Investigations Into the Regulation of Dominance Behaviour and of the Division of Labour in Bumblebee Colonies (Bombus Terrestris)

Abstract

During the first part of colony life Bombus terrestris queens have a strong regulating influence on worker dominance. Dominant workers from queenless groups which are introduced into the colony are immediately dominated by the queen. They drop to a low position in the dominance hierarchy of the colony and may start foraging. The queen's dominance signal decreases at a certain, queen-specific time after she has switched to the laying of unfertilized eggs. Then an introduced dominant worker will supersede her and become the 'false-queen'. The false-queen apparently does not produce the complete dominance signal since she usually has to carry out attacks on the workers to establish and maintain her dominance. The workers' flexibility with respect to the tasks they perform (foraging or nest duties) decreases with their age and in the course of colony development. House bees, especially those which have achieved a high position in the dominance hierarchy, are less inclined to change their tasks after removal of the foragers than foragers after removal of the house bees. But, in both cases, most of the work is taken over by young workers (less than 10 days old). Foragers which change to nest duties may substantially increase their dominance and may become egglayers. Juvenile hormone (JH) treatment does not affect the division of labour, but it does influence the activity of the workers. The influence of JH on worker dominance, as observed in queenless workers, seems to be overridden by colony factors such as domination by the queen. JH treatment stimulates oogenesis, but ovary development decreases again (by degeneration of the developed oocytes) down to the level that corresponds to the worker's dominance status as soon as the exogenous JH has been broken down and excreted. Ovariectomy influences neither the division of labour nor the establishment of a dominance hierarchy. Dominant ovariectomized workers perform all behaviours normally displayed by egglaying workers (including among others egg eating and aggressiveness), except constructing and testing of egg cells. The latter two behaviours seem to depend both upon dominance and upon the presence of ripe eggs in the ovaries. It is concluded that a Bombus terrestris colony is a stable social structure, in which the dominance of the queen and colony needs for food determine the differentiation in foragers or house bees, some of which will become egglayer. The physiology of a worker reflects this social context rather than autonomous developments at the individual level.