Geographic differences in body size in the big-headed ant, Pheidole megacephala

Body size is a key life history characteristic and is the focus of behavioral, ecological and evolutionary studies. However, the majority of work in this area has examined life history traits and subsequent trade-offs, in solitary organisms. By comparison relatively little work has been done using highly social organisms such as the eusocial insects. We examined body size variation in the dimorphic ant, Pheidole megacephala (big-headed ant), an invasive species found in temperate and tropical climates throughout the world. We compared body mass and morphology of major and minor workers from four distinct populations that vary in abiotic and biotic conditions including the competitive environment. We predicted that 1) each population would show caste specific variation in morphology and this variation would relate to environmental conditions, and 2) differences in the critical size differentiation between majors and minors would vary according to the competitive environment (number of coexisting ant species). This study will serve as a foundation for further examinations of how the environment shapes body size in social insects.