Worker Policing Research Articles

Worker Reproduction in the Highly Polygynous Ant Crematogaster pygmaea Forel, 1904 (Hymenoptera: Formicidae)

In most ant species, workers have retained functional ovaries, allowing them to potentially lay viable unfertilized eggs that develop into males. Mechanisms (ex.: queen and/or worker policing) have nevertheless evolved to control worker reproduction when the queen is present. In many species with a high degree of polygyny, especially in tramp species, complete sterility of workers has evolved, presumably to “trap” them within their “worker phenotype”. Our study showed for the first time that in the highly polygynous and polydomous ant Crematogaster pygmaea, workers retained the full capacity to produce reproductive eggs in queenless colonies, with at least some of them developing in adult males. We provide evidence that worker-produced males are reproductively functional. Although most queenless colonies produced eggs, few larvae developed into pupae and adult males. We conclude that workers strongly police the worker-produced offspring, even in the queen's absence. Probable high relatedness between queens of C. pygmaea colonies and strong genetic proximity between brood and workers could force the workers in their helper, non-reproductive function even if they keep the ability to reproduce. Our observations indicate that the production of adult males and gynes in C. pygmaea is controlled by seasonal factors related to the rainy season.

Conserved worker policing in African carpenter ants with drastically different egg chemotypes

Distinctive chemical signatures have the potential to serve as discriminatory cues for olfactory recognition mechanisms. Cuticular hydrocarbon (CHC) profiles are among the most prominent chemical signatures in insects that can be highly diverse even among closely related species and between populations with similar ecology. Particularly within the major insect order Hymenoptera, CHC profiles are characterized by high complexity and variation with the potential to evolve rapidly. In this study, we found two very distinct CHC chemotypes distinguishing sympatric colonies of the African carpenter ant Camponotus maculatus (Hymenoptera: Formicinae). These chemotypic differences were mainly detected on the surface profiles of eggs produced by either queens or isolated worker groups. In one chemotype, queen- and worker-laid eggs are very similar. This is largely contrasted by the other chemotype, where queen-laid eggs clearly differ from worker-laid eggs with several prominent queen-exclusive compounds. However, workers display a stable behavior of discriminating against and selectively disposing of worker-laid eggs i.e., worker policing, independent of egg chemotype. Furthermore, genetic barcoding of workers revealed a clear separation between colonies characterized by producing these two distinct egg chemotypes, which may indicate that these colonies belong to a cryptic species complex. Interestingly, worker policing behaviour appears to be evolutionarily conserved, despite the strikingly different egg surface profiles.

Policing is more effective against eggs of non‐natal versus natal workers at early colony stages in a bumblebee

AbstractEusocial insect colonies are vulnerable to exploitation by egg‐laying workers arising either as natal reproductive workers or as non‐natal reproductive ‘drifting’ workers (intraspecific social parasites). Worker egg‐laying is potentially costly to the colony, but queens and workers can counter its costs via egg eating (queen or worker policing). Bumblebee colonies exhibit egg laying by both natal and non‐natal workers: natal workers collectively lay more eggs but do so only after a specific point in the colony cycle, the ‘competition point’, whereas non‐natal workers potentially lay eggs throughout the colony cycle. These features create a special opportunity to investigate whether policing of worker‐laid eggs is context‐dependent (i.e. depends on worker origin of eggs and/or colony stage). We introduced artificial egg cells containing eggs laid by either natal or non‐natal workers into colonies of the bumblebee Bombus terrestris both before and after their competition points and observed the fate of introduced egg cells and eggs. In both colony stages, the majority of introduced egg cells and eggs were policed, demonstrating that policing was not activated only after the competition point. However, in the pre‐competition point stage alone, a smaller proportion of non‐natal workers' eggs (15%) remained after 20 h compared to the proportion of natal workers' eggs remaining (24%). More effective policing of non‐natal workers' eggs early in the colony cycle potentially represents an adaptive, context‐dependent response to the stage in the cycle when all worker‐laid eggs are normally unrelated to the natal colony.

Reproduction and signals regulating worker policing under identical hormonal control in social wasps

In social Hymenoptera, fertility and fertility signalling are often under identical hormonal control, and it has been suggested that such hormonal pleiotropies can help to maintain signal honesty. In the common wasp Vespula vulgaris, for example, fertile queens have much higher juvenile hormone (JH) titers than workers, and JH also controls the production of chemical fertility cues present on the females’ cuticle. To regulate reproductive division of labour, queens use these fertility cues in two distinct ways: as queen pheromones that directly suppress the workers’ reproduction as well as to mark queen eggs and enable the workers to recognize and police eggs laid by other workers. Here, we investigated the hormonal pleiotropy hypothesis by testing if experimental treatment with the JH analogue methoprene could enable the workers to lay eggs that evade policing. In support of this hypothesis, we find that methoprene-treated workers laid more eggs, and that the chemical profiles of their eggs were more queen-like, thereby causing fewer of their eggs to be policed compared to in the control. Overall, our results identify JH as a key regulator of both reproduction and the production of egg marking pheromones that mediate policing behaviour in eusocial wasps.

