Dominance orders,worker reproduction,and queen-worker conflict in the slave-making ant Harpagoxenus sublaevis

Summary In a queenright colony of the monogynous slave-making ant Harpagoxenus sublaevis, a subset of workers formed a linear dominance order in which dominance was corrlated with ovarian development, frequency of trophallaxis, length of time spent in the nest, but not body size. Identical dominance orders occurred in queenless colonies. Experiments in which the top-ranking workers were removed from queenless colonies demonstrated that worker dominance behaviour inhibits egg-laying in subordinates. A similar removal experiment showed queens restrict dominance behaviour and egg-laying in workers, probably pheromonally. Observations of slave raids indicated ovary-developed workers spent significantly less time scouting for slaves, and tended to participate less in slave raids, than workers without ovarian development. These findings suggest that potentially fertile H. sublaevis workers aggressively compete for egg-laying rights, consume extra food for egg development, and safeguard their reproductive futures by avoiding risks outside the nest. Hence worker reproduction in this species strongly influences the colony's social structure, nutrient flow, and division of labour, even though all workers in a colony are full sisters. I hypothesize that worker reproduction was formerly even more prevalent in H. sublaevis, with workers following the strategy of raising sisters and producing sons predicted by kinship theory. Its continued existence despite queen opposition conceivably results from selection on orphaned workers to reproduce, and the inability of slave-maker workers to raise female-biased broods. The social organization of H. sublaevis therefore highlights the importance both of worker reproduction and of the concomitant queen-worker conflict over male parentage in Hymenopteran social evolution.     

Patterns of male parentage in the fungus-growing ants

Ant queens from eight species, covering three genera of lower and two genera of higher attine ants, have exclusively or predominantly single mating. The ensuing full-sib colonies thus have a strong potential reproductive conflict between the queen and the workers over male production. This is because, all other things being equal, relatedness incentives should favour traits expressed in both workers and the queen to monopolise the production of the colony's male offspring. Microsatellite genotyping of males from these attine species shows that workers in queenless colonies are able to produce males, but that no worker-produced males were found in queenright colonies. Our results suggest that worker reproduction is rare or even absent in colonies with a fertile queen. This indicates that either the queen directly prevents the workers from raising their own sons, or that worker reproduction is absent in the presence of a fertile queen due to high ergonomic costs of this behaviour.     

Queen acceptance and the complexity of nestmate discrimination in the Argentine ant

In most social insect species, individuals recognize and behave aggressively towards non-nestmate conspecifics to maintain colony integrity. However, introduced populations of the invasive Argentine ant, Linepithema humile, exhibit pronounced variation in intraspecific aggression denoting diversity in nestmate recognition behavior, which possibly shapes their social structure and the varying levels of unicoloniality observed among these populations. One approach to better understand differential aggression behaviors towards conspecifics and recognition cue perception and response in L. humile is to examine variation in nestmate discrimination capability among genetically distinct colonies under different social contexts. Consequently, we investigated the dynamics of queen and worker recognition in southeastern US L. humile queenless and queenright colonies by measuring rates of non-nestmate worker and queen adoption and intercolony genetic similarity. Aggression levels between colony pairs differed and were associated with non-nestmate worker, but not queen adoption. Adoption of queens and workers was a function of host colony origin, while colony queen number affected adoption of queens, but not workers, with queens more readily accepted by queenless hosts. Fecundity of adopted non-nestmate queens was comparable to that of rejected non-nestmate and host colony queens, suggesting that queen fecundity did not affect adoption decisions. Genetic similarity between colonies ranged from 30 to 77% alleles shared, with more genetically similar colonies showing lower levels of intraspecific aggression. Non-nestmate queens and workers that were more genetically similar to host colony workers were more likely to be adopted. We provide the first evidence for the role of L. humile colony queen number on queen discrimination and suggest an effect of resident queens on worker conspecific acceptance thresholds. Our findings indicate a role for genetically based cues in L. humile nestmate recognition. However, subtle discrimination capability seems to be influenced by the social context, as demonstrated by more frequent recognition errors in queenless colonies.     

