Distribution and evolution of genetic caste determination in Pogonomyrmex seed-harvester ants

We examined the distribution and ancestral relationships of genetic caste determination (GCD) in 46 populations of the seed-harvester ants Pogonomyrmex barbatus and P. rugosus across the east-to-west range of their distributions. Using a mtDNA sequence and two nuclear markers diagnostic for GCD, we distinguished three classes of population phenotypes: those with GCD, no evidence of GCD, and mixed (both GCD and non-GCD colonies present). The GCD phenotype was geographically widespread across the range of both morphospecies, occurring in 20 of 46 sampled populations. Molecular data suggest three reproductively isolated and cryptic lineages within each morphospecies, and no present hybridization. Mapping the GCD phenotype onto a mtDNA phylogeny indicates that GCD in P. rugosus was acquired from P. barbatus, suggesting that interspecific hybridization may not be the causal agent of GCD, but may simply provide an avenue for GCD to spread from one species (or subspecies) to another. We hypothesize that the origin of GCD involved a genetic mutation with a major effect on caste determination. This mutation generates genetic conflict and results in the partitioning and maintenance of distinct allele (or gene set) combinations that confer differences in developmental caste fate. The outcome is two dependent lineages within each population; inter-lineage matings produce workers, while intra-lineage matings produce reproductives. Both lineages are needed to produce a caste-functional colony, resulting in two reproductively isolated yet interdependent lineages. Pogonomyrmex populations composed of dependent lineages provide a unique opportunity to investigate genetic variation underlying phenotypic plasticity and its impact on the evolution of social structure.     

Genetic caste determination in harvester ants: possible origin and maintenance by cyto-nuclear epistasis

While reproductive caste in eusocial insects is usually determined by environmental factors, in some populations of the harvester ants, Pogonomyrmex barbatus and P. rugosus, caste has been shown to have a strong genetic component. This system of genetic caste determination (GCD) is characterized by between-caste nuclear variation and high levels of mitochondrial haplotype variation between alternative maternal lineages. Two previous genetic models, involving a single nuclear caste-determining locus or interactions between two nuclear loci, respectively, have been proposed to explain the GCD system. We propose a new model based on interactions between nuclear and mitochondrial genes that can better explain the co-maintenance of distinct nuclear and mitochondrial lineages. In our model, females with coevolved cyto-nuclear gene complexes, derived from intra-lineage mating, develop into gynes, while females with disrupted cyto-nuclear complexes, derived from inter-lineage mating, develop into workers. Both haplodiploidy and inbreeding facilitate the buildup of such coevolved cyto-nuclear complexes within lineages. In addition, the opportunity for both intra-lineage and inter-lineage mating in polyandrous populations facilitates the accumulation of gyne-biasing genes. This model may also help to explain the evolution of workerless social, parasites. We discuss similarities of GCD and cytoplasmic male sterility in plants and how worker production of males would affect the stability of GCD. Finally, we propose experiments and observations that might help resolve the origin and maintenance of this unusual system of caste determination.     

Reproductive isolation between Pogonomyrmex rugosus and two lineages with genetic caste determination

Hybrid speciation occurs when combination of two interspecific genomes results in individuals that are of high fitness but reproductively incompatible with the parental species. Although hybrid speciation is a relatively common source of new species in plants, it appears to be a much rarer occurrence in animal taxa. Here we report on reproductive isolation and range overlap between the rough harvester ant Pogonomyrmex rugosus and two lineages with hybrid genotypes (H 1 and H2). Both lineages obligately interbreed and produce genetically distinct queen and worker offspring, a phenomenon referred to as genetic caste determination (GCD). Diploid offspring produced by gametes of the same lineage develop only into queens, whereas diploid offspring derived from gametes of distinct lineages develop into workers. We investigated small-scale patterns of gene flow between the parent and the two H lineages by sampling along an 80-km transect between a pure P. rugosus population and a two-lineage population. Microsatellite and mitochondrial markers both indicated virtually no gene flow between the parent species and either lineage even at sites where parental and H-lineage colonies co-occurred. The geographic ranges of the parental species and the two-lineage population were essentially parapatric, with a surprisingly narrow band of overlap and evidence of spatial structuring even at microgeographic scales within the transition zone. This suggests that ecological competition with the parent species plays a significant role in determining the evolutionary persistence and current distribution of the hybrid lineages and the genetic caste system.     

