The Effect of Queen and Worker Adoption on Weaver Ant (Oecophylla smaragdina F.) Queen Fecundity

Incipient ant colonies are often under fierce competition, making fast growth crucial for survival. To increase production, colonies can adopt multiple queens (pleometrosis), fuse with other colonies or rob brood from neighboring colonies. However, different adoption strategies might have different impacts such as future queen fecundity or future colony size. O. smaragdina queen production was measured in incipient colonies with 2, 3 or 4 founding queens, following the transplantation of 0, 30 or 60 pupae from a donor colony. Pupae developed into mature workers, resulting in increased worker/queen ratios in pupae transplanted treatments and leading to increases in the per capita queen production. Conversely, more queens did not induce increased per capita fecundity. Thus, brood robbing added individuals to the worker force and increased future production of resident queens, whereas queen adoption increased the colony’s future production, but not the production of individual queens.     

Larger laboratory colonies consume proportionally less energy and have lower per capita brood production in Temnothorax ants

Colony size can affect individual- and colony-level behavioral and physiological traits in social insects. Changes in behavior and physiology in response to colony growth and development can affect productivity and fitness. Here, we used respirometry to study the relationship between colony size and colony energy consumption in Temnothorax rugatulus ants. In addition, we examined the relationship between colony size and worker productivity measured as per capita brood production. We found that colony metabolic rate scales with colony size to the 0.78 power and the number of brood scales with the number of workers to the 0.49 power. These regression analyses reveal that larger ant colonies use proportionally less energy and produce fewer brood per worker. Our findings provide new information on the relationships between colony size and energetic efficiency and productivity in a model ant genus. We discuss the potential mechanisms giving rise to allometric scaling of metabolic rate in ant colonies and the influence of colony size on energy consumption and productivity in general.     

Energetics of newly-mated queens and colony founding in the fungus-gardening ants Cyphomyrmex rimosus and Trachymyrmex septentrionalis (Hymenoptera: Formicidae)

Abstract.  The energetics of colony founding is investigated in the fungus gardening ants (Attini) Trachymyrmex septentrionalis and Cyphomyrmex rimosus . Similar to most ants, inseminated queens of these two species found nests independently unaccompanied by workers (haplometrosis). Whereas most ant founding queens seal themselves in a chamber and do not feed when producing a brood entirely from metabolic stores (claustral founding), the majority of fungus gardening ants must forage during the founding phase (semiclaustral founding). Laboratory-reared T. septentrionalis individuals comprise 84 dealate females collected after mating flights in June 2004. Twenty are immediately killed to obtain values for queen traits and another 20 after worker emergence for queen, fungus garden and worker traits. Cyphomyrmex rimosus comprise 22 dealate females collected in June 2005; ten of which are immediately killed and similarly prepared. Newly-mated T. septentrionalis queens have 25% of their dry weight as fat; whereas newly-mated C. rimosus queens contain 11% fat. These amounts are 50–75% less than most independently founding ant species. Trachymyrmex septentrionalis queens lose merely 5% of their energetic content during colony founding, whereas the total energetic content of their brood is more than three-fold the amount lost by the queen. Incipient T. septentrionalis colonies produce approximately half as much ant biomass per gram of fungus garden as do mature colonies. Similar to most ants, T. septentrionalis produces minim workers that are approximately 40% lighter than workers from mature colonies. Regardless of their size, T. septentrionalis workers contain much lower fat than do workers of claustral species. These data indicate that fungus gardening is adaptive because colonies can produce much cheaper offspring, making colony investment much lower.     

Fitness and the level of homozygosity in a social insect

To date very few studies have addressed the effects of inbreeding in social Hymenoptera, perhaps because the costs of inbreeding are generally considered marginal owing to male haploidy whereby recessive deleterious alleles are strongly exposed to selection in males. Here, we present one of the first studies on the effects of queen and worker homozygosity on colony performance. In a wild population of the ant Formica exsecta, the relative investment of single‐queen colonies in sexual production decreased with increased worker homozygosity. This may either stem from increased homozygosity decreasing the likelihood of diploid brood to develop into queens or a lower efficiency of more homozygous workers at feeding larvae and thus a lower proportion of the female brood developing into queens. There was also a significant negative association between colony age and the level of queen but not worker homozygosity. This association may stem from inbreeding affecting queen lifespan and/or their fecundity, and thus colony survival. However, there was no association between queen homozygosity and colony size, suggesting that inbreeding affects colony survival as a result of inbred queens having a shorter lifespan rather than a lower fecundity. Finally, there was no significant association between either worker or queen homozygosity and the probability of successful colony founding, colony size and colony productivity, the three other traits studied. Overall, these results indicate that inbreeding depression may have important effects on colony fitness by affecting both the parental (queen) and offspring (worker) generations cohabiting within an ant colony.     

