When a fertile female worker produces drones, a conflict arises between her interests and those of the queen. The worker shares half her genes with the drone and one-quarter with her brothers, favouring her offspring over those of the queen.
The queen shares half her genes with her sons and one-quarter with the sons of fertile female workers.
This pits the worker against the queen and other workers, who try to maximize their reproductive fitness by rearing the offspring most related to them.
This relationship leads to a phenomenon known as “worker policing”. In these rare situations, other worker bees in the hive who are genetically more related to the queen’s sons than those of the fertile workers will patrol the hive and remove worker-laid eggs. Another form of worker-based policing is aggression toward fertile females.
Some studies have suggested a queen pheromone which may help workers distinguish worker- and queen-laid eggs, but others indicate egg viability as the key factor in eliciting the behavior.
Worker policing is an example of forced altruism, where the benefits of worker reproduction are minimized and that of rearing the queen’s offspring maximized.
In very rare instances workers subvert the policing mechanisms of the hive, laying eggs which are removed at a lower rate by other workers; this is known as anarchic syndrome. Anarchic workers can activate their ovaries at a higher rate and contribute a greater proportion of males to the hive.
Although an increase in the number of drones would decrease the overall productivity of the hive, the reproductive fitness of the drones’ mother would increase.
Anarchic syndrome is an example of selection working in opposite directions at the individual and group levels for the stability of the hive.
Under ordinary circumstances the death (or removal) of a queen increases reproduction in workers, and a significant proportion of workers will have active ovaries in the absence of a queen.
The workers of the hive produce a last batch of drones before the hive eventually collapses. Although during this period worker policing is usually absent, in certain groups of bees it continues.
According to the strategy of kin selection, worker policing is not favored if a queen does not mate multiple times. Workers would be related by three-quarters of their genes, and the difference in relationship between sons of the queen and those of the other workers would decrease.
The benefit of policing is negated, and policing is less favored. Experiments confirming this hypothesis have shown a correlation between higher mating rates and increased rates of worker policing in many species of social hymenoptera.
Worker policing is a behavior seen in colonies of social hymenopterans (ants, bees, and wasps) whereby worker females eat or remove eggs that have been laid by other workers rather than those laid by a queen. Worker policing ensures that the offspring of the queen will predominate in the group.
In certain species of bees, ants and wasps, workers or the queen may also act aggressively towards fertile workers. Worker policing has been suggested as a form of coercion to promote the evolution of altruistic behavior in eusocial insect societies.
Proposed mechanisms for the recognition of worker-laid eggs or active reproductive workers include marker hydrocarbons on the surface of queen-laid eggs, cuticle hydrocarbons on reproductive workers, and recognition of nest-mates.
In rare cases, worker-laid eggs carry mimicked queen hydrocarbons and escape policing, a condition known as the anarchic syndrome.
Not all forms of policing require the presence of a queen; it also occurs in a few species of ants which establish a dominance hierarchy of reproductive female workers, where top-ranking individuals reproduce.
In many social insect communities, sex is determined through haplodiploidy. Haploid male drones develop from unfertilized eggs while diploid females develop from fertilized eggs.
Queens of a colony may mate with one or several drones. In many species of ants, bees, and wasps, workers retain functional ovaries, but cannot mate.
Thus, they only produce male offspring. On average, a queen shares half of her genes with her sons, but only shares a quarter of her genes with the sons of fertile female workers.
Therefore it is in the queen’s best interests to have her own sons raised, and not the sons of workers.
By contrast, the worker shares half of her genes with her sons and only a quarter with her brothers. Furthermore, the workers in colonies with a single once-mated queen are related to their nephews by three eighths, higher than they are related to brothers.
This results in a conflict in which the queen and worker females are at odds over the proportion of male offspring they contribute, since each side tries to maximize its reproductive fitness.
Queens favor production of their own sons while workers favor production of their own sons, and do not stop their sisters from laying because of the favorable relatedness to nephews.
