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.     

Differential reproduction in multiple-queen colonies of the fire ant Solenopsis invicta (Hymenoptera: Formicidae)

Summary The contribution to maternity of workers and female sexuals over time by queens in six multiple-queen laboratory colonies of Solenopsis invicta was directly assessed by use of enzyme genetic markers. Queens contributed more equally to the worker pool than to the pool of sexuals in virtually all samples (Fig. 1), and individuals producing a substantial proportion of the workers often had low or no representation of their daughters in the pool of sexuals. Signficant disparity among queens in their relative production of sexual daughters was often evident, with dominance in production of sexuals by a given queen commonly occurring in association with a pronounced loss of weight followed shortly by her death. The results suggest that significant variability in short-as well as long-term reproductive success may occur among the distantly related queens associating in natural polygyne S. invicta nests. Variance in apportionment of maternity of sexuals did not appear to be simply related to varying levels of fecundity, suggesting that the common presumption that reproductive success can be equated with fecundity in polygyne social Hymenoptera may not be well founded. The observed variance also did not appear to result from a simple mechanism of kin recognition and discrimination by workers in the process of brood rearing. Rather, this variance may have largely resulted from either, 1) recognition of certain queens and their progeny coupled with preferential sexualization of these immatures by nurse workers, or, 2) queen biasing of eggs toward development as sexuals. The frequent association of weight loss and death of mother queens with high levels of sexual daughter production may be best explained by the latter mechanism.     

Diploid male production — a significant colony mortality factor in the fire ant Solenopsis invicta (Hymenoptera: Formicidae)

Summary Two forms of the fire ant, Solenopsis invicta, occur in North America; the monogyne form has colonies with a single functional queen while the polygyne form has colonies containing many functional queens. Field surveys indicate that diploid males are common in natural populations of the polygyne form but absent from monogyne populations, in contrast to laboratory data showing that similar frequencies of queens producing such males occur in the two types of populations. Our results show that mature monogyne colonies with adopted queens rear diploid males in the laboratory, so it is unlikely that the absence of these males from monogyne colonies in the field is due to discrimination against them by monogyne workers. On the other hand, incipient monogyne colonies that produce diploid males exhibit significantly higher mortality and significantly slower rates of growth (Figs. 1–3) than colonies producing workers only. These results suggest that the observed distribution of male diploidy in S. invicta can be explained by differential mortality of diploid male producing colonies of the two forms, with such colonies of the monogyne form experiencing 100% mortality early in development. The mortality differences due to this factor are shown to be related to the different social structures and modes of colony founding characterizing the two forms.     

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.     

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     

Multiple paternity and colony homeostasis in<Emphasis Type="Italic"> Lasius niger</Emphasis> ants

Multiple mating by social insect queens is a common phenomenon despite likely imposing substantial costs on queens. Mating with several males could be adaptive if a more genetically diverse worker force is better able to always handle any task sufficiently well, leading to a higher colony homeostasis. If multiple-paternity colonies are more homeostatic, then I propose that they may constitute less stressful rearing environments for developing sexuals. The effective stress levels experienced by developing males and queens may, however, also depend on colony productivity and sex-ratio preferences. I tested these hypotheses in the ant Lasius niger by examining whether the fluctuating asymmetry, means and coefficients of variation of a set of phenotypic traits in males and new queens co-varied with the effective number of patrilines per colony, colony productivity or sex ratio.Little support was found that the level of intra-colonial genetic diversity affects the variation of phenotype in sexuals. In 1 out of 2 years, however, females from colonies with high effective patriline numbers were heavier relative to their head width than were females from colonies with few patrilines. Support was found for the hypothesis that colonies with more resources may invest more in individual sexuals, and tendencies suggested that sexuals may receive better treatment when they belong to the majority sex of their colony.Communicated by J. Heinze     

Pheromonal and behavioral queen control over the production of gynes in the Argentine ant Iridomyrmex humilis (Mayr)