Social parasitism of queens and workers in the Cape honeybee (<Emphasis Type="Italic">Apis mellifera capensis</Emphasis>)

Workers of a queenless honeybee colony can requeen the colony by raising a new queen from a young worker brood laid by the old queen. If this process fails, the colony becomes hopelessly queenless and workers activate their ovaries to lay eggs themselves. Laying Cape honeybee workers (Apis mellifera capensis) produce female offspring as an additional pathway for requeening. We tested the frequency of successful requeening in ten hopelessly queenless colonies. DNA genotyping revealed that only 8% of all queens reared in hopelessly queenless colonies were the offspring of native laying worker offspring. The vast majority of queens resulted from parasitic takeovers by foreign queens (27%) and invading parasitic workers (19%). This shows that hopelessly queenless colonies typically die due to parasitic takeovers and that the parasitic laying workers are an important life history strategy more frequently used than in providing a native queen to rescue the colony. Parasitism by foreign queens, which might enter colonies alone or accompanied by only a small worker force is much more frequent than previously considered and constitutes an additional life history strategy in Cape honeybees.     

Polydomy and sexual allocation ratios in the ant Myrmica punctiventris

Summary Colony structure and reproductive investment were studied in a population of Myrmica punctiventris. This species undergoes a seasonal cycle of polydomy. A colony overwinters in entirety but fractionates into two or more nest sites during the active season and then coalesces in the fall. Colony boundaries were determined by integrating data on spatial pattern, behavioral compatability, and genetic relatedness as revealed by protein electrophoresis. Colonies contained at most one queen. Consequently, a colony consisted of one queenright nest and one or more queenless nests. Furthermore, estimates of relatedness were fully consistent, with queens being single mated. M. punctiventris therefore has a colony genetic structure that conforms to the classical explanation of the maintenance of worker sterility by kin selection. Kin selection theory predicts that workers would favor a female-biased allocation ratio while selection on queens would favor equal investment in males and females. We predicted that in polydomous populations, queenless nests would rear more female reproductives from diploid larvae than queenright nests. There was a significant difference between queenright and queenless nests in sexual allocation; queenless nests allocated energy to reproductive females whereas queenright nests did not. At neither the nest nor colony levels did worker number limit sexual production. We also found that nests tended to rear either males or females but when colony reproduction was summed over nests, the sexes were more equally represented. The difference in allocation ratios between queenless and queenright nests was attributed solely to queen presence/absence. Our work shows that polydomy provides an opportunity for workers to evade queen control and thereby to sexualize brood.Offprint requests to: L.E. Snyder at the current address     

Worker reproduction in honey-bees (Apis) and the anarchic syndrome: a review

Honey-bees, Apis, are an important model system for investigating the evolution and maintenance of worker sterility. The queen is the main reproductive in a colony. Workers cannot mate, but they can lay unfertilized eggs, which develop into males if reared. Worker reproduction, while common in queenless colonies, is rare in queenright colonies, despite the fact that workers are more related to their own sons than to those of the queen. Evidence that worker sterility is enforced by 'worker policing' is reviewed and worker policing is shown to be widespread in Apis. We then discuss a rare behavioural syndrome, 'anarchy', in which substantial worker production of males occurs in queenright colonies. The level of worker reproduction in these anarchic colonies is far greater than in a normal queenright honey-bee colony. Anarchy is a counterstrategy against worker policing and an example of a 'cheating' strategy invading a cooperative system.     

