The ecology of communal breeding: the case of multiple-queen leptothoracine ants

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Abstract
Multiple-queen societies of ants are key subjects in the study of communal breeding. Societies of leptothoracine ants may be obligately monogynous (contain a single reproductive queen), functionally monogynous (only one of several mated queens lays eggs), or facultatively polygynous (some colonies contain more than one egg-laying, mated queen). This paper presents a framework for understanding these diverse social systems as a function of leptothoracine ecology. The framework is derived from a synthesis of empirical information - in particular, a link between the social system and the degree of habitat patchiness - with three bodies of theory. These are ecological constraints theory, ESS (evolutionarily stable strategy) models of dispersal, and kin selection models predicting the stable reproductive skew (allocation of reproduction). In contrast to several previous ecological hypotheses, multiplequeening in leptothoracines almost certainly results from high costs to single queens of dispersal and colony foundation (high ecological constraints), which select for queens to seek adoption in their natal colony. Factors raising these costs include nest-site limitation, cold climate, and habitat patchiness. ESS models suggest that high dispersal costs lead to a larger stable fraction of non-dispersers and hence to higher relatedness between queens. Skew models predict that high ecological constraints and high relatedness promote high skew (one or a few individuals dominate reproduction) and high within-colony aggression. Therefore, (i) extensive habitats with moderate costs of solitary colony foundation should promote multiple-queening with high dispersal levels, moderate queen relatedness, low reproductive skew, and low queen aggression. By contrast, (ii) patchy habitats should induce multiple-queening with less dispersal, higher queen relatedness, higher skew, and higher aggression. In addition, (iii) habitats with small or widely spaced nest-sites, or with low costs of founding colonies alone, should lead to universal dispersal without multiple-queening. These associations of traits occur in facultatively polygynous, functionally monogynous and obligately monogynous leptothoracines respectively. Therefore, the framework in this paper explains a substantial amount of the social and ecological diversity of leptothoracine ants.