Head and mandible shapes are highly integrated yet represent two distinct modules within and among worker sub-castes of the ant genus Pheidole

Ants use their mandibles for a wide variety of tasks related to substrate
manipulation, brood transport, food processing, and colony defence. Due to
constraints involved in colony upkeep, ants evolved a remarkable diversity
of mandibular forms, often related to specific roles such as specialized
hunting and seed milling. Considering these varied functional demands, we
focused on understanding how the mandible and head shape vary within and
between Pheidole sub-castes. Using x-ray microtomography and 3D geometric
morphometrics, we tested if these structures are integrated and modular,
and how ecological predictors influenced these features. Our results
showed that mandible and head shape of majors and minor workers tend to
vary from robust to slender, with some more complex changes related to the
mandibular base. Additionally, we found that head and mandible shapes are
characterized by a high degree of integration, but with little correlation
with feeding and nesting habits. Our results suggest that a combination of
structural (allometric) constraints and the behavioural flexibility
conferred by caste dimorphism might largely buffer selective pressures
that would otherwise lead to a fine-tuning between ecological conditions
and morphological adaptation.