Summary Both field observations and laboratory experiments have suggested that queens of I. humilis inhibit the production of new queens (gynes). Using small colony fragments, laboratory experiments were conducted to determine the means by which this inhibition is achieved. The addition of queen corpses to queenless fragments effectively inhibited the production of gynes, suggesting that a queen inhibitory primer pheromone is involved. This inhibitory influence was removed when corpses were washed in pentane, lending further support to the pheromonal hypothesis. Adult gynes (winged virgin queens) were not inhibitory, whereas young dealated mated queens of the same age were, suggesting that only inseminated queens produce the pheromone. Daily addition of eggs to queenless units did not appear to have a strong inhibitory influence, indicating that the lower worker/larva ratios associated with the presence of an egg-laying queen in such colony fragments does not greatly influence the production of sexuals. Pheromonal inhibition of gyne development appears to be achieved mainly by preventing the sexualization of bipotent female larvae, probably by affecting the brood-rearing behavior of workers. In addition, queens may also cause the execution of female larvae after they have become sexualized. In nearly all cases, the addition of a living queen to previously queenless units containing gyne larvae caused workers to execute one or more of these larvae within 24 h. In some cases queens were also seen attacking gyne larvae. The addition of queen corpses resulted in the execution of gyne larvae, suggesting that a queen pheromone mediates, at least in part, this execution behavior of workers. These results show that I. humilis queens exert control over the production of gynes in two ways: (1) by preventing the sexualization of female larvae and (2) by killing female larvae after they have become sexualized. A queen primer pheromone appears to be involved in both processes. Queen behavior also plays a role, at least in the execution of gyne larvae. This queen control over the production of gynes, probably mostly pheromonal, appears to operate strongly in the field where gynes are produced only in spring just after a sharp drop in the inhibitory queen influence due to the massive execution of queens by the workers. Offprint requests to: E.L. Vargo at his present address     

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     

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.     

Queen life-span and total reproductive success are positively associated in the ant Cardiocondyla cf. kagutsuchi

While reproduction and longevity are negatively correlated in a large number of animals, this fundamental trade-off appears to be absent from social insect queens. Here, we examine the distribution of the life-spans of queens and their mates and the age-trajectory of reproduction in experimental colonies of the Pacific tramp ant Cardiocondyla cf. kagutsuchi. Queens of Cardiocondyla are generally short-lived (0.5–2 years), which allows determining their lifetime reproductive success and establishing mortality tables. We show that the queen’s total number of sexual offspring is positively associated with its life-span. Regular counts of eggs, larvae, and pupae suggest that more fecund queens live longer than less fecund queens and that an early onset of sexual production does not negatively affect the queen’s life-span. This corroborates the view that the costs of reproduction are not born by the queen itself. The number of eggs present in colonies increased with queen’s age until shortly before death, indicating negligible reproductive senescence. Sex ratios were strongly affected by the origin of queens but neither by colony size nor total productivity. Several queens produced only males late in their lives, suggesting the occurrence of sperm depletion. Male life-span was not correlated with any other studied trait. Our study shows that the ant genus Cardiocondyla provides a suitable model system to investigate the evolution of life-span in social insect queens and males.     

The independent origin of a queen number bottleneck that promotes cooperation in the African swarm-founding wasp, Polybioides tabidus

When cooperation is based on shared genetic interests, as in most social insect colonies, mechanisms which increase the genetic similarity of group members may help to maintain sociality. Such mechanisms can be especially important in colonies with many queens because within-colony relatedness drops quickly as queen number increases. Using microsatellite markers, we examined the Old World, multiple-queen, swarm-founding wasp Polybioides tabidus which belongs to the ropalidiine tribe, and found that relatedness among the workers was four times higher than what would be expected based on queen number alone. Relatedness was elevated by a pattern of queen production known as cyclical oligogyny, under which, queen number varies, and daughter queens are produced only after the number of old queens has reduced to one or a very few. As a result, the queens are highly related, often as full sisters, elevating relatedness among their progeny, the workers. This pattern of queen production is driven by collective worker control of the sex ratios. Workers are three times more highly related to females than to males in colonies with a single queen while they are more equally related to males and females in colonies with more queens. As a result of this difference, workers will prefer to produce new queens in colonies with a single queen and males in colonies with many queens. Cyclical oligogyny has also evolved independently in another group of swarm-founding wasps, the Neotropical epiponine wasps, suggesting that collective worker control of sex ratios is widespread in polistine wasps. Received: 22 May 2000 / Revised: 24 August 2000 / Accepted: 4 September 2000     