Worker reproductive parasitism and drift in the western honeybee Apis mellifera

When a honeybee (Apis spp.) colony loses its queen and is unable to rear a new one, some of the workers activate their ovaries and produce eggs. When a colony has a queen (i.e., it is queenright) almost all worker-laid eggs are eaten, but when hopelessly queenless, the workers become more tolerant of worker-laid eggs and rear some of them to adult drones. This increased tolerance renders a queenless colony vulnerable to worker reproductive parasitism, wherein unrelated workers enter the colony and lay eggs. Here, we show that the proportion of unrelated (non-natal) workers significantly decreases after an Apis mellifera colony becomes queenless. The remaining non-natal workers are as likely to have activated ovaries as natal workers, yet they produce more eggs than natal workers, resulting in significantly higher reproductive success for non-natal workers. In a second experiment, we provided queenless and queenright workers with a choice to remain in their own colony or to join a queenless or queenright colony nearby. The experiment was set up such that worker movement was unlikely to be due to simple orientation errors. Very few workers joined another colony, and there was no preference for workers to drift into or out of queenless or queenright colonies, in accordance with the proportion of non-natal workers declining significantly after becoming queenless in the first experiment.     

Worker drift and egg dumping by queens in wild Bombus terrestris colonies

Wild bumblebee colonies are hard to find and often inaccessible, so there have been few studies of the genetic structure of bumblebees within natural colonies, and hence, it is not clear how frequently events such as worker reproduction, worker drift and queen usurpation take place. This study aimed to quantify the occurrence of natal-worker reproduction, worker drift and drifter reproduction within 14 wild colonies of Bombus terrestris in Central Scotland. Four unlinked microsatellites were used to identify patterns of relatedness of the colonies’ adults and broods. In colonies with queens (queenright colonies), worker reproduction accounted for just 0.83 % of males, increasing to 12.11 % in queenless colonies. Four colonies contained a total of six workers which were not daughters of the queen, and were assumed to be drifters, and four male offspring of drifters. Drifting is clearly not common and results in few drifter offspring overall, although drifters produced approximately seven times more offspring per capita than workers that remained in their natal colony. Unexpectedly, two colonies contained clusters of sister workers and juvenile offspring that were not sisters to the rest of the adults or brood found in the colonies, demonstrating probable egg dumping by queens. A third colony contained a queen which was not a sister or daughter to the other bees in the colony. Although usurping of bumblebee colonies by queens in early season is well documented, this appears to be the first record of egg dumping, and it remains unclear whether it is being carried out by old queens or newly mated young queens.     

Reproductive plasticity in bumblebee workers (<Emphasis Type="Italic">Bombus terrestris</Emphasis>)—reversion from fertility to sterility under queen influence

Worker sterility in the bumblebee Bombus terrestris is conditional and is linked to the social development of the colony. Workers refrain from reproducing or overtly challenging the queen until gyne production has initiated, at the so-called competition point (CP). It is not known whether this behavior is hard-wired or workers show reproductive plasticity. It also remains unclear whether worker reproductive decision is under queen and/or worker control. In this study, we tested worker reproductive plasticity in an attempt to assess whether and under which conditions worker sterility/fertility are reversible. We introduced egg-laying workers into colonies with different social structures for 1 week then monitored their reproductive status. We revealed a remarkable reproductive plasticity in the introduced workers that was social-condition-dependent. In the presence of a pre-CP queen, the introduced workers reverted to sterility, whereas in the presence of a post-CP queen, such workers remained egg-layer. Reversion to sterility does not occur when direct contact with the queen is prevented, as the introduced workers remained egg-layer in the queenright colonies with a confined queen. Egg-laying workers that were introduced into queenless colonies mostly maintained their fertility regardless of colony social phase. This shows that worker transition from cooperative to selfish behavior is reversible depending on the social context.     

