Guarding specialisation in pre-reproductive colonies of the allodapine bee Exoneura bicolor

The allodapine bee Exoneura bicolor constructs linear nests in pithy centres of dead plant material. Brood is reared progressively. During winter, nests usually contain one mated female with large ovaries and several subordinate females with small ovaries. The dominant female pheromonally inhibits ovarian enlargement of subordinate females. During the foraging season, dominant females guard the nest, while subordinate females undertake the more risk prone foraging activities. During winter, virtually no foraging occurs. Therefore, guarding might be the most dangerous activity during winter. In this paper we investigate whether the position of females in the nest during winter is related to dominance. Females did not change relative position frequently and the position of females decreased near monotonically with ovary size, with the dominant female closest to the nest entrance. It is concluded that: (1) dominant females do not move about in the nest to ensure spread of their pheromones among nestmates, or to behaviourally inhibit ovarian enlargement of subordinates; (2) dominant females do not force subordinates to guard the nest. Because the guard is probably the female that is most subject to predation during winter, guarding by the dominant should have some payoffs. Potential benefits are: (a) contamination of incoming air with volatile pheromones; (b) preventing intruders from entering the nest; (c) controlling nestmates leaving and entering the nest. The latter may enable females to control food that is brought back into the nest, and to prevent mating by subordinate females. Our results indicate that social structure within these nests is influenced by the linear structure of the nest burrows.