Lack of detectable nepotism in multiple-queen colonies of the fire ant Solenopsis invicta (Hymenoptera: Formicidae)

Multiple-queen (polygyne) colonies of the introduced fire ant Solenopsis invicta present a paradox for kin selection theory. Egg-laying queens within these societies are, on average, unrelated to one another, and the numbers of queens per colony are high, so that workers appear to raise new sexuals that are no more closely related to them than are random individuals in the population. This paradox could be resolved if workers discriminate between related and unrelated nestmate sexuals in important fitness-related contexts. This study examines the possibility of such nepotism using methods that combine the following features: (1) multiple relevant behavioral assays, (2) colonies with an unmanipulated family structure, (3) multiple genetic markers with no known phenotypic effects, and (4) a statistical technique for distinguishing between nepotism and potentially confounding phenomena. We estimated relatedness between interactants in polygyne S. invicta colonies in two situations, workers tending egg-laying queens and workers feeding maturing winged queens. In neither case did we detect a significant positive value of relatedness that would implicate nepotism. We argue that the non-nepotistic strategies displayed by these ants reflect historical selection pressures experienced by native populations, in which nestmate queens are highly related to one another. The markedly different genetic structure in native populations may favor the operation of stronger higher-level selection that effectively opposes weaker individual-level selection for nepotistic interactions within nests. Received: 28 June 1996 / Accepted after revision: 6 October 1996     

Conflicts,social economics and life history strategies in ants

Summary This paper presents a life history model for a perennial social insect colony. The model's purpose is to explore the evolutionary consequences, in terms of fitness of different parties within the colony, of alternative life history strategies. The model has been specifically developed for colonies of the slave-making ant, Harpagoxenus sublaevis, which has reproductive workers organized in dominance orders. It incorporates empirically obtained parameters, and uses computer algorithms based on numerical optimization to determine the optimum policy for a colony queen in allocating resources between workers, queens, and males. Variants of the model also consider alternative situations in which either (1) orphaned workers do not slave-raid, or (2) workers are sterile. The results correspond closely to data on colony growth and reproductive allocation obtained from the field. They suggest that a colony queen would suffer reduced fitness in the two theoretical cases as compared to the real situation. Reproduction by orphaned workers posthumously enhances the colony queen's fitness because a queen with sterile workers cannot produce enough extra sexuals in her lifetime to balance her loss in grandson production. The results also suggest that the division of labour between slave-raiding and nonraiding workers observed in H. sublaevis colonies can be explained as an outcome of worker-worker reproductive conflict: reproductively-inhibited subordinate workers can increase their inclusive fitness by slaveraiding for dominant, nonraiding egg-layers. These findings emphasize the evolutionary importance of the orphanage period and of intracolony conflict in monogynous social insect colonies.     

Sex allocation ratios in the facultatively polygynous ant, Leptothorax acervorum

We investigated sex allocation in a central European population of the facultatively polygynous ant Leptothorax acervorum. The population-wide sex ratio was found to be quite balanced, with a proportional investment in female sexuals of 0.49. Sex allocation varied considerably between colonies, resulting in split sex ratios. The productivity of colonies was negatively correlated with queen number and positively with colony size. In contrast, the sex ratio (proportional investment in female sexuals) was neither correlated with queen number, colony size, nor total sexual production, but with worker relatedness. The uncoupling of the genetic colony structure and queen number presumably results from frequent queen turnover and colony splitting.     