Multiple mating and facultative polygyny in the Panamanian leafcutter ant Acromyrmex echinatior

Queen mating frequency of the facultatively polygynous ant Acromyrmex echinatior was investigated by analysing genetic variation at an (AG)_n repeat microsatellite locus in workers and sexuals of 20 colonies from a single Panamanian population. Thirteen colonies were found to be monogynous, 5 colonies contained multiple queens, whereas the queen number of 2 colonies remained unresolved. Microsatellite genotypes indicated that 12 out of 13 queens were inseminated by multiple males (polyandry). The mean queen mating frequency was 2.53 and the mean genetically effective paternity frequency was 2.23. These values range among the highest found in ants, and the results are in keeping with the high mating frequencies reported for other species of leafcutter ants. Consistent skew in the proportional representation of different patrilines within colonies was found, and this remained constant in two consecutive samples of offspring. Dissections showed that all examined queens from multiple-queen colonies were mated egg-layers. The mean relatedness value among nestmate workers in polygynous colonies was lower than that for monogynous colonies. No diploid males were detected in a sample of 70 genotyped males. Worker production of males was detected in one queenless colony. We discuss our findings in relation to known patterns of multiple maternity and paternity in other eusocial Hymenoptera. Received: 2 September 1998 / Received in revised form: 3 February 1999 / Accepted: 7 February 1999     

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.     

Mixed messages: fertility signaling interferes with nestmate recognition in the monogynous ant Camponotus floridanus

Chemical communication is crucial for the organization of social insect colonies. However, with the heavy use of one communication modality, problems may arise such as the interference of different types of information. This study investigated how information about fertility and colony membership is integrated in the ant Camponotus floridanus. We introduced into mature, queenright colonies (a) the nestmate queen, (b) a nestmate worker, (c) a foreign, high-fertility queen, (d) a foreign, low-fertility queen, and (e) a foreign worker. As expected, workers did not attack their nestmate queen or a nestmate worker but responded aggressively to foreign workers and foreign, low-fertility queens. Surprisingly, workers did not attack foreign, high-fertility queens. Chemical analysis demonstrated that the cuticular hydrocarbon profile of C. floridanus encodes information about fertility status in queens and workers and colony membership in workers. We suggest that ants respond to this information in the cuticular hydrocarbon profile: individuals with strong fertility signals are accepted regardless of their colony membership, but individuals without strong fertility signals are tolerated only if their cuticular hydrocarbon profile matches that of colony members. Learning how social insects respond to multiple types of information presented together is critical to our understanding of the recognition systems that permit the complex organization of social insect colonies.     

Differential reproductive success among subfamilies in queenless honeybee (<Emphasis Type="Italic">Apis mellifera</Emphasis> L.) colonies

With very rare exceptions, queenright worker honeybees (Apis mellifera L.) forego personal reproduction and suppress reproduction by other workers, preferring to rear the queens sons. This is in stark contrast to colonies that have lost their queen and have failed to rear a replacement. Under these conditions workers activate their ovaries and lay many eggs that develop parthenogenetically into a final brood of males (drones) before the colony perishes. Interestingly, not all workers contribute equally to this final generation of drones in queenless colonies. Some subfamilies (workers that share the same father) contribute a disproportionately greater number of offspring than other subfamilies. Here we explore some of the mechanisms behind this reproductive competition among subfamilies. We determined the relative contribution of different subfamilies present in colonies to laying workers, eggs, larvae and pupae by genotyping samples of all life stages using a total of eight microsatellite loci. Our colonies were headed by free-mated queens and comprised 8–17 subfamilies and therefore differed significantly from colonies used in an earlier study investigating the same phenomena where colonies comprised an artificially low number of subfamilies. We show that, first, subfamilies vary in the speed with which they activate their ovaries after queen-loss and, second, that the survival of eggs to the larval stage is unequal among subfamilies suggesting that some subfamilies lay eggs that are more acceptable than others. However, there is no statistically significant difference among subfamilies in the survival of larvae to pupae, indicating that ovary activation and egg survival are the critical components to reproductive competition among subfamilies of queenless honeybee workers.Communicated by R. Page     

Queen primer pheromone affects conspecific fire ant (<Emphasis Type="Italic">Solenopsis invicta</Emphasis>) aggression

