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.     

Fertility signaling in queens of a North American ant

In most species of advanced eusocial insects, the partitioning of reproduction between nestmates is thought to be regulated by means of primer pheromones or other chemical cues, which presumably influence the behavior of co-queens and workers such that they maximize their own inclusive fitness. Here we show that in multi-queen colonies of the Nearctic ant, Leptothorax sp. A, physical dominance in concert with chemical cues, which signal the ovarian development of a queen, are used to control reproduction of competing queens and to influence worker behavior. The analysis of ranks obtained during two fighting periods in the annual colony cycle revealed a strong link between individual aggressiveness of a queen and her fertility. During the adoption of newly mated queens in autumn, the resident, egg-laying α-queen was more likely to start aggression first and keep her high rank position compared to the fighting period after hibernation. We suggest that this is proximately caused by the α-queen having much stronger developed ovaries in autumn than the young queens, whereas after hibernation, the ovaries of all queens are similarly inactive. Interactions during the first weeks after the end of hibernation and intrinsic, individual differences in aggressiveness appear to be crucial for the dominance rank achieved later. Queens which were allowed to become fertile when their nestmate queens still were kept under prolonged hibernation, were immediately socially dominant over the latter when all queens were reunited, though no aggression occurred. In another experiment, queen antagonism was prevented by spatial separation in different parts of the same nest and all queens began to lay eggs. Workers stayed preferentially with queens with high actual fecundity rather than with those which had had high social status before separation. This and further evidence suggest that ovarian status is communicated, most likely by a chemical cue perceived by co-queens and workers, affects the direction of their aggressive behavior, and allows them to discriminate among queens. Received: 5 May 1998 / Accepted after revision: 29 August 1998     

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.     

Extended family structure in the ant Formica paralugubris: the role of the breeding system

In populations of various ant species, many queens reproduce in the same nest (polygyny), and colony boundaries appear to be absent with individuals able to move freely between nests (unicoloniality). Such societies depart strongly from a simple family structure and pose a potential challenge to kin selection theory, because high queen number coupled with unrestricted gene flow among nests should result in levels of relatedness among nestmates close to zero. This study investigated the breeding system and genetic structure of a highly polygynous and largely unicolonial population of the wood ant Formica paralugubris. A microsatellite analysis revealed that nestmate workers, reproductive queens and reproductive males (the queens' mates) are all equally related to each other, with relatedness estimates centring around 0.14. This suggests that most of the queens and males reproducing in the study population had mated within or close to their natal nest, and that the queens did not disperse far after mating. We developed a theoretical model to investigate how the breeding system affects the relatedness structure of polygynous colonies. By combining the model and our empirical data, it was estimated that about 99.8% of the reproducing queens and males originated from within the nest, or from a nearby nest. This high rate of local mating and the rarity of long-distance dispersal maintain significant relatedness among nestmates, and contrast with the common view that unicoloniality is coupled with unrestricted gene flow among nests. Received: 8 February 1999 / Received in revised form: 15 June 1999 / Accepted: 19 June 1999     

Queen fertility, egg marking and colony size in the ant Camponotus floridanus

In ant societies, workers do not usually reproduce but gain indirect fitness benefits from raising related offspring produced by the queen. One of the preconditions of this worker self-restraint is sufficient fertility of the queen. The queen is, therefore, expected to signal her fertility. In Camponotus floridanus, workers can recognize the presence of a highly fertile queen via her eggs, which are marked with the queen's specific hydrocarbon profile. If information on fertility is encoded in the hydrocarbon profile of eggs, we expect workers to be able to differentiate between eggs from highly and weakly fertile queens. We found that workers discriminate between these eggs solely on the basis of their hydrocarbon profiles which differ both qualitatively and quantitatively. This pattern is further supported by the similarity of the egg profiles of workers and weakly fertile queens and the similar treatment of both kinds of eggs. Profiles of queen eggs correspond to the cuticular hydrocarbon profiles of the respective queens. Changes in the cuticular profiles are associated with the size of the colony the queen originates from and her current egg-laying rate. However, partial correlation analysis indicates that only colony size predicts the cuticular profile. Colony size is a buffered indicator of queen fertility as it is a consequence of queen productivity within a certain period of time, whereas daily egg-laying rate varies due to cyclical oviposition. We conclude that surface hydrocarbons of eggs and the cuticular profiles of queens both signal queen fertility, suggesting a major role of fertility signals in the regulation of reproduction in social insects.     

