Age-dependent physiological changes and subcaste roles in a worker dimorphic ant Camponotus yamaokai

Some ants have morphologically distinct subcastes among workers, divided into minor workers, which are smaller, and major workers, which are larger and often have enlarged heads. Minor workers perform general tasks, while major workers often have more specialized roles, such as nest defense and nutritional storage. In addition to morphological caste differences, age plays an important role in polyethism. Generally, young workers having high fat mass become nurses, while old workers with reduced fat mass shift to foragers. However, few studies have examined the relationship between subcaste and age-dependent division of labor, and associated physiological changes. In this study, we used Camponotus yamaokai, which exhibits clear dimorphism among workers, to clarify the specialized roles of major workers in terms of behavior and physiology. We found that major workers rarely forage or leave the nest but instead defend the nest by plugging the nest entrance when heterospecific competitors are present in the feeding area. In histological observation of the lipid storage organ (i.e. fat body), minor workers showed a reduction in fat body size with aging, the typical of worker age-dependent changes. On the other hand, physiological changes with aging were minimal in major workers. These findings indicate that major workers of C. yamaokai maintain a stable, ‘young’ physiological state and are specialized for intranidal nest defense, rarely shifting to extranidal tasks with age.