Monogyne fire ant, Solenopsis invicta, colony workers are territorial and are aggressive toward members of other fire ant colonies. In contrast, polygyne colony workers are not aggressive toward non-nestmates, presumably due to broader exposure to heritable and environmentally derived nestmate recognition cues (broad template). Workers from both monogyne and polygyne fire ant colonies execute newly mated queens after mating flights. We discovered that monogyne and polygyne queens have a remarkable effect on conspecific recognition. After removal of their colony queen, monogyne worker aggression toward non-nestmate conspecifics quickly drops to merely investigative levels; however, heterospecific recognition/aggression remains high. Queenless monogyne or polygyne worker groups were also not aggressive toward newly mated queens. Queenless worker groups of both forms that adopted a monogyne-derived newly mated queen became aggressive toward non-nestmate workers and newly mated queens. We propose that the powerful effect of fire ant queens on conspecific nestmate recognition is caused by a queen-produced recognition primer pheromone that increases the sensitivity of workers to subtle quantitative differences in nestmate recognition cues. This primer pheromone prevents the adoption of newly mated queens (regulation of reproductive competition) in S. invicta and when absent allows queenless workers to adopt a new queen readily. This extraordinary discovery has broad implications regarding monogyne and polygyne colony and population dynamics.     

Reproductive conflicts and egg discrimination in a socially polymorphic ant

The ability to discriminate against competitors shapes cooperation and conflicts in all forms of social life. In insect societies, workers may detect and destroy eggs laid by other workers or by foreign queens, which can contribute to regulate reproductive conflicts among workers and queens. Variation in colony kin structure affects the magnitude of these conflicts and the diversity of cues used for discrimination, but the impact of the number of queens per colony on the ability of workers to discriminate between eggs of diverse origin has so far not been investigated. Here, we examined whether workers from the socially polymorphic ant Formica selysi distinguished eggs laid by nestmate workers from eggs laid by nestmate queens, as well as eggs laid by foreign queens from eggs laid by nestmate queens. Workers from single- and multiple-queen colonies discriminated worker-laid from queen-laid eggs, and eliminated the former. This suggests that workers collectively police each other in order to limit the colony-level costs of worker reproduction and not because of relatedness differences towards queens’ and workers’ sons. Workers from single-queen colonies discriminated eggs laid by foreign queens of the same social structure from eggs laid by nestmate queens. In contrast, workers from multiple-queen colonies did not make this distinction, possibly because cues on workers or eggs are more diverse. Overall, these data indicate that the ability of F. selysi workers to discriminate eggs is sufficient to restrain worker reproduction but does not permit discrimination between matrilines in multiple-queen colonies.     

Production of sexuals in a fission-performing ant: dual effects of queen pheromones and colony size

Models based on the kin selection theory predict that in social hymenopterans, queens may favor a lower investment in the production of sexuals than workers. However, in perennial colonies, this conflict may be tuned down by colony-level selection because of the trade off between colony survival and reproductive allocation. In this study, we present a survey of sexual production in colonies of Aphaenogaster senilis, a common species of ant in the Iberian Peninsula. Similar to most species that reproduce by fission, males were found in large excess compared to gynes (172:1). Sexuals were more likely to be found in queenless than in queenright (QR) field colonies. However, we also found a few gynes and numerous males in very large QR colonies. We compared these data with those available in the literature for A. rudis, a congeneric species from North America that has independent colony founding. The sex ratio in this species was only five males for each female, and sexuals were mostly found in QR nests, irrespective of colony size. We confirmed queen inhibition of sexual production in A. senilis in laboratory experiments and provide evidence that this inhibition is mediated by a nonvolatile pheromone. To seek the potential source of such a queen pheromone, we analyzed the secretions of two conspicuous exocrine glands, the Dufour’s and postpharyngeal glands (DG and PPG, respectively) in both queens and workers. Both secretions were composed of hydrocarbons, but that of DG also contained small quantities of tetradecanal and hexadecanal. The hydrocarbon profile of the DG and PPG showed notable caste specificity suggesting a role in caste-related behavior. The PPG secretions also differed between colonies suggesting its role in colony-level recognition. We suggest that in A. senilis, there are two modes of colony fission: First, in very large colonies, gynes are produced, probably because of the dilution of the queen pheromone, and consequently one or more gynes leave the mother colony with workers and brood to found a new nest. This is beneficial at the colony level because it avoids the production of costly sexuals in small colonies. However, because the queen and workers have different optima for sexual production, we hypothesize that queens tend to overproduce the pheromone to delay their production. This in turn may drive workers to leave the mother colony during nest relocation and to produce sexuals once they are away from the queen’s influence, creating a second mode of colony fission.     