Genetic population structure and sociogenetic organisation in Formica truncorum (Hymenoptera; Formicidae)

Summary The genetic population structure and the sociogenetic organization of the red wood ant Formica truncorum were compared in two populations with monogynous colonies and two populations with polygynous colonies. The genetic population structure was analysed by measuring allele frequency differences among local subsets of the main study populations. The analysis of sociogenetic organisation included estimates of nestmate queen and nestmate worker relatedness, effective number of queens, effective number of matings per queen, relatedness among male mates of nestmate queens and relatedness between queens and their male mates. The monogynous populations showed no differentiation between subpopulations, whereas there were significant allele frequency differences among the subpopulations in the polygynous population. Workers, queens and males showed the same genetical population structure. The relatedness among nestmate workers and among nestmate queens was identical in the polygynous societies. In three of the four populations there was a significant heterozygote excess among queens. The queens were related to their male mates in the polygynous population analysed, but not in the monogynous ones. The data suggest limited dispersal and partial intranidal mating in the populations with polygynous colonies and outbreeding in the populations having monogynous colonies. Polyandry was common in both population types; about 50% of the females had mated at least twice. The males contributed unequally to the progeny, one male fathering on average 75% of the offspring with double mating and 45–80% with three or more matings. Correspondence to: L. Sundström     

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     

Worker discrimination among queens in newly founded colonies of the fire ant Solenopsis invicta

Although colonies of the fire ant Solenopsis invicta are often founded by small groups of queens, all but one of the queens are soon eliminated due to worker attacks and queen fighting. The elimination of supernumerary queens provides an important context for tests of discrimination by the workers, since the outcome of these interactions strongly affects the workers' inclusive fitness. To test whether workers in newly founded colonies discriminate among nestmate queens, paired cofoundresses were narrowly separated by metal screens that prevented direct fighting, but through which the workers could easily pass. Soon after the first workers completed development, they often attacked one of the queens; these attacks were strongly associated with queen mortality. When one queen's brood was discarded, so that the adult workers were all the daughters of just one queen, the workers were significantly less likely to bite their mother than the unrelated queen; however, this tendency was comparatively weak. Queens kept temporarily at a higher temperature to increase their rate of investment in brood-rearing lost weight more rapidly than paired queens and were subsequently more likely to be attacked and killed by workers. Workers were more likely to bite queens that had been temporarily isolated than queens that remained close to brood and workers. When queens were not separated by screens, the presence of workers stimulated queen fights. These results show that workers discriminate strongly among equally familiar queens and that discrimination is based more on the queens' condition and recent social environment than on kinship. Received: 9 June 1998 / Accepted after revision: 10 October 1998     

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.     

Two independent mechanisms of egg recognition in worker <Emphasis Type="Italic">Formica fusca</Emphasis> ants

Informational constraints can be an important limitation on the accuracy of recognition. One potential constraint is the use of recognition information from the same sources in multiple discriminatory contexts. Worker wood ants, Formica fusca, discriminate eggs based on their maternal sources of origin in two main contexts: recognition of eggs laid by nestmate versus non-nestmate queens and recognition of worker-laid versus queen-laid eggs. We manipulated the experience of F. fusca workers in laboratory colonies to both worker-laid and queen-laid eggs by transferring eggs between colonies in order to investigate whether these two contexts of egg discrimination are independent. Experience of non-nestmate queen-laid eggs significantly increased worker acceptance of both familiar (18% accepted) and unfamiliar (10%) queen-laid eggs compared to control workers without experience of eggs other than those laid by their own colony’s queen (2%). In contrast, worker acceptance of worker-laid eggs was not affected by variation in the egg experience of workers (14% in workers from control colonies exposed only to eggs from their own colony’s queen versus 19% and 17% in workers from colonies which had received eggs laid by either a non-nestmate queen or nestmate workers, respectively). Our results suggest that these two recognition contexts do not strongly constrain each other and are different in their ontogeny. In particular, worker-laid eggs are universally discriminated against by workers from colonies with a queen whatever the egg experience of the workers, while non-nestmate queen-laid eggs are strongly discriminated against only by workers without experience of eggs laid by more than one queen.     