Experimental conversion of colony social organization by manipulation of worker genotype composition in fire ants (<Emphasis Type="Italic">Solenopsis invicta</Emphasis>)

Previous studies have shown that colony social organization in Solenopsis invicta is under strong genetic control. Colonies containing some proportion of workers with the Bb or bb genotypes at the gene Gp-9 display polygyne social organization (multiple reproductive queens per colony), whereas colonies with only BB workers express monogyne organization (single reproductive queen per colony). The hypothesis that the presence of workers bearing the b allele confers the polygyne social phenotype on a colony leads to the prediction that social organization can be manipulated by experimentally altering frequencies of adult workers bearing this allele. We did this by replacing queens in colonies of each social form with single queens of the alternate form, which differ in Gp-9 genotype. As worker Gp-9 genotype compositions changed, experimental colonies switched to the alternate social organization. These switches occurred when frequencies of workers with the b allele passed an identifiable threshold, such that colonies with fewer than 5% such workers behaved like monogyne colonies and those with more than 10% behaved like polygyne colonies. Our data thus confirm the prediction that colony social organization in this ant can be altered by manipulating adult worker genotype compositions, and thereby support the hypothesis that the expression of polygyny requires the presence of adult workers bearing the b allele at Gp-9.     

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     

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     

Comparative study of genetic and social structure in two forms of the fire ant Solenopsis invicta (Hymenoptera: Formicidae)

Summary The genetic and social structures of polygyne and monogyne forms of the fire ant, Solenopsis invicta, are investigated in a comparative manner using allozyme data from two polymorphic loci. Foundress queens of the monogyne form are signly inseminated and appear to produce all males present in the colony during the major summer mating flights. The average regression coefficient of relatedness (b) among female nestmates of the monogyne form is 0.714 (Fig. 2), statistically indistinguishable from the pedigree coefficient of relatedness (G) of 0.75. We suggest that the evolution of obligate worker sterility in Solenopsis is associated with this high relatedness between workers and the queens they rear. Functional queens in polygyne nests also are singly inseminated and are no more closely related to nestmate queens than to other queens (within-nest b=0). Within-nest relatedness of workers in the polygyne population is similarly low (Fig. 2). Both the monogyne and polygyne populations from northcentral Georgia are in Hardy-Weinberg equilibrium at both allozyme loci and we found no evidence of significant population subdividion or inbreeding in the polygyne population. These results do not support the view that kin selection has promoted the evolution of polygyny in North American S. invicta. Rather, mutualism appears to be the most likely selective factor mediating queen associations inthis ant.     

Effects of foundress number on brood raids and queen survival in the fire ant Solenopsis invicta

In several ant species, colonies are founded by small groups of queens (pleometrosis), which coexist until the first workers eclose, after which all but one queen is killed. It has been hypothesized that, by producing a larger cohort of workers, cooperating queens may increase colony success during brood raids, a form of competition in which brood and workers from losing nests are absorbed into winning colonies. To test whether this benefit is sufficient to favor pleometrosis, newly mated queens of the fire ant Solenopsis invicta were assembled in groups of one, two, three, or four, reared in the laboratory until the first workers eclosed, then planted in the field in replicated assemblages. The proportion of colonies engaging in brood raids increased with average foundress number per nest and with colony density but was unaffected by variance in foundress number among interacting colonies. Within mixed assemblages of single-queen and multiple-queen colonies, queen number had no effect on the likelihood of engaging in raids or the probability of nest survival through the brood raiding period. However, following nearly 30% of raids, queens moved to new nests and displaced the resident queens. When queen relocation and subsequent mortality were accounted for, it was found that the survival of queens from four-queen groups was substantially higher than that of solitary queens. By contrast, the survival of queens from two-queen colonies was no greater than that of solitary queens. These results show that the competitive advantages of multiple-queen colonies are sufficient to counterbalance the increased mortality of queens within groups only when the number of foundresses is greater than two and when colonies are founded at high density. When colonies lose brood raids, the workers appear to abandon their mothers to join surviving colonies. However, in laboratory experiments, queens attempting to enter foreign nests were significantly more likely to displace the resident queen if their own daughters were present within the invaded nest. Thus, workers may be able to bias the probability that their mother rejoins them and displaces competing queens.     

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