Worker policing occurs when worker bees in the colony are genetically more closely related to the queen than the reproductive female.
In many instances, the eggs of the female worker are eaten, or she is repeatedly attacked by other workers. This mechanism of egg removal ensures that the queen’s sons predominate.
Experiments confirming the role of kin selection in worker policing demonstrate the effects of multiple matings, which can lead to lower average relatedness between workers.
Queens of the social wasp Dolichovespula saxonica mate singly or multiply. Researchers who study these wasps have observed a strong positive correlation between worker relatedness and male production.
After controlling for the absolute number of eggs laid, these scientists conclude that the queen’s multiple matings favor mutual worker policing.
Although early theories of worker policing focused on the kin selection component, evidence from honey bees suggests that altruism is sometimes enforced.
Fewer workers reproduce as policing effectiveness rises, and policing effectiveness decreases with increasing relatedness except in colonies with no queen. This suggests that worker policing is a social sanction imposed on selfish individuals.
Worker policing has evolved convergently in several social insect species. The following cases are examples:
One of the first examples of worker policing to be discovered was in the honey bee, Apis mellifera. Worker policing is prevalent in most honey bee colonies, and worker reproduction is minimal (0.12%) in this species.Worker policing occurs via egg-eating in Apis mellifera colonies.
In many of these hives, the activation of ovaries in fertile females is diminished, suggesting that there are disincentives to laying eggs.
Workers in colonies of the dwarf honey bee, Apis florea, are also reported to engage in oophagy. Using microsatellite analysis, researchers concluded that no mature drones had non-queen alleles in the colonies they investigated.
Thus, even though workers had activated ovaries and were capable of laying eggs, worker policing ensured the functional sterility of otherwise fertile workers.
A higher percentage of female workers of the bee species Apis cerana are reported to activate their ovaries than those of Apis mellifera or Apis florea.
When queens were removed, up to 40% of the workers activated their ovaries in subsequent days. However, policing workers continued to eat the worker-laid eggs, suggesting that the mechanism of policing in this species does not involve direct intervention from the queen.
The observation that all these Apis species engage in polyandry has led researchers to conclude that worker policing is plesiomorphic for the Apis genus.
Worker policing also occurs in the primitively eusocial bumblebees such as Bombus terrestris.
The tree wasp Dolichovespula sylvestris also displays worker policing. Both egg-eating and aggression are reported in these wasp communities, and the queen also engages in the policing process.
Similar behavior has been observed in the closely related species Dolichovespula media.
In colonies of the paper wasp, Polistes chinensis antennalis, workers can lay up to a quarter of the male eggs in the colony.
The number of eggs that survived to hatching, however, was minimal compared to the number produced by the queen. Analysis of the microsatellite markers showed that both queens and workers contributed to policing of worker and queen-laid eggs even in monogynous and monandrous colonies.
The common wasp, Vespula vulgaris, engages in worker policing, and it is known that a significant number of workers have active ovaries.
However, studies by researchers have suggested that relatedness may not be the key factor in the development of worker policing. Rather, it is hypothesized that worker policing has been selected due to conflict suppression in the colony.
In a test of worker policing, researchers examined the removal of worker-laid eggs in Vespula rufa, a wasp that has low paternity. Under the testing conditions, worker policing was not as efficient, and some of the drones appeared to be from worker-laid eggs.
The European Hornet, Vespa crabro, was previously thought to be under reproductive pheromone control by the queen, thus explaining why the other females didn’t reproduce, even though they were capable of doing so.
Experiments by Foster showed that the workers were instead regulating sterility in each other, thus instead exhibiting worker policing. While each worker is capable of reproducing, the colony as a whole is more efficient and organized if the workers allow only the queen to lay larvae.
Synoeca cyanea also engage in worker policing during times when queen repopulation is not needed. They will prevent females from laying eggs through aggressive behavior and egg-eating.