Stable relatedness structure of the large-colony swarm-founding wasp Polybia paulista

In social-insect colonies, cooperation among nestmates is generally stabilized by their high genetic similarity. Thus, fitness gained through cooperation drops quickly as the number of reproductive females (queens) increases. In this respect, wasps of the tribe Epiponini have attracted special attention, because the colonies have tens, or even hundreds of queens. It has been empirically or genetically confirmed that relatedness within nestmates can be elevated by a mechanism known as cyclical monogyny, under which new queens are produced only after the number of old queens is reduced to one. Another likely factor that can increase relatedness within nestmates under polygyny is comb partitioning by queens. If queens concentrate their egg laying on one or a subset of the available combs, then workers may be able to rear closer relatives by focusing their work on the comb where they emerged. Using microsatellite markers, we tested the hypotheses of cyclical monogyny and comb partitioning by queens increasing relatedness within nestmates under polygyny in the large-colony epiponine wasp, Polybia paulista. There were no significant differences between relatedness within combs and between combs, and thus we ruled out the possibility that each queen partitions reproduction between combs. However, as cyclical monogyny predicts, a lower effective number of queens contributed to queen production than to worker production. Cyclical monogyny explained well the observed smaller effective number of queens for new queens than that for workers, but failed to explain the stable relatedness values throughout colony cycles.Communicated by L. Keller     

Chemical camouflage of the slave-making ant Polyergus samurai queen in the process of the host colony usurpation (Hymenoptera: Formicidae)

Founding queens of the obligatory social parasite ant Polyergus samurai usurp the host ant Formica japonica colony. The aggressive behaviors of F. japonica workers on the parasite queen disappear after the parasite queen kills the resident queen. To determine whether the parasite queen chemically mimics the host ants, we examined the aggressive behavior of F. japonica workers toward glass dummies applied with various extracts of the parasite queen and host workers. The crude extracts and hydrocarbon fraction reproduced the host workers’ behavior to the live ants. The extracts of the post-adoption parasite queen, as well as the nestmate extracts of F. japonica, did not elicit the aggressive behavior, but the extract of the pre-adoption parasite queen triggered attacks by the host workers. The nestmate recognition of host workers did not change, regardless of contact with the parasite. The gas chromatography and gas chromatography–mass spectrometry analyses indicated that the cuticular hydrocarbon (CHC) profile of the parasite queen drastically changed during the process of usurpation. Discriminant analysis showed the successfully usurped P. samurai queen had colony-specific CHC profiles. CHC profiles of the P. samurai queen who killed the host queen were more similar to those of the host queen than the workers, while the P. samurai queen who usurped the queenless colony had a profile similar to those of host workers. These results suggest that the P. samurai queen usually acquires the CHCs from the host queen during the fight, but from host wokers in queenless host colonies.     

Recognition of caste and mating status maintains monogyny in the ant Aphaenogaster senilis

In ants dispersing through colony fission, queens mate near their natal nest and found a new society with the help of workers. This allows potential future queens to challenge the mother queen’s reproductive monopoly. Conflicts might be resolved if the mated queen signals her presence and the workers control the developmental fate of the diploid larvae (whether they develop to worker or queen). In this study we sought to determine whether, in the fission-performing ant Aphaenogaster senilis, conflicts between queens for control of the colony are resolved by the resident queen signalling her mating status. Virgin queens were less effective than newly mated queens in inhibiting queen rearing. Moreover, potential challenger queens were recognized and heavily aggressed independent of mating status. Chemical analyses showed that mating status was associated with changes in cuticular hydrocarbon and poison gland composition, but not in Dufour’s gland composition. Cuticular dimethylalkanes were identified as potential constituents that signal both caste (present in queens only) and mating status (mated queens have higher amounts). We hypothesised that pheromone emission by virgin queens did not reach the threshold needed to fully inhibit larval development into queens but was sufficiently high to stimulate overt aggression by mated queens. These findings provide evidence for the complexity of chemical communication in social insects, in which a small number of signals may have a variety of effects, depending on the context.     