REPRODUCTIVE SKEW AND SPLIT SEX RATIOS IN SOCIAL HYMENOPTERA

Abstract I present a model demonstrating that, in social Hymenoptera, split sex allocation can influence the evolution of reproductive partitioning (skew). In a facultatively polygynous population (with one to several queens per colony), workers vary in their relative relatedness to females (relatedness asymmetry). Split sex‐ratio theory predicts that workers in monogynous (single‐queen) colonies should concentrate on female production, as their relatedness asymmetry is relatively high, whereas workers in the polygynous colonies should concentrate on male production, as their relatedness asymmetry is relatively low. By contrast, queens in all colonies value males more highly per capita than they value females, because the worker‐controlled population sex ratio is too female‐biased from the queens' standpoint. Consider a polygynous colony in a facultatively polygynous population of perennial, social Hymenoptera with split sex ratios. A mutant queen achieving reproductive monopoly would gain from increasing her share of offspring but, because the workers would assess her colony as monogynous, would lose from the workers rearing a greater proportion of less‐valuable females from the colony's brood. This sets an upper limit on skew. Therefore, in social Hymenoptera, skew evolution is potentially affected by queen‐worker conflict over sex allocation.     

The influence of social structure on brood survival and development in a socially polymorphic ant: insights from a cross‐fostering experiment

Animal societies vary in the number of breeders per group, which affects many socially and ecologically relevant traits. In several social insect species, including our study species Formica selysi, the presence of either one or multiple reproducing females per colony is generally associated with differences in a suite of traits such as the body size of individuals. However, the proximate mechanisms and ontogenetic processes generating such differences between social structures are poorly known. Here, we cross‐fostered eggs originating from single‐queen (= monogynous) or multiple‐queen (= polygynous) colonies into experimental groups of workers from each social structure to investigate whether differences in offspring survival, development time and body size are shaped by the genotype and/or prefoster maternal effects present in the eggs, or by the social origin of the rearing workers. Eggs produced by polygynous queens were more likely to survive to adulthood than eggs from monogynous queens, regardless of the social origin of the rearing workers. However, brood from monogynous queens grew faster than brood from polygynous queens. The social origin of the rearing workers influenced the probability of brood survival, with workers from monogynous colonies rearing more brood to adulthood than workers from polygynous colonies. The social origin of eggs or rearing workers had no significant effect on the head size of the resulting workers in our standardized laboratory conditions. Overall, the social backgrounds of the parents and of the rearing workers appear to shape distinct survival and developmental traits of ant brood.     

Development rate and brood production in haplo- and pleometrotic colonies of <Emphasis Type="Italic">Oecophylla smaragdina</Emphasis>

Pleometrosis (colony founding by multiple queens) may improve life history characteristics that are important for early colony survival. When queens unite their initial brood, the number of workers present when incipient colonies open may be higher than for single queen colonies. Further, the time until the first worker emerges may shorten. For territorial species and species that rob brood from neighbouring colonies, a faster production of more workers may improve the chance of surviving intraspecific competition. In this study, the time from the nuptial flight to the emergence of the first worker in incipient Oecophylla smaragdina Fabr. colonies founded by 1–5 queens was compared and the production of brood during the first 68 days after the nuptial flight was assessed. Compared to haplometrotic colonies, pleometrotic colonies produced 3.2 times more workers, their first worker emerged on average 4.3 days (8%) earlier and the queen’s per capita egg production almost doubled. Further, colony production was positively, correlated with the number of founding queens and time to worker emergence was negatively correlated. These results indicate that pleometrotic O. smaragdina colo-nies are competitively superior to haplometrotic colonies as they produce more workers faster and shorten the claustral phase, leading to increased queen fecundity.     

Proximate Determinants of Reproductive Skew in Polygyne Colonies of the Ant Formica fusca

Understanding the determinants of reproductive skew (the partitioning of reproduction among co‐breeding individuals) is one of the major questions in social evolution. In ants, multiple‐queen nests are common and reproductive skew among queens has been shown to vary tremendously both within and between species. Proximate determinants of skew may be related to both queen and worker behaviour. Queens may attempt to change their reproductive share through dominance interactions, egg eating and by changing individual fecundity. Conversely, workers are in a position to regulate the reproductive output of queens when rearing the brood. This paper investigates queen behaviour at the onset of egg laying and the effect of queen fecundity and worker behaviour on brood development and reproductive shares of multiple queens in the ant Formica fusca. The study was conducted in two‐queen laboratory colonies where the queens produced only worker offspring. The results show that in this species reproductive apportionment among queens is not based on dominance behaviour and aggression, but rather on differences in queen fecundity. We also show that, although the queen fecundity at the onset of brood rearing is a good indicator of her final reproductive output, changes in brood composition occur during brood development. Our results highlight the importance of queen fecundity as a major determinant of her reproductive success. They furthermore suggest that in highly derived polygyne species, such as the Formica ants, direct interactions as a means for gaining reproductive dominance have lost their importance.     