Behavioral regulation of genetic caste determination in a Pogonomyrmex population with dependent lineages

The fate of a social insect colony is partially determined by its ability to allocate individuals to the caste most appropriate for the requirements for growth, maintenance, and reproduction. In pairs of dependent lineages of Pogonomyrmex barbatus, the allocation of individuals to the queen or worker caste is constrained by genotype, a system known as genetic caste determination (GCD). In mature GCD colonies, interlineage female eggs develop into sterile workers, while intralineage eggs become reproductively capable queens. Although the population-level consequences of this system have been intensively studied, the proximate mechanisms for GCD remain unknown. To elucidate these mechanisms, we brought newly mated queens into the laboratory and allowed them to establish colonies, nearly half of which unexpectedly produced virgin queens only seven months after colony founding. We genotyped eggs, workers, and the virgin queens from these colonies. Our results showed that queens in young colonies produce both interlineage and intralineage eggs, demonstrating that queens of GCD colonies indiscriminately use sperm of at least two lineages to fertilize their eggs. Intralineage eggs were more frequent in colonies producing virgin queens. These findings suggest that intralineage eggs are predetermined to become queens and that workers may cull these eggs when colonies are not producing queens. Virgin queens produced by young GCD colonies were smaller than field-caught virgin queens, and often had developmental problems. Hence, they are probably nonfunctional and represent an intense resource drain for developing colonies, not a contribution to colony fitness.     

Brood production and lineage discrimination in the red harvester ant (Pogonomyrmex barbatus)

In contrast to the system of caste determination in most social insects, reproductive caste determination in some populations of Pogonomyrmex barbatus has a genetic basis. Populations that exhibit genetic caste determination are segregated into two distinct, genetic lineages. Same-lineage matings result in female reproductives, while inter-lineage matings result in workers. To investigate whether founding P. barbatus queens lay eggs of reproductive genotype, and to determine the fate of those eggs, we genotyped eggs, larvae, and pupae produced by naturally inseminated, laboratory-raised queens. We show that founding dependent lineage queens do lay eggs of reproductive genotype, and that the proportion of reproductive genotypes decreases over the course of development from eggs to larvae to pupae. Because queens must mate with a male of each lineage to produce both workers and female reproductives, it would benefit queens to be able to distinguish males of the two lineages. Here we show that P. barbatus males from the two genetic lineages differ in their cuticular hydrocarbon profiles. Queens could use male cuticular hydrocarbons as cues to assess the lineage of males at the mating aggregation, and possibly keep mating until they have mated with males of both lineages.     

Individual responsiveness to shock and colony-level aggression in honey bees: evidence for a genetic component

The phenotype of the social group is related to phenotypes of individuals that form that society. We examined how honey bee colony aggressiveness relates to individual response of male drones and foraging workers. Although the natural focus in colony aggression has been on the worker caste, the sterile females engaged in colony maintenance and defense, males carry the same genes. We measured aggressiveness scores of colonies and examined components of individual aggressive behavior in workers and haploid sons of workers from the same colony. We describe for the first time, that males, although they have no stinger, do bend their abdomen (abdominal flexion) in a posture similar to stinging behavior of workers in response to electric shock. Individual worker sting response and movement rates in response to shock were significantly correlated with colony scores. In the case of drones, sons of workers from the same colonies, abdominal flexion significantly correlated but their movement rates did not correlate with colony aggressiveness. Furthermore, the number of workers responding at increasing levels of voltage exhibits a threshold-like response, whereas the drones respond in increasing proportion to shock. We conclude that there are common and caste-specific components to aggressive behavior in honey bees. We discuss implications of these results on social and behavioral regulation and genetics of aggressive response.     

Ontogeny of nestmate recognition cues in the red carpenter ant (Camponotus floridanus)

Summary A combination of behavioral and chemical analyses was used to investigate the nature of nestmate recognition cues and the effects of worker age and social experience on these cues in the ant Camponotus floridanus. Five categories of workers were tested: foragers, 5-day old and 0-day old callows, 5-day old and 0-day old naive callows. Bioassays consisted of introductions of dead workers from these categories into their own colonies or into an alien colony after the following treatments: 1) killed by freezing, 2) solvent-washed, 3) solvent-washed and coated with a nestmate soak, 4) solvent-washed and coated with a non-nestmate soak. Soaks were obtained from individual ants immersed in hexane and were applied individually to washed workers from the same category. Soaks were analyzed by gas chromatography (GC) and compared by multivariate analyses. Freeze-killed workers from each category elicited more aggressive behavior in the alien colony than in its own. By comparing GC profiles, a worker from any category can be assigned to its colony of origin. Thus all studied worker categories are colony-specific. Solvent-washed ants did not induce more aggressive behaviors in the alien colony than in their own, but they induced significantly less aggressivity in an alien colony than non-treated dead ants from the same category. Washed ants indced more aggressive behaviors when coated with a soak from a different colony as opposed to a soak from the colony in which they were introduced. The combination of behavioral and chemical results lead to the following conclusions: 1) Information contained in soak derived from workers was sufficient to allow nestmate recognition. 2) Nestmate recognition cues, and consequently the recognition response displayed to their bearer, change with age. 3) Social experience is necessary to develop or acquire a colony-specific label. The role of age and social experience on nestmate recognition in social Hymenoptera is discussed.     