A mathematical model of anarchy in bees

Functional worker sterility in queenright honey bees (Apis mellifera) is fostered by the colony-level costs of this behavior. However, anarchy in the beehive can occur when a fraction of workers lay eggs, which, if reared, will develop into males (drones). Because males do not gather resources but still consume them, they represent an additional energic cost for the colony. This small number of anarchic worker-laid eggs is normally found and eaten by non-anarchic bees via worker policing, a controlling mechanism induced by queen’s pheromones. However, worker policing by anarchic bees is possible by mimicking the pheromone marker of the queen. Here, we show that the intensity of worker policing has an upper-bound limit set by the loss in its population-level benefits. We developed a mathematical model to assess the benefits of worker policing at different intensities and costs of anarchy in the beehive. We found that there is an optimal rate in which workers should police in order to maximize colony-level benefits. The adjustment of worker policing intensity depending on the degree of anarchy in the population can keep the number of drones descending from workers low. However, this control can collapse when worker policing gets saturated due to high frequencies of anarchism. Our findings can help to advance the understanding on the maintenance of eusociality in bees by showing how the balance between anarchic behavior and worker policing regulates productivity in the beehive.

Do rebel workers in the honeybee Apis mellifera avoid worker policing?

A recent study showed that worker larvae fed in a queenless colony develop into another female polyphenic form—rebel workers. The rebel workers are more queen-like than normal workers because they have higher reproductive potential revealed by more ovarioles in their ovaries. However, it was unclear whether eggs laid by rebel workers avoided worker policing. Worker-laid eggs are normally eaten by other workers in a queenright colony. The aim of this study was to compare the survival of three classes of eggs, namely, those laid by normal workers, rebel workers, and the queen. All eggs were tested in queenright colonies. We expected that rebel workers would avoid policing by laying more queen-like eggs. Contrary to our expectations, eggs laid by rebel workers were eaten by other workers, as were eggs laid by normal workers, and only a few worker-laid eggs (both normal and rebel) survived for more than 3 h. Therefore, in a queenright colony, eggs laid by rebel workers do not avoid policing.

Apis florea and Apis cerana workers do not discriminate between queen-laid and worker-laid Apis mellifera eggs

While honey bee workers have functional ovaries, worker reproduction is rare in colonies with a laying queen. An important mechanism by which functional worker sterility is enforced is worker policing—any behaviour of workers that prevents other workers from reproducing. In honey bees, policing workers identify worker-laid eggs and eat them. Policing workers apparently distinguish queen-laid eggs from eggs laid by workers using as yet unidentified chemical signals placed on eggs. These signals are conserved across evolutionary time because the Western hive bee Apis mellifera can distinguish queen- and worker-laid eggs of the phylogenetically distant dwarf honey bee Apis florea. However A. mellifera cannot distinguish queen- and worker-laid eggs of the more closely related species Apis cerana. Here we show that A. cerana and A. florea workers do not distinguish queen-laid from worker-laid eggs of A. mellifera, even though A. cerana (but not A. florea) tolerate A. mellifera eggs for extended periods.

Population structure and sociogenetic organisation in a species with ergatoid queens, the desert ant Ocymyrmex robustior

In ants, reproductive division of labour is typically associated with queen-worker dimorphism. In some species with ergatoid queens (wingless worker-like queens), this polymorphism is drastically reduced and virgin queens may integrate the worker force. While ergatoid queens have been described in several species, their colony and population genetic structure remain largely unstudied. Here, we investigated the population structure and sociogenetic organisation of the desert ant Ocymyrmex robustior. All Ocymyrmex species have only ergatoid queens that are worker-sized. Workers, queens, and males from a large population were genotyped at ten polymorphic microsatellite loci. Our results show that the study population is genetically structured, consistent with dependent colony foundation. Genetic analyses revealed that 17.6% of the males were diploid; diploid males are fertile, siring triploid females. Nests were typically headed by a single queen, and queens were strictly monandrous. However, several nests in the population shared matrilines, indicating polygyny, polydomy, dependent colony foundation, serial polygyny, or a combination of these processes. Dissections reveal that workers lay eggs in both queenright and queenless nests, while virgin ergatoid queens lay eggs in queenright nests only. However, our genetic analyses show that male offspring in queenright nests are all queen-produced, suggesting worker policing and/or trophic egg laying.

