How do queens and workers influence queen replacement in honey bee colonies?
When a queen honeybee (Apis mellifera) dies, the workers rear a number of potential replacements (virgin queens), which are then eliminated in conflicts with one another until only one remains; this survivor becomes the colony's new laying queen. The factors determining the outcome of these contests are poorly understood, although overt fighting between virgin queens undoubtedly plays a major part. An individual virgin queen's fighting success may depend on a number of attributes, including her size and weight, pheromonal signals, and use of 'piping signals' (characteristics sounds made by a queen that could act as an advertisement of fighting ability and suppress the emergence of rivals). Virgin queens are also at an advantage if they emerge earlier, since they have the opportunity to eliminate younger rivals that are still developing (Scheinder et al 2001).
Nepotism and the role of the workers
Mortal contests between rivals fighting for the right to reproduce are common in nature. What makes conflicts between the queens of social insect colonies particularly interesting is that they are not simply battles between individuals. The workers of the colony may also influence the outcome of the contest, and will be under selection pressure to do so in ways that maximise the workers' inclusive fitness - the extent to which their genes are passed to the next generation (Schneider et al 2001). The workers could do this by favouring queens that are more closely related to them (nepotism), or by favouring 'higher-quality' queens with a greater future reproductive potential.
Though the workers appear to show "little, if any, nepotism" (Tarpy et al 2000) during the rearing of larval queen cells, studies have shown that a queen's relatedness to the workers does affect the outcome of the 'polygyny reduction' stage of the queen replacement process (the elimination of competing queens). This demonstrates that the workers do indeed have some influence upon queen replacement, and that some nepotism occurs (Tarpy et al 2000).
The mechanism by which workers might influence the outcome of contests between emerged virgin queens is not known. An intriguing behaviour that may play a part is the 'vibration signal', in which a worker grasps a virgin queen and rapidly vibrates her body for a second or two. Schneider et al (2001) found a relationship between the rate at which emerged virgin queens were vibrated and their chance of ultimate success over their rivals (although differences in the rate at which queen cells were vibrated prior to emergence did not have any measurable effect upon the queens' future success). This raises the possibility that the vibration signal could act as a means through which workers influence the result of contests between virgin queens, although the exact mechanism by which vibration signals might affect fighting success is not known. However, Schneider et al did not prove a cause-and-effect relationship - it may be that the observed variation in workers' vibration behaviour was a response to differences in virgin queens' fighting abilities rather than a cause of them.
Though nepotism by workers during queen replacement is easily-demonstrated and hardly surprising, examining whether or not 'queen quality' influences the outcome of queen replacement is more complicated. In addition to being under evolutionary pressure to produce a highly-fecund queen, the colony has another priority: to install a new queen as quickly as possible, in order to minimise the interruption to the colony's breeding activities. Since younger virgin queens are believed to have higher reproductive potential (Tarpy et al 2000), but older ones will mature sooner and thus lessen the period in which the colony is without a queen, a conflict arises between these two priorities.
From their experimental manipulations of honeybee colonies, Tarpy et al (2000) concluded that age matters - older larval queens are reared preferentially by workers - but that queen quality is not an important factor in queen replacement. (However, the assessments of queen quality used were based entirely on indirect measures of fecundity such as ovary size, and it is quite possible that worker bees were judging queens on the basis of features that the experiment did not take into account.) This suggests that the rapidity of queen replacement is more important to a colony than the fecundity of the new queen. If their ability to eliminate younger rivals still within their cells is taken into account, the advantage enjoyed by older queens may be even greater than this study suggests.
In the controlled experiments carried out by Tarpy et al (2000), older queens also enjoyed an advantage during polygyny reduction, being more likely to win fights with younger queens. However, the results obtained by Scheinder et al (2001) contradict this: they show no relationship between the order in which queens emerged and their ultimate chances of survival.
Queen replacement is a process of such importance to the long-term success and survival of a honeybee colony (Schneider et al 2001) that it must create strong selection pressure not only upon the prospective queens themselves (to maximise their individual chances of success) but upon the workers of the colony (to maximise their inclusive fitness by encouraging the success of the 'highest-quality' or the most closely-related queen). It appears that workers have been selected to minimise the time that the colony is without a queen by favouring older queens during larval rearing (and perhaps also during polygyny reduction). The workers also increase their inclusive fitness by nepotistically favouring more closely-related contenders in contests between virgin queens. The mechanism by which workers influence the outcome of these contests is not understood, although the 'vibration signals' given by workers to virgin queens may play a role.
Tarpy D R, Hatch S and Fletcher D J (2000). The influence of queen age and quality during queen replacement in honey bee colonies. Animal Behaviour 59, 97-101.
Schneider S S, Paint-Kurt S and Degrandi-Hoffman G (2001). The role of the vibration signal during queen competition in colonies of the honeybee Apis mellifera. Animal Behaviour 61, 1173-1180.
This was originally written as a university biology essay