Colony fusion causes within-colony variation in a parthenogenetic ant

The evolutionary stability of cooperation and altruism in colonies of social insects requires that nestmates be to some extent related. An efficient system of discrimination against non-nestmates protects the nest against unrelated conspecifics, which might exploit or parasitize the colony. The co-occurrence of unrelated individuals in mature colonies therefore is a rare event that deserves more attention. Here, we report on the relatively common incidence of colony fusion in the ant Platythyrea punctata. Workers of this ant can produce genetically identical female offspring from unfertilized eggs through thelytokous parthenogenesis. Consequently, the majority of colonies has a “clonal structure” and consists of individuals with identical multilocus genotypes. Nevertheless, field observations indicate that a surprisingly large percentage of colonies contain workers belonging to two or more different genetic lineages. Much of this genetic heterogeneity is incompatible with eventual recombination or mutation events, but instead appears to result from colony fusion or the adoption of unrelated individuals. Indeed, colonies of P. punctata from the Dominican Republic and Barbados readily merged in the laboratory and, after elimination of one of the two reproductive workers, formed stable, genetically heterogeneous colonies. We discuss the possible causes and benefits of colony fusion in natural populations.     

Genetic influence on caste in the ant Camponotus consobrinus

Genetic influences on polyethism within social insect colonies are well known, suggesting that the determination of caste (soldiers and minor workers) may also be genetically mediated. The Australian sugar ant Camponotus consobrinus is suitable for such a study, having soldiers and minor workers that follow a complex allometry. Further, although most C. consobrinus colonies are monogynous, 13 of 42 surveyed using microsatellites were found to be polygynous. Thus, although a minority of colonies were polygynous, the great majority of queens live in polygynous colonies. From the 29 monogynous colonies studied, we inferred that the queens are monandrous. Ants from four polygynous colonies were assigned to families on the basis of microsatellite genotypes, after measurements had been taken of head width and scape length. These measurements reflect a complex allometry interpretable as soldier and minor worker growth curves with a large changeover zone. Genetic influence on caste determination was examined by testing for differences between families within colonies in the distribution of scape lengths, residuals from the overall colony allometric curve, and proportions of soldiers and minor workers (as determined by head width falling above or below the inflection point of the overall colony allometric curve). Families in all four colonies differed significantly in caste proportions and in head-width distributions, and three of the four colonies showed significant differences between families in residuals from the overall colony growth curve. Nested ANOVAs using head widths and scape-length residuals showed that when the effect of family is removed, intercolony differences in allometry are negligible. This evidence indicates genetic rather than environmental causes for the observed differences between families. We speculate that this variation may reflect some selective advantage to within-colony heterogeneity between families or that selective differences are few between a wide array of family growth patterns. Received: 16 June 1999 / Received after revision: 13 September 1999 / Accepted: 25 September 1999     

Foraging and spatiotemporal territories in the honey ant Myrmecocystus mimicus wheeler (Hymenoptera: Formicidae)