Social enforcement depending on the stage of colony growth in an ant.

Altruism is a paradox in Darwinian evolution. Policing is an important mechanism of the evolution and maintenance of altruism. A recently developed dynamic game model incorporating colony demography and inclusive fitness predicts that, in hymenopteran social insects, policing behaviour enforcing reproductive altruism in group members depends strongly on the colony growth stage, with strong policing as the colony develops and a relaxation of policing during the reproductive phase. Here, we report clear evidence supporting this prediction. In the ant Diacamma sp., reproduction by workers was suppressed by worker policing when the colony was small, whereas in large, mature colonies worker policing was relaxed and worker-produced males emerged. Conditional expression of traits can provide strong empirical evidence for natural selection theory if the expression pattern is precisely predicted by the theory, and our results illustrate the importance of intracolony population dynamics in the evolution of social systems.

Individual and genetic task specialization in policing behaviour in the European honeybee

Cooperation in biological systems is frequently maintained by social enforcement mechanisms, where individually egoistic and group-costly behaviour is mutually suppressed by other group members. One of the best examples in nature is worker policing in the honeybee, Apis mellifera, where workers selectively remove or ‘police’ eggs laid by workers that egoistically try to produce their own offspring instead of working for the good of the colony. It has long been suggested that worker policing behaviour should be genetically determined, as theory has shown that queen polyandry in the honeybee would be expected to give rise to clear indirect genetic or ‘inclusive fitness’ benefits of worker policing, thereby causing genes for policing to spread in the population. In the present study, we tested the theory that worker policing should have a genetic component by determining whether workers belonging to different patrilines, derived from different fathers, differ in their tendency to police eggs. This analysis showed that variation in policing behaviour indeed has a genetic basis, with the trait having an estimated broad-sense heritability of 0.25 ([0.013–0.46] 95% confidence limits). In addition, there was clear individual specialization in policing, as just a few individuals within each patriline were observed to police. Remarkably though, there was no evidence for age specialization, as workers of all ages, except those younger than 10 days and older than ca. 40 days, engaged in policing. This contrasts with most other behaviours in the honeybee, which usually follow a strict age-linked pattern of division of labour. Overall, we conclude that worker policing behaviour in the honeybee is genetically heritable and that workers of all ages engage in policing to help maintain the social order in the colony.

Evolution of worker policing

Workers in insect societies are sometimes observed to kill male eggs of other workers, a phenomenon known as worker policing. We perform a mathematical analysis of the evolutionary dynamics of policing. We investigate the selective forces behind policing for both dominant and recessive mutations for different numbers of matings of the queen. The traditional, relatedness-based argument suggests that policing evolves if the queen mates with more than two males, but does not evolve if the queen mates with a single male. We derive precise conditions for the invasion and stability of policing alleles. We find that the relatedness-based argument is not robust with respect to small changes in colony efficiency caused by policing. We also calculate evolutionarily singular strategies and determine when they are evolutionarily stable. We use a population genetics approach that applies to dominant or recessive mutations of any effect size.

Social coercion of larval development in an ant species.

Ants provide one of the best examples of the division of labor in animal societies. While the queens reproduce, workers generally refrain from laying eggs and dedicate themselves exclusively to domestic tasks. In many species, the small diploid larvae are bipotent and can develop either into workers or queens depending mostly on environmental cues. This generates a conflicting situation between the adults that tend to rear a majority of larvae into workers and the larvae whose individual interest may be to develop into reproductive queens. We tested the social regulation of larval caste fate in the fission-performing ant Aphaenogaster senilis. We first observed interactions between resident workers and queen- and worker-destined larvae in presence/absence of the queen. The results show that workers tend to specifically eliminate queen-destined larvae when the queen is present but not when she is absent or imprisoned in a small cage allowing for volatile pheromone exchanges. In addition, we found that the presence of already developed queen-destined larvae does not inhibit the development of younger still bipotent larvae into queens. Finally, we analyzed the cuticular hydrocarbon profiles of queen- and worker-destined larvae and found no significant quantitative or qualitative difference. Interestingly, the total amount of hydrocarbons on both larval castes is extremely low, which lends credence on the chemical insignificance hypothesis of larval ants. Overall, our results suggest that workers control larval development and police larvae that would develop into queens instead of workers. Such policing behavior is similar in many aspects to what is known of worker policing among adults.

