Worker policing

Worker policing is a method of controlling reproduction in many species of social hymenopterans (ants, bees, wasps). To ensure that the offspring of the queen predominate, workers police the colony by eating or removing worker-laid eggs. In some instances, workers or the queen will also act aggressively towards fertile workers.

Honey bee hive with queen

Evolutionary basis

See also: Haplodiploidy

In many, but not all, 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 will mate singly or multiply. On average, a queen will share half of her genes with her sons, but will only share a quarter of her genes with the sons of fertile female workers. Therefore, it is in the queen's best interests to have her sons propagate the colony. By contrast, the worker will share half of her genes with her sons and only a quarter with her nearest brother. This results in a conflict wherein both the queen and worker female are at odds over the proportion of male offspring they contribute since each side will try to maximize its reproductive fitness.

Queen and male ants mating

Worker policing occurs when worker bees in the colony are more genetically related to the queen than the reproductive female. In many instances, the eggs of the female worker are eaten, or she is aggressively targeted with repeated attacks from other workers. This former mechanism of egg removal ensures the queen's sons will predominate.

Experiments confirming the role of kin selection in worker policing demonstrate the effects of multiple matings. 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 have further concluded that the queen's multiple matings favor mutual worker policing.

Although early theories of worker policing focused on the kin selection component, recent evidence from honey bees suggests enforced altruism plays a role. Fewer workers reproduce as policing effectiveness rises, and policing effectiveness decreases with increasing relatedness except in colonies that do not contain a queen. This has led to a new theory that views worker policing as a social sanction imposed on selfish individuals.

Examples from social Hymenoptera

Worker policing has evolved convergently in several social insect species. The following cases are examples:

Bees

One of the first discovered examples of worker policing was in the hives of the honey bee Apis mellifera. Worker policing is prevalent in most colonies of these bees, and worker reproduction is minimal (0.12%). 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.

Dolichovespula sylvestris, the tree wasp

Workers in colonies of the Asian honey bee Apis florea are 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 fertile workers.

Female workers of the bee species Apis cerana are reported to activate their ovaries at higher rates than those of Apis mellifera or Apis florea. When queens were removed, workers activated their ovaries at high percentages in subsequent days, as high as forty percent. However, policing workers continued to eat the worker-laid eggs, suggesting that the mechanism of policing in this species may not involve direct intervention from the queen.

The observation that all the species mentioned above engage in polyandry has lead researchers to conclude that worker policing is pleismorphic for the Apis genus.

Ants

In colonies of the ponerine ant, Pachycondyla inversa, workers eat the eggs of fertile female workers and display aggressive behavior towards egg-laying females.

In colonies of the ant Gnamptogenys menadensis, workers will sometimes mate and lay eggs while others remain as virgins to lay trophic eggs. In a display of policing, workers can immobilize female reproductive workers by biting their limbs; it is reported that 50% of the victims die from this treatment. The attacking workers may also drag offending workers outside the colony. Since workers can lay both male and female eggs, worker policing could be favored since the cost to the community is large.

The ant species Aphaenogaster smythiesi japonica also display evidence of policing. When researchers separated and then reunited workers from colonies that did or did not have queens, workers from the queen-right colonies attacked the workers with activated ovaries from the queen-less colonies.

Wasps

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.

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 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 worker policing in Vespula rufa, a wasp that has shown evidence of low paternity. Under the testing conditions, worker policing was not as efficient, and some of the drones were believed to be worker derived.

Proposed mechanisms

Many hypotheses have been generated on the mechanisms behind egg discrimination in worker policing.

Queen hydrocarbons

A study on the ant Camponotus floridanus found a special surface hydrocarbon on queen-laid eggs. Workers in the colony with the queen's eggs refrained from egg-laying, whereas other groups that did not have queen-laid eggs showed worker reproduction. Thus, the hydrocarbon could act as a signal to alert workers to halt reproduction. More generally, however, the authors concluded that the hydrocarbons were a fertility signal. It is still unknown as to how the decision to restrain reproduction occurs or what minimum threshold of fertility in the queen is needed to reduce worker reproduction.

Fertile worker hydrocarbons

The study of cuticle hydrocarbons has also been applied to the reproductive workers. In some ant colonies of Platythyrea punctata, the presence of CHCs in new reproductive workers triggered aggressive behavior from the rest of the colony, often between new and old reproductive workers. Specifically, the old reproductive workers rubbed a marker hydrocarbon on the antennae of the new females, identifying them as rivals and targets for aggression.

Nestmate recognition

In contrast to the other ant species, worker policing in the ant Formica fusca appears to incorporate nestmate recognition in addition to the queen hydrocarbons. Workers displayed higher levels of aggression toward non-nest mates and also removed some queen-laid eggs. Thus, queen hydrocarbons were not the only contributor to discrimination of the eggs.

Exceptions

Anarchic syndrome

In extremely rare cases, female worker bees can lay eggs that escape worker policing in a process known as the anarchic syndrome. Female reproductive workers in these colonies activate their ovaries even in the presence of a queen. It is believed that the worker eggs are allowed due to mimicry of the queen hydrocarbons. Thus, female workers can maximize their reproductive fitness at the expense of the colony. Anarchic syndrome is an example of selection working at in opposite directions at individual and group levels.

Selfish worker policing

Temnothorax unifasciatus, a species of ant, has been shown not to have collective worker policing. However, when a queen is removed, there appears to be a rank order for reproduction where top ranking reproductives displayed aggression towards lower-order female workers. Thus, reductions in egg laying are created by dominance hierarchies. Individuals do not appear to act in benefit of the colony, instead opting to increase their own fitness by laying their eggs and reducing the contributions of opponents.

Worker policing without genetic conflict

In the thelytokous ant Platythyrea punctata, colonies are clonal, therefore worker genetic relatedness is equal and worker policing is not expected due to lack of genetic conflicts. However, this would severely affect the condition of the colony if the reproductive workers are left unchecked. For example, more time spent on individual reproduction would detract from care of the queen's offspring. In studies of these ants, workers did actually display worker policing, as evidenced by increased aggression towards the reproductive workers. Worker policing here is favored due to group efficiency.