Kin discriminators in the eusocial sweat bee <Emphasis Type="Italic">Lasioglossum malachurum</Emphasis>: the reliability of cuticular and Dufour’s gland odours

The ability to discriminate degrees of relatedness may be expected to evolve if it allows unreciprocated altruism to be preferentially directed towards kin (Hamilton in J Theor Biol 7:1–16, 1964). We explored the possibility of kin recognition in the primitively eusocial halictid bee Lasioglossum malachurum by investigating the reliability of worker odour cues that can be perceived by workers to act as indicators of either nest membership or kinship. Cuticular and Dufour’s gland compounds varied significantly among colonies of L. malachurum, providing the potential for nestmate discrimination. A significant, though weak, negative correlation between chemical distance and genetic relatedness (r = −0.055, p < 0.001) suggests a genetic component to variation in cuticular bouquet, but odour cues were not informative enough to discriminate between different degrees of relatedness within nests. This pattern of variation was similar for Dufour’s gland bouquets. The presence of unrelated individuals within nests that are not chemically different from their nestmates suggests that the discrimination system of L. malachurum is prone to acceptance errors. Compounds produced by colony members are likely combined to generate a gestalt colony chemical signature such that all nestmates have a similar smell. The correlation between odour cues and nest membership was greater for perceived compounds than for non-perceived compounds, suggesting that variability in perceived compounds is a result of positive selection for nestmate recognition despite potentially stabilising selection to reduce variability in odour differences and thereby to reduce costs derived from excessive intracolony nepotistic behaviour.     

Maternity of replacement queens in the thelytokous Cape honey bee Apis mellifera capensis

Unlike workers of all other honey bee (Apis mellifera) subspecies, workers of the Cape honey bee of South Africa (A. mellifera capensis) reproduce thelytokously and are thus able to produce female offspring that are pseudoclones of themselves. This ability allows workers to compete with their queen over the maternity of daughter queens and, in one extreme case, has led to a clonal lineage of workers becoming a social parasite in commercially managed populations of A. mellifera scutellata. Previous work (Jordan et al., Proc R Soc Lond B Biol Sci 275:345, 2008) showed that, in A. mellifera capensis, 59% of queen cells produced during swarming events contained the offspring of workers and that, of these, 65% were the offspring of non-natal workers. Here, we confirm that a substantial proportion (38.5%) of offspring queens is worker-laid. We additionally show that: (1) Although queens produce most diploid female offspring sexually, we found some homozygous or hemizygous queen offspring, suggesting that queens also reproduce by thelytoky. These parthenogenetic individuals are probably nonviable beyond the larval stage. (2) Worker-laid offspring queens are viable and become the resident queen at the same frequency as do sexually produced queen-laid offspring queens. (3) In this study, all but one of the worker-derived queens were laid by natal workers rather than workers from another nest. This suggests that the very high rates of social parasitism observed in our previous study were enhanced by beekeeping manipulations, which increased movement of parasites between colonies.     

Prior experience with eggs laid by non-nestmate queens induces egg acceptance errors in ant workers

We studied the effect of prior experience to eggs laid by nestmate and non-nestmate queens on the acceptance of queen-laid eggs by worker wood ants, Formica fusca. We transferred eggs from a non-nestmate queen into colonies during early spring, when their own queen was recommencing egg laying. A few weeks later, workers from these “experienced” colonies accepted eggs of both familiar (44% acceptance) and unfamiliar (40%) non-nestmate queens much more than workers from control colonies (2%) that had only had previous contact with their own queen’s eggs. Thus, prior exposure to eggs laid by a non-nestmate queen induces much greater acceptance of all non-nestmate queen-laid eggs. Mechanistically, we hypothesize that exposure to eggs from several queens may increase acceptance by causing a highly permissive acceptance threshold of non-nestmate queen-laid eggs rather than by widening the template for acceptable queen-laid eggs. These novel results show that egg-discrimination behaviour in F. fusca is flexible and that workers respond to the diversity of eggs experienced in their colony.     

Mating and reproduction in the wasp Vespula germanica

We used polymorphic microsatellite markers to study patterns of queen and worker reproduction in annual nests of the wasp Vespula germanica in its introduced range in Australia. We found that queens were typically polyandrous (at least 85.4% mated multiply), with the minimum number of male mates ranging from 1 to 7. Calculations based on nestmate worker relatedness (r=0.46) yielded an estimate of effective queen mating frequency of 2.35. Queens were unrelated to their mates (r=-0.01), indicating that mating occurred at random within Australian V. germanica populations. In addition, the distribution of the minimum number of male mates of queens followed a Poisson distribution. This result suggested that the probability of a queen remating was not affected by previous copulations. We also discovered that mates of polyandrous queens contributed unequally to progeny production leading to significant male reproductive skew within nests. Analyses of nestmate male genotypes revealed that queens usually produced most or all males. However, workers were responsible for the production of many males in a few nests, and, in contrast to theoretical expectations, two of these nests were apparently queenright.     