Dual Effect of Wasp Queen Pheromone in Regulating Insect Sociality

Eusocial insects exhibit a remarkable reproductive division of labor between queens and largely sterile workers [1, 2]. Recently, it was shown that queens of diverse groups of social insects employ specific, evolutionarily conserved cuticular hydrocarbons to signal their presence and inhibit worker reproduction [3]. Workers also recognize and discriminate between eggs laid by the queen and those laid by workers, with the latter being destroyed by workers in a process known as "policing" [4, 5]. Worker policing represents a classic example of a conflict-reducing mechanism, in which the reproductive monopoly of the queen is maintained through the selective destruction of worker-laid eggs [5, 6]. However, the exact signals used in worker policing have thus far remained elusive [5, 7]. Here, we show that in the common wasp, Vespula vulgaris, the pheromone that signals egg maternity and enables the workers to selectively destroy worker-laid eggs is in fact the same as one of the sterility-inducing queen signals that we identified earlier [3]. These results imply that queen pheromones regulate insect sociality in two distinct and complementary ways, i.e., by signaling the queen's presence and inhibiting worker reproduction, and by facilitating the recognition and policing of worker-laid eggs.

Killing and replacing queen-laid eggs: low cost of worker policing in the honeybee.

Worker honeybees, Apis mellifera, police each other's reproduction by killing worker-laid eggs. Previous experiments demonstrated that worker policing is effective, killing most (∼98%) worker-laid eggs. However, many queen-laid eggs were also killed (∼50%) suggesting that effective policing may have high costs. In these previous experiments, eggs were transferred using forceps into test cells, mostly into unrelated discriminator colonies. We measured both the survival of unmanipulated queen-laid eggs and the proportion of removal errors that were rectified by the queen laying a new egg. Across 2 days of the 3-day egg stage, only 9.6% of the queen-laid eggs in drone cells and 4.1% in worker cells were removed in error. When queen-laid eggs were removed from cells, 85% from drone cells and 61% from worker cells were replaced within 3 days. Worker policing in the honeybee has a high benefit to policing workers because workers are more related to the queen's sons (brothers, r = 0.25) than sister workers' sons (0.15). This study shows that worker policing also has a low cost in terms of the killing of queen-laid eggs, as only a small proportion of queen-laid eggs are killed, most of which are rapidly replaced.

Abnormal behaviors mediated by nest cell size: a case study with Yemeni honey bees

In the Yemeni honey bee, Apis mellifera jementica, as in other species of honey bees in the genus Apis, the queen is responsible for the egg-laying duties, while workers are facultatively sterile and perform other colony-wide tasks. Under certain conditions, however, workers can lay unfertilized eggs that develop into male drones. In this study, the effect of cell size on queen egg laying and worker policing behaviorswas investigated. Yemeni honey bee queens were allowed to lay eggs in different cell size combs. Abnormal queen-laid eggs in large cell size combs were found to be removed by workers firstly followed by worker-laid eggs within 24 hours while abnormal queen-laid eggs, in normal cell size combs, survived up to three days. Nest cell size seemed to contribute in the alteration of queen egg laying and policing behaviors.

No facultative worker policing in the honey bee (Apis mellifera L.)

Kin selection theory predicts that in colonies of social Hymenoptera with multiply mated queens, workers should mutually inhibit ("police") worker reproduction, but that in colonies with singly mated queens, workers should favor rearing workers' sons instead of queens' sons. In line with these predictions, Mattila et al. (Curr Biol 22:2027-2031, 2012) documented increased ovary development among workers in colonies of honey bees with singly mated queens, suggesting that workers can detect and respond adaptively to queen mating frequency and raising the possibility that they facultative police. In a follow-up experiment, we test and reject the hypothesis that workers in single-patriline colonies prefer worker-derived males and are able to reproduce directly; we show that their eggs are policed as strongly as those of workers in colonies with multiply mated queens. Evidently, workers do not respond facultatively to a kin structure that favors relaxed policing and increased direct reproduction. These workers may instead be responding to a poor queen or preparing for possible queen loss.