Body size and sperm quality in queen‐ And worker‐produced ant males

Workers of many species of social Hymenoptera have functional ovaries and are capable of laying haploid, unfertilized eggs, at least in the absence of a queen. Except for honeybees, it remains largely unknown whether worker‐produced males have the same quality as queen‐produced males and whether workers benefit in direct fitness by producing their sons. Previous studies in the monogynous ant Temnothorax crassispinus revealed that a high proportion of males in natural and laboratory colonies are worker offspring. Here, we compare longevity, body size, sperm length and sperm viability between queen‐ and worker‐produced males. We either split queenright colonies into queenright and queenless halves or removed the queen from a fraction of the queenright colonies and then examined the newly produced males. Male quality traits varied considerably among colonies but differed only slightly between queen‐ and worker‐produced males. Worker‐produced males outnumbered queen‐produced males and also had a longer lifespan, but under certain rearing conditions sperm from queen‐produced males had a higher viability.     

Effects of colony-level attributes on larval feeding in the fire ant, Solenopsis invicta

Summary: Do colony attributes modulate individual behavior? The effects of colony size and worker:brood ratio on the rate of worker-to-larva trophallaxis in the fire ant, Solenopsis invicta, were investigated. Neither colony size ranging from 100 to 10,000 nor worker:brood ratio ranging from 1:1 to 16:1 affected the density of workers on the brood pile, nor the rate or duration of worker-to-larva trophallaxis. The demands of hungry larvae were met even in groups as small as 100 workers in worker:brood ratios as small as 1. Only when the worker:brood ratio was less than 1, were larvae tended or fed at reduced rates. Under natural conditions, this occurs only in incipient colonies. Otherwise, in post-incipient colonies, the flow of food to larvae was unmodified by colony attributes. The implications of this finding are two-fold: First, it reinforces previous research demonstrating that social feeding in the fire ant emerges from localized interactions rather than mass communication. Second, it highlights the resiliency of this weedy species. Hypothetically, colonies drastically reduced by catastrophic events such as flooding should still be able to produce sexuals.     

Polyandry and colony genetic structure in the primitive ant Nothomyrmecia macrops

The Australian endemic ant Nothomyrmecia macrops is considered one of the most ‘primitive’ among living ants. We investigated the genetic structure of colonies to determine queen mating frequencies and nestmate relatedness. An average of 18.8 individuals from each of 32 colonies, and sperm extracted from 34 foraging queens, were genotyped using five highly variable microsatellite markers. Queens were typically singly (65%) or doubly mated (30%), but triple mating (5%) also occurred. The mean effective number of male mates for queens was 1.37. No relationship between colony size and queen mate number was found. Nestmate workers were related by b=0.61 ± 0.03, significantly above the threshold under Hamilton’s rule over which, all else being equal, altruistic behaviour persists, but queens and their mates were unrelated. In 25% of the colonies we detected a few workers that could not have been produced by the resident queen, although there was no evidence for worker reproduction. Polyandry is for the first time recorded in a species with very small mature colonies, which is inconsistent with the sperm‐limitation hypothesis for the mediation of polyandry levels. Facultative polyandry is therefore not confined to the highly advanced ant genera, but may have arisen at an early stage in ant social evolution.     

Semi-claustral colony founding in the seed-harvester ant Pogonomyrmex californicus: a comparative analysis of colony founding strategies

The evolution of queens that rear their first brood solely using body reserves, i.e. fully claustral, is viewed as a major advance for higher ants because it eliminated the need for queens to leave the nest to forage. In an apparently unusual secondary modification, the seed-harvester ant Pogonomyrmex californicus displays obligate queen foraging, i.e. queens must forage to garner the resources necessary to survive and successfully rear their first brood. I examined the potential benefits of queen foraging by comparing ecological and physiological traits between P. californicus and several congeners in which the queen can rear brood using only body reserves. The primary advantage of foraging appears to lie in providing the queens of P. californicus with the energy to raise significantly more brood than possible by congeners that use only body reserves; the workers reared in the first brood were also heavier in mass than that predicted by their head width. Other correlates of queen foraging in P. californicus relative to tested congeners included a significantly lower total fat content for alate queens, a small queen body size, and a low queen to worker body mass ratio. Queens also forage in several other well-studied species of Pogonomyrmex, suggesting the possibility that queen foraging may be more common than previously thought in higher ants.     

