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Non‐native Ants Are Smaller than Related Native Ants

I compare the sizes of non‐native and native ants to evaluate how worker size may be related to the ability of a species to invade new habitats. I compare the size of 78 non‐native ant species belonging to 26 genera with the size of native congeneric species; native ants are larger than non‐native ants in 22 of 26 genera. Ants were sorted by genera into fighting and nonfighting groups, based on observations of interspecific interactions with other ant species. In all of the genera with monomorphic worker castes that fight during competition, the non‐native species were smaller than the native species. The genera that engage in combat had a higher frequency of significantly smaller size in non‐native ants. I selected Wasmannia auropunctata for further studies, to compare native and non‐native populations. Specimens of W. auropunctata from non‐native populations were smaller than conspecific counterparts from its native habitat. I consider hypotheses to explain why non‐native ants are smaller in size than native ants, including the role of colony size in interspecific fights, changes in life history, the release from intraspecific fighting, and climate. The discovery that fighting non‐natives are smaller than their closest native relatives may provide insight into the mechanisms for success of non‐native species, as well as the role of worker size and colony size during interspecific competition.