Mating system evolution and worker caste diversity inPheidoleants

The efficiency of social groups is generally optimized by a division of labour, achieved through behavioural or morphological diversity of members. In social insects, colonies may increase the morphological diversity of workers by recruiting standing genetic variance for size and shape via multiply mated queens (polyandry) or multiple-breeding queens (polygyny). However, greater worker diversity in multi-lineage species may also have evolved due to mutual worker policing if there is worker reproduction. Such policing reduces the pressure on workers to maintain reproductive morphologies, allowing the evolution of greater developmental plasticity and the maintenance of more genetic variance for worker size and shape in populations. Pheidole ants vary greatly in the diversity of worker castes. Also, their workers lack ovaries and are thus invariably sterile regardless of the queen mating frequency and numbers of queens per colony. This allowed us to perform an across-species study examining the genetic effects of recruiting more patrilines on the developmental diversity of workers in the absence of confounding effects from worker policing. Using highly variable microsatellite markers, we found that the effective mating frequency of the soldier-polymorphic P. rhea (avg. meN = 2.65) was significantly higher than that of the dimorphic P. spadonia (avg. meN = 1.06), despite a significant paternity skew in P. rhea (avg. B = 0.10). Our findings support the idea that mating strategies of queens may co-evolve with selection to increase the diversity of workers. We also detected patriline bias in the production of different worker sizes, which provides direct evidence for a genetic component to worker polymorphism.

Enforcement of Reproductive Synchrony via Policing in a Clonal Ant

In insect societies, worker policing controls genetic conflicts between individuals and increases colony efficiency. However, disentangling relatedness from colony-level effects is usually impossible. We studied policing in the parthenogenetic ant Cerapachys biroi, where genetic conflicts are absent due to clonality and reproduction is synchronized through stereotyped colony cycles. We show that larval cues regulate the cycles by suppressing ovarian activity and that individuals that fail to respond to these cues are policed and executed by their nestmates. These individuals are genetically identical to other colony members, confirming the absence of intracolonial genetic conflicts. At the same time, they bear distinct cuticular hydrocarbon profiles, which could serve as proximate recognition cues for policing. Policing in C. biroi keeps uncontrolled reproduction at bay and thereby maintains the colony-level phenotype. This study shows that policing can enforce adaptive colony-level phenotypes in societies with minimal or no potential genetic conflicts. In analogy to immunosurveillance on cancer cells in genetically homogeneous multicellular organisms, colony efficiency is improved via the control of individuals that do not respond properly to regulatory signals and compromise the functioning of the higher-level unit.

Worker self-restraint and policing maintain the queen’s reproductive monopoly in a pseudomyrmecine ant

Division of reproductive labor in insect societies is often based on worker self-restraint and both queen and worker policing. Workers of many hitherto studied wasps, bees and ants do not lay eggs in the presence of a queen. However, it is presently unclear how far these observations in a few select clades can be generalized. We investigated if and how queens maintain a reproductive monopoly in colonies of the elongate twig ant, Pseudomyrmex gracilis, a member of the previously unstudied ant subfamily Pseudomyrmecinae. Colonies are usually headed by a single, singly mated queen (monogyny, monandry). Workers therefore would be more closely related to males produced by other workers (r = 0.375) than to the sons of queens (r = 0.25). Nevertheless, workers appear to refrain from laying male-destined eggs in the presence of the queen. In queenless conditions, workers form dominance hierarchies by antennal boxing, and only one or a few high-ranking individuals readily begin to lay eggs. When returned into a queenright colony, egg-laying workers are immediately bitten, stung and expelled or killed by other workers. While the composition of cuticular hydrocarbons clearly differed between castes, it less clearly reflected worker ovarian development. An association with worker ovarian development that would allow workers to monitor the reproductive status of nestmates could only be tentatively postulated for certain substances. Our study broadens our knowledge about reproductive conflict in social Hymenoptera and shows that worker sterility in the presence of a queen is more common in monogynous, monandrous ants than expected from relatedness alone.

Experimental evidence that workers recognize reproductives through cuticular hydrocarbons in the ant Odontomachus brunneus

Eusociality is characterized by a reproductive division of labor, wherein workers respond to the presence of reproductive individuals by refraining from reproduction themselves and restricting the reproductive efforts of others. Our understanding of how eusociality is maintained therefore depends on characterizing the mechanism by which workers detect the presence of a reproductive. Variations in cuticular hydrocarbons correspond to changes in reproductive ability in ants, and experimental studies are beginning to reveal the function of hydrocarbons as signals. In this study, we compare the cuticular hydrocarbon profiles of dominant and reproductive workers and queens of the ant Odontomachus brunneus with profiles of non-reproductive workers. Using split/reunification tests we document the existence of worker policing in both queenless and queenright colonies; supernumerary reproductives were treated aggressively by nestmates. Finally, we induce aggression and replicate queen-like submissive nestmate responses by supplementing the hydrocarbon profile of workers with (Z)-9-nonacosene, a compound that was significantly more abundant on the cuticles of reproductives. In three bioassays, we compare this manipulation to various control manipulations of the hydrocarbon profile and demonstrate that workers gauge the reproductive activity of nestmates through changes in their cuticular hydrocarbon profiles.