Sex ratio determination and worker reproduction in the slave-making ant Harpagoxenus sublaevis

Summary In a population of the monogynous slave-making ant Harpagoxenus sublaevis in S.E. Sweden, the mean proportion of dry weight investment in queens was 0.54. This result differed significantly from 0.75 but not from 0.5, matching the prediction from the genetic relatedness hypothesis of sex ratio applied to slave-makers, given (as confirmed by this study) single mating of queens, population-wide mate competition, and relatively low levels of worker male production. Sex investment appeared unaffected by resource availability. In the same 47 colony population sample, fertile slave-maker workers were found in every queenless colony (ca. 30% of all colonies), and in 58% of queen-right colonies. Fertile workers occurred at a significantly higher frequency in the queenless colonies (19.2%) than in the queenright ones (9.8%), confirming that queenless conditions promote worker fertility. Fertile and sterile workers were similar in size. Electrophoretic allozyme analysis of ants from 49 colonies showed that: 1) queens mated singly; 2) female nestmates were full sisters (their regression coefficient of relatedness (±SE) was 0.735±0.044); 3) inbreeding did not occur; 4) queen and worker siblings were not genetically differentiated. Worker male production in queenright colonies was neither confirmed nor ruled out by the genetic data. However, production data indicated that queenless workers produced between 4.4 and 21.6% of all males. Overall colony productivity was largely determined by slave number, itself positively correlated with the number of slave-maker workers. There was an abrupt switch from all worker to all sexual production as colony size rose, as predicted by life history models. In queenright colonies, fertile slave-makers did not discernibly reduce colony productivity. Such workers occurred in queenright colonies with most slaves, suggesting they exploited energetic surpluses. Worker reproduction in H. sublaevis therefore appears to have greater influence at the level of individual behaviour than at colony or population level.     

Parentage and sex allocation in the facultatively polygynous ant Myrmicatahoensis

Most social groups have the potential for reproductive conflict among group members. Within insect societies, reproduction can be divided among multiple fertile individuals, leading to potential conflicts between these individuals over the parentage of sexual offspring. Colonies of the facultatively polygynous ant Myrmicatahoensis contain from one to several mated queens. In this species, female sexuals were produced almost exclusively by one queen. The parentage of male sexuals was more complex. In accordance with predictions based on worker sex-allocation preferences, male-producing colonies tended to have low levels of genetic relatedness (i.e., high queen numbers). Correspondingly, males were often reared from the eggs of two or more queens in the nest. Further, over half of the males produced appeared to be the progeny of fertile workers, not of queens. Overall investment ratios were substantially more male biased than those predicted by genetic relatedness, suggesting hidden costs associated with the production of female sexuals. These costs are likely to include local resource competition among females, most notably when these individuals are adopted by their maternal nest. Received: 3 March 1998 / Accepted after revision: 20 June 1998     

The dissolution of cooperative groups: mechanisms of queen mortality in incipient fire ant colonies

In several species of ants, queens often form temporary cooperative associations during colony foundation. These associations end soon after the eclosion of the first workers with the death or expulsion of all but one of the queens. This study examined competition between foundress queens of the fire ant Solenopsis invicta. Although attacks by the workers contributed to queen mortality, queens gained no advantage by producing more workers than their co-foundresses. Restriction fragment length polymorphism analysis of mitochondrial DNA showed that the queen producing more workers during colony founding was no more likely to survive than the less productive queen. In experimentally manipulated colonies in which all the workers were daughters of only one of the queens, the mother of the workers was no more likely to survive than the unrelated queen. Queens producing diploid males reared fewer offspring but were as likely to survive as queens producing only workers. These results suggest that workers do not discriminate between related and unrelated queens within colonies. Aggressive encounters between queens were common. Queens were more likely to die or be expelled if paired with heavier queens or if they lost more weight than their co-foundress during the claustral period. Finally, when queens were separated by screens through which workers could pass, the workers usually attacked and killed the queen farther from the brood. These results suggest that queen survival is promoted by a high fighting ability relative to co-foundresses, rather than by increased worker production, and that workers respond to queen differences that are independent of kinship. Received: 8 September 1995/Accepted after revision: 5 March 1996