The evolution of ant worker polymorphism correlates with multiple social traits

In eusocial insects, worker polymorphism is shaped by several factors, including colony size, queen mating frequency, and the timing of queen-worker differentiation during larval development. In a comparative study of 18 species of Cataglyphis desert ants representing a wide range of worker sizes, we used phylogenetically controlled analyses to examine correlations between worker head width variation (i.e., worker polymorphism) and multiple social traits, namely, mature colony size, mean worker head width, queen head width, queen-worker head width dimorphism, and within-colony genetic relatedness, resulting from multiple mating by queens. We found that worker polymorphism was positively correlated with mature colony size, mean worker head width, and queen head width. In contrast, worker polymorphism was not correlated with queen-worker dimorphism and within-colony genetic relatedness. These results underscore that evolution of worker polymorphism and social traits are correlated. They also illustrate that additional research using multivariate approaches is needed to further clarify the evolution of insect societies. Significance statement In eusocial insects, worker morphological variation (i.e., worker polymorphism) is tightly linked to division of labor. Multiple factors are supposed to shape the evolution of worker polymorphism. Using phylogenetically controlled analyses of worker head width variation from 18 species of Cataglyphis desert ants, we show that worker polymorphism positively correlates with mature colony size, mean worker head width, and queen size. These results highlight that the evolution of worker polymorphism and social traits are correlated. Identifying the mechanisms underlying these relationships could provide major insights into the development and evolution of insect societies.