Summary The honey ant Myrmecocystus mimicus is a scavenger, forages extensively on termites, collects floral nectar, and tends homoptera. Individual foragers of M. mimicus usually disperse in all directions when leaving the nest, but there are also groups of foragers that tend to swarm out of the nest primarily in one direction. Such massive departues are usually at irregular intervals, which may last several hours. The results of field and laboratory experiments suggest that these swarms of foragers are organized by a group recruitment process, during which recruiting scout ants lay chemical orientation trails with hindgut contents and simultaneously stimulate nestmates with a motor display and secretions from the poison gland. Usually these columns travel considerable distances (4–48 m) away from the nest, frequently interfering with the foraging activity of conspecific neighboring colonies.To prevent a neighboring colony from access to temporal food sources or to defend spatiotemporal borders, opposing colonies engage in elaborate display tournaments. Although hundreds of ants are often involved during these tournaments almost no physical fights occur. Instead, individual ants confront each other in highly sterotyped aggressive displays, during which they walk on stilt legs while raising the gaster and head. Some of the ants even seem to inflate their gasters so that the tergites are raised and the whole gaster appears to be larger. In addition, ants involved in tournament activities are on average larger than foragers.The dynamics of the tournament interactions were observed in several colonies over several weeks-mapping each day the locations of the tournaments, the major directions of worker routes away from the nest, and recording the general foraging activities of the colonies. The results indicate that a kind of dominance order can occur among neighboring colonies. On the other hand, often no aggressive interactions among neighboring colonies can be observed, even though the colonies are actively foraging. In those cases the masses of foragers of each colony depart in one major direction that does not bring them into conflict with the masses of foragers of a neighboring colony. This stability, however, can be disturbed by offering a new rich food source to be exploited by two neighboring colonies. This invariably leads to tournament interactions.When a colony is considerably stronger than the other, i.e., with a much larger worker force, the tournaments end quickly and the weaker colony is raided. The foreign workers invade the nest, the queen of the resident colony is killed or dirven off, while the larvae, pupae, callow workers, and honey pot workers are carried or dragged to the nest of the raiders. From these and other observations we conclude that young M. mimicus queens are unlikely to succeed in founding a colony within approximately 3 m of a mature M. mimicus colony because they are discovered and killed, or driven off by workers of the resident colony. Within approximately 3–15 m queens are more likely to start colonies, but these incipient groups run a high risk of being raided and exterminated by the mature colony.Although populations of M. mimicus and M. depilis tend to replace each other, there are areas where both species overlap marginally. Foraging areas and foraging habitats of both species also overlap broadly, but we never observed tournament interactions between M. mimicus and M. depilis.The adaptive significance of the spatiotemporal territories in M. mimicus is discussed.     

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.     

Convergent development of ecological,genetic, and morphological traits in native supercolonies of the red ant Myrmica rubra

Ant supercolonies (large networks of interconnected nests) represent the most extreme form of multi-queen breeding (polygyny) and have been found across ant lineages, usually in specific long-term stable populations. Many studies on the genetic population structure and demography of ant supercolonies have been done in recent decades, but they have lacked multicolonial control patches with separated colonies headed by a single or few queens so the origin of the supercolonial trait syndrome has remained enigmatic. Here, we set out to compare sympatric supercolonial and multicolonial patches in two natural Danish populations of the common red ant Myrmica rubra. We used DNA microsatellites to reconstruct genetic colony/population structure and obtained morphological and density measurements to estimate life history and ecology covariates. We found that supercolonies in both populations completely dominated their patches whereas colonies in multicolonial patches coexisted with other ant species. Supercolony patches had very low genetic differentiation between nests, negligible relatedness within nests, and lower inbreeding than multicolonial patches, but there were no significant morphological differences. One population also had nests that approached true outbred monogyny with larger workers and males but smaller queens than in the two other social nest types. Our results suggest that once smaller colonies start to adopt additional queens, they also gain the potential to ultimately become supercolonial when the habitat allows rapid expansion through nest budding. This is relevant for understanding obligate polygyny in ants and for appreciating how and why introduced North American populations of M. rubra have recently become invasive.     

Genetic influence on caste determination underlying the asexual queen succession system in a termite

The question of how reproductives and sterile workers differentiate within eusocial groups has long been a core issue in the study of social insects. Recent studies have shown that not only environmental factors but also genetic factors affect caste differentiation. In the termite Reticulitermes speratus, queens produce their replacements (neotenics) asexually but use normal sexual reproduction to produce other colony members. Here, we demonstrate a genetic influence on caste determination underlying the asexual queen succession system in this termite species. Thelytoky in termites is accomplished by automixis with terminal fusion, yielding almost completely homozygous offspring; thus, parthenogenetically and sexually produced offspring profoundly differ in heterozygosity. An analysis of the relationship between the reproductive dominance of female neotenics obtained from experimentally orphaned colonies and their genotypes at five microsatellite loci showed that homozygosity at two loci influenced the developmental priority and/or reproductive quality of neotenics. These results suggest the existence of a multi-locus system affecting the queen fecundity and explain why parthenogens have genetic priority to become neotenics in this termite species.     

