Task-specific and seasonal differences in individual nutritional status of red wood ant workers

Diet plays a crucial role in caste differentiation and division of labour in ants, but relationships between behavioural specialization and the nutritional status of workers in monomorphic species remain understudied. We used the red wood ant Formica aquilonia, a key species of forest communities in North Eurasia, to examine whether (i) nurses are fatter than foragers; (ii) task groups collected on the nest surface, ground surface and trees have different fat reserves; and (iii) task groups differ in the diet and trophic level. Higher fat content in nurses including sunbathing workers compared to foragers was shown by abdomen: thorax dry mass, C:N ratio and delta 13C values in the abdomen and thorax. Colony fat reserves were minimal in summer and maximum in autumn, especially in nurses. Ground foragers restoring the foraging territory in spring showed signs of starvation according to the thoracic delta 15N values. Nest guards and foragers were similarly lean in summer. Tree foragers collecting honeydew were leaner than ground foragers (hunters) in autumn. Workers were of similar trophic level. Differences in crop content between nurses and foragers were indicated by the delta 15N value in the abdomen. Our study suggests that the seasonal dynamics of energy reserves and nutritional differences between task groups are linked to the annual life cycle of the colony, contributing to its reproductive success.