Social regulation of ovary activation in ’anarchistic’ honey-bees (Apis mellifera)

Honey-bee (Apis mellifera) colonies exhibit extreme reproductive division of labour. Workers almost always have inactive ovaries and the queen monopolises egg laying. Although extremely rare, ’anarchistic’ colonies exist in which workers produce male offspring despite the presence of the queen. By comparing the rates of ovary activation in anarchistic and wild-type bees fostered to host colonies of different genotype (i.e. anarchist and non-anarchist) and queen status (i.e. queenless and queenright), we investigated the factors involved in inhibiting ovary activation. Fostered anarchist workers always had a higher level of ovary development than fostered wild-type bees in both anarchist and non-anarchist host colonies. Fostered workers of both genotypes had more active ovaries in anarchistic than in wild-type hosts. Fostered workers of both strains also had more active ovaries in queenless than in queenright hosts. The results suggest that selection for worker reproduction in the anarchistic line has both reduced the effects of brood and queen pheromones on worker ovary inhibition and increased the likelihood that workers of the anarchistic line will develop ovaries compared to wild-type workers. Received: 14 June 2000 / Revised: 26 September 2000 / Accepted: 7 October 2000     

Sociogenetic organization of the red ant Myrmica rubra

Knowledge of the sociogenetic organization determining the kin structure of social insect colonies is the basis for understanding the evolution of insect sociality. Kin structure is determined by the number and relatedness of queens and males reproducing in the colonies, and partitioning of reproduction among them. This study shows extreme flexibility in these traits in the facultatively polygynous red ant Myrmica rubra. Relatedness among worker nestmates varied from 0 to 0.82. The most important reason for this variation was the extensive variation in the queen number among populations. Most populations were moderately or highly polygynous resulting in low relatedness among worker nestmates, but effectively monogynous populations were also found. Polygynous populations also often tend to be polydomous, which is another reason for low relatedness. Coexisting queens were positively related in two populations out of five and relatedness was usually similar among workers in the same colonies. Due to the polydomous colony organization and short life span of queens, it was not possible to conclusively determine the importance of unequal reproduction among coexisting queens, but it did not seem to be important in determining the relatedness among worker nestmates. The estimates of the mating frequency by queens remained ambiguous, which may be due to variation among populations. In some populations relatedness among worker nestmates was high, suggesting monogyny and single mating by queens, but in single-queen laboratory nests relatedness among the worker offspring was lower, suggesting that multiple mating was common. The data on males were sparse, but indicated sperm precedence and no relatedness among males breeding in the same colony. A comparison of social organizations and habitat requirements of M. rubra and closely related M. ruginodis suggested that habitat longevity and patchiness may be important ecological factors promoting polygyny in Myrmica. Received: 15 May 1995/Accepted after revision: 17 October 1995     

Nestmate recognition in the ant Cataglyphis niger : do queens matter?