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     

Transitioning from unstable to stable colony growth in the desert leafcutter ant Acromyrmex versicolor

Like organisms, cohesive social groups such as insect colonies grow from a few individuals to large and complex integrated systems. Growth is driven by the interplay between intrinsic growth rates and environmental factors, particularly nutritional input. Ecologically inspired population growth models assume that this relationship remains constant until maturity, but more recent models suggest that it should be less stable at small colony sizes. To test this empirically, we monitored worker population growth and fungal development in the desert leafcutter ant, Acromyrmex versicolor, over the first 6 months of colony development. As a multitrophic, symbiotic system, leafcutter colonies must balance efforts to manage both fungus production and the growth of the ants consuming it. Both ants and fungus populations grew exponentially, but the shape of this relationship transitioned at a size threshold of 89?±?9 workers. Above this size, colony mortality plummeted and colonies shifted from hypometric to hypermetric growth, with a distinct stabilization of the relationship between the worker population and fungus. Our findings suggest that developing colonies undergo key changes in organizational structure and stability as they grow, with a resulting positive transition in efficiency and robustness.     

Relatedness,recognition errors,and colony fusion in the termite <Emphasis Type="Italic">Nasutitermes corniger</Emphasis>

Loss of aggression between social groups can have far-reaching effects on the structure of societies and populations. We tested whether variation in the genetic structure of colonies of the termite Nasutitermes corniger affects the probability of aggression toward non-nestmates and the ability of unrelated colonies to fuse. We determined the genotypes of workers and soldiers from 120 colonies at seven polymorphic microsatellite loci. Twenty-seven colonies contained offspring of multiple founding queens or kings, yielding an average within-colony relatedness of 0.33. Genotypes in the remaining 93 colonies were consistent with reproduction by a single queen and king or their progeny, with an average within-colony relatedness of 0.51. In standardized assays, the probability of aggression between workers and soldiers from different colonies was an increasing function of within-colony relatedness. The probability of aggression was not affected significantly by the degree of relatedness between colonies, which was near zero in all cases, or by whether the colonies were neighbors. To test whether these assays of aggression predict the potential for colony fusion in the field, we transplanted selected nests to new locations. Workers and soldiers from colonies that were mutually tolerant in laboratory assays joined their nests without fighting, but workers and soldiers that were mutually aggressive in the assays initiated massive battles. These results suggest that the presence of multiple unrelated queens or kings promotes recognition errors, which can lead to the formation of more complex colony structures.     

Division of labor and slave raid initiation in slave-making ants

In social insect societies, division of labor, i.e., workers of a colony specializing in different tasks, is thought to improve colony performance. Workers of social parasitic slave-making ants focus on a single task, searching for and raiding host colonies to replenish their slave workforce. However, in the North American slavemaker Protomognathus americanus, some workers do not partake in raids but remain inside the colony. We analyzed raid participation, fertility, and cuticular hydrocarbon profiles of slavemaker workers and slaves to understand these behavioral differences and the regulation of division of labor in slavemaker colonies. Raid observations showed that some workers were repeatedly involved in raiding activities (exterior workers), whereas others stayed inside the nest (interior workers). Exterior workers were always infertile, while half of the interior workers were fertile. Analysis of cuticular hydrocarbons demonstrated differences between the groups. We also detected chemical differences between interior and exterior slaves, indicating an influence of the individuals’ tasks on their cuticular profiles. Task- and fertility-related profiles may allow selective nestmate recruiting. Division of labor should also adapt to varying conditions. Since slave raids are dangerous, they should only be initiated when the colony needs additional slaves. Exclusively fed by their slaves, slavemaker workers could determine this need via their nutritional status. In an experiment with various feeding regimes, colonies subjected to a lower food provisioning rate showed increased proportions of slavemaker workers searching for host colonies. Division of labor in slave-making ants, therefore, might be flexible and can change depending on the colonies’ needs.     

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.     

The red and the black: habituation and the dear-enemy phenomenon in two desert Pheidole ants

Many species of territorial animals are more aggressive toward strangers than neighbors, a pattern of aggression referred to as the ’dear-enemy phenomenon.’ In many cases, the mechanism by which neighbors are discriminated from strangers and the function of neighbor-stranger discrimination remain controversial. We investigated the spatial patterns of inter-colony aggression within and between two Pheidole species of seed-harvesting ants in the Mojave Desert of California by quantifying aggression between colonies in standardized staged encounters. We also tested whether the level of fighting between workers of two colonies is affected by previous exposure to each other. We show that neighbors (i.e., colonies less that 2.6 m away) of either species are treated less aggressively than more distant colonies and that habituation may be a mechanism by which this discrimination is achieved. The variation in aggression among spatially distant colonies also suggests that additional genetic or environmental factors are involved in recognition. The function of the dear-enemy phenomenon in these ant species may be related to the greater risk to the resources of a colony presented by strange workers than workers from a neighboring colony. Received: 18 November 1999 / Received in revised form: 3 April 2000 / Accepted: 3 May 2000     

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