony kin structure, queen or worker control over sex ratio and the type of conflict between colony members.

An Organismal Perspective on the Evolution of Insect Societies

“…The predominantly female biased ratios of especially ants have traditionally been explained as worker control of sex allocation (Trivers and Hare, 1976;Bourke and Franks, 1995), and this view has extensive support from more detailed analyses (Sundström et al, 1996;Chapuisat et al, 1997;Kümmerli and Keller, 2009), although worker control is not universal (Helms, 1999;Helms et al, 2000;Passera et al, 2001). In an alternative superorganismal treatment, population structures and sex biased dispersal would be the key factors that explain sex allocation.…”

“…Split sex ratios, which can be seen as the superorganism equivalent of separate sexes, have been observed in many species. These patterns have been largely explained by scenarios invoking queen worker conflict over sex allocation, and variation in either relatedness asymmetries, or variation in the relative power of the conflict parties (Meunier et al, 2008;Helanterä and Ratnieks, 2009;Kümmerli and Keller, 2009), and organismal theories of evolution of separate sexes have not been invoked as an explanation. An exception is the harvester ant Pogonomyrmex occidentalis, where colonies are treated as organisms when explaining the absence of split sex ratios, through careful assessment of the sex-specific fitness return functions (Wiernasz and Cole, 2009).…”

“…Sex ratios in social insects have been under intense studies in the framework of within colony conflicts, and are fairly well understood both theoretically and mechanistically Grafen, 1990, 1991;Meunier et al, 2008;Helanterä and Ratnieks, 2009;Kümmerli and Keller, 2009). Split sex ratios, which can be seen as the superorganism equivalent of separate sexes, have been observed in many species.…”

Sexual conflict and sex allocation

“…For example, the remarkable phenomenon of sex allocation via split sex ratios, where colonies specialize on the production of either female or male sexuals, has usually been explained in terms of kin structure. However, Kü mmerli & Keller (2009) show how split sex ratios may be caused by a wider variety of factors and are perhaps, therefore, a more general phenomenon. In social insects, kin structure is well studied and it predicts the existence of conflicts within social groups and also optimal sex ratios.…”

Colony growth and reproductive ability of feral nests of the introduced bumblebee Bombus terrestris in northern Japan

“…This has been explained based on the relatedness asymmetry hypothesis Grafen, 1990, 1991) or based on a combination of factors that determine a colony's kin structure and the corresponding types of controls and conflicts between colony members (Kümmerli and Keller, 2009). However, differences in relatedness asymmetry cannot account for split sex ratios in B. terrestris, because gynes only mate with a single male and monogyne colonies lack variation in relatedness asymmetry among colonies (Estoup et al, 1995).…”