Abstract: In social insects, natural selection operates at the level of the colony, rather than the individual, but our understanding of how colony-level phenotypes arise and vary between species is lacking. Here, we test how colony-level phenotypes vary within the fire ants by measuring the composition of colonies of the tropical fire ant, Solenopsis geminata, over a wide range of sizes at multiple times throughout the year. Similar to the well-studied fire ant species S. invicta, we find that S. geminata colony composition varies strongly with colony size, such that as colonies grow they produce increasingly large workers as well as queens and males. However, major production increases more rapidly with colony size in S. geminata than in S. invicta, which may explain our observation that S. geminata also possesses a smaller maximum colony size. Unlike S. geminata or S. invicta in the USA, we find no evidence for seasonal variation in colony composition in S. geminata in Costa Rica. Solenopsis geminata colonies from Costa Rica also exhibit continuous variation in queen number, from one to two to hundreds. Overall, this research describes how colony-level features vary within the fire ants, providing basic data that can be used to study the mechanisms underlying the development and evolution of colony-level phenotypes in social insects.
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