This study compares two basic models for the origin and maintenance of colony gestalt odor in the polygynous ant species Cataglyphis niger. In the first model, queens are centers of de novo biosynthesis and distribution of recognition odors (“queen-centered” model); in the second, colony odors are primarily synthesized and distributed by workers (“worker-centered” model). We tested the behavioral patterns that are predicted from each model, and verified by biochemical means the distributional directionality of these signals. Encounters between nestmates originating from split colonies were as amicable as between nestmates from non-split colonies; queenless ants were as aggressive as their queenright nestmates, and both were equally aggressed by alien ants. These results indicate that queens have little impact on the recognition system of this species, and lend credence to the worker-centered model. The queen-centered model predicts that unique queen substances should be produced in appreciable quantities and that, in this respect, queens should be more metabolically active than workers. Analysis of the chemical composition of postpharyngeal glands (PPGs) or cuticular extracts of queens and workers revealed high similarity. Quantitatively, queens possessed significantly greater amounts of hydrocarbons in the PPG than workers, but the amount on the thoracic epicuticle was the same. Queens, however, possess a lower hydrocarbon biosynthesis capability than workers. The biochemical evidence thus refutes the queen-centered model and supports a worker-centered model. To elucidate the directionality of cue distribution, we investigated exchange of hydrocarbons between the castes in dyadic or group encounters in which selective participants were prelabeled. Queens tended to receive more and give less PPG content, whereas transfer to the epicuticle was low and similar in all encounters, as predicted from the worker-centered hypothesis. In the group encounters, workers transferred, in most cases, more hydrocarbons to the queen than to a worker. This slight preference for the queen is presumably amplified in a whole colony and can explain their copious PPG content. We hypothesize that preferential transfer to the queen may reflect selection to maintain her individual odor as close to the average colony odor as possible. Received: 4 November 1997 / Accepted after revision: 5 February 1998     

Colony life history and demography of a swarm-founding social wasp

Colonies of social insects are sometimes viewed as superorganisms. The birth, reproduction, and death of colonies can be studied with demographic measures analogous to those normally applied to individuals, but two additional questions arise. First, how do adaptive colony demographies arise from individual behaviors? Second, since these superorganisms are made up of genetically distinct individuals, do conflicts within the colony sometimes modify and upset optima for colonies? The interplay between individual and superindividual or colony interests appears to be particularly complex in neotropical, swarm-founding, epiponine wasps such as Parachartergus colobopterus. In a long-term study of this species, we censused 286 nests to study colony-level reproduction and survivorship and evaluated individual-level factors by assessing genetic relatedness and queen production. Colony survivorship followed a negative exponential curve very closely, indicating type II survivorship. This pattern is defined by constant mortality across ages and is more characteristic of birds and other vertebrates than of insects. Individual colonies are long-lived, lasting an average of 347 days, with a maximum of over 4.5 years. The low and constant levels of colony mortality arise in part from colony initiation by swarming, nesting on protected substrates, and an unusual expandable nest structure. The ability to requeen rapidly was also important; relatedness data suggest that colonies requeen on average once every 9–12 months. We studied whether colony optima with respect to the timing of reproduction could be upset by individual worker interests. In this species, colonies are normally polygynous but new queens are produced only after a colony reaches the monogynous state, a result which is in accord with the genetic interests of workers. Therefore colony worker interests might drive colonies to reproduce whenever queen number happens to cycled down to one rather than at the season that is otherwise optimal. However, we found reproduction to be heavily concentrated in the rainy season. The number of new colonies peaked in this season as did the percentages of males and queens. Relatedness among workers reached a seasonal low of 0.21–0.27, reflecting the higher numbers of laying queens. This seasonality was achieved in part by a modest degree of synchrony in the queen reduction cycle. Worker relatedness reached peaks of around 0.4 in the dry season, reflecting a decrease to a harmonic mean queen number of about 2.5. Thus, a significant number of colonies must be approaching monogyny entering the rainy season. Coupled with polygynous colonies rearing only males (split sex ratios), this makes it possible for a colony cycle driven by selfish worker interests to be consistent with concentrating colony reproduction during a favorable season.