Caste regulation in the pharaoh's ant Monomorium pharaonis: recognition and cannibalism of sexual brood by workers

Abstract In colonies of the pharaoh's ant Monomorium pharaonis (L), new males and queens can be produced at any time by the removal of the inhibitory effect of the presence of existing fertile queens. When queens are absent, workers rear sexual larvae and will also accept sexual larvae introduced from other nests. However, in the presence of fertile queens workers cannibalize sexual brood, and will not accept male or queen larvae or pre-pupae from other nests, although worker brood is always accepted and reared. Worker larvae are covered in bifurcated hairs, whereas sexual larvae are essentially hairless. Workers may use these morphological cues to distinguish between sexual and worker brood stages.     

Bait distribution among multiple colonies of Pharaoh ants (hymenoptera: Formicidae)

Pharaoh ant, Monomorium pharaonis (L.), infestations often consist of several colonies located at different nest sites. To achieve control, it is desirable to suppress or eliminate the populations of a majority of these colonies. We compared the trophallactic distribution and efficacy of two ant baits, with different modes of action, among groups of four colonies of Pharaoh ants. Baits contained either the metabolic-inhibiting active ingredient hydramethylnon or the insect growth regulator (IGR) pyriproxyfen. Within 3 wk, the hydramethylnon bait reduced worker and brood populations by at least 80%, and queen reductions ranged between 73 and 100%, when nests were in proximity (within 132 cm) to the bait source. However, these nest sites were reoccupied by ants from other colonies located further from the bait source. The pyriproxyfen bait was distributed more thoroughly to all nest locations with worker populations gradually declining by 73% at all nest sites after 8 wk. Average queen reductions ranged from 31 to 49% for all nest sites throughout the study. Even though some queens survived, brood reductions were rapid in the pyriproxyfen treatment, with reductions of 95% at all locations by week 3. Unlike the metabolic inhibitor, the IGR did not kill adult worker ants quickly, thus, more surviving worker ants were available to distribute the bait to all colonies located at different nest sites. Thus, from a single bait source, the slow-acting bait toxicant provided gradual, but long-term control, whereas the fast-acting bait toxicant provided rapid, localized control for a shorter duration.     

Colony fusion and worker reproduction after queen loss in army ants

Theory predicts that altruism is only evolutionarily stable if it is preferentially directed towards relatives, so that any such behaviour towards seemingly unrelated individuals requires scrutiny. Queenless army ant colonies, which have anecdotally been reported to fuse with queenright foreign colonies, are such an enigmatic case. Here we combine experimental queen removal with population genetics and cuticular chemistry analyses to show that colonies of the African army ant Dorylus molestus frequently merge with neighbouring colonies after queen loss. Merging colonies often have no direct co-ancestry, but are on average probably distantly related because of overall population viscosity. The alternative of male production by orphaned workers appears to be so inefficient that residual inclusive fitness of orphaned workers might be maximized by indiscriminately merging with neighbouring colonies to increase their reproductive success. We show that worker chemical recognition profiles remain similar after queen loss, but rapidly change into a mixed colony Gestalt odour after fusion, consistent with indiscriminate acceptance of alien workers that are no longer aggressive. We hypothesize that colony fusion after queen loss might be more widespread, especially in spatially structured populations of social insects where worker reproduction is not profitable.     

Effects of social conditions on Juvenile Hormone mediated reproductive development in Bombus terrestris workers

Abstract. During the annual life cycle of the bumble bee Bombus terrestris (L.) colony, there is a stage characterized by worker reproduction in the presence of the queen. It has been proposed that this is a result of a decrease in queen inhibition. This hypothesis was examined by studying the effects of queens taken from colonies at different stages of development on several aspects of worker physiology and behaviour: rates of Juvenile Hormone (JH) release in vitro , ovary development, and behaviour associated with reproduction. After optimizing and validating the radiochemical assay for JH release for bumble bee workers, we found that queenless workers had significantly more developed ovaries and higher rates of release of JH than did queenright workers, confirming and extending previous findings that suggest that bumblebee ovarian development is under JH control. Mated queens, separated from their colony and brood, can have the same inhibitory effect on the reproductive development of callow workers. In contrast, workers confined with virgin queens or in queenless groups demonstrated a significantly higher rate of release of JH, overt aggression and threatening behaviours. However, there were no differences in rates of release of JH between workers confined in groups in the laboratory with queens taken from colonies either before or after the onset of worker reproduction. Furthermore, overt aggression and threatening behaviours were similar and low in both types of groups. These results gave no support to the hypothesis that a decrease in queen inhibition is associated with the onset of worker reproduction. We also show that young workers reared in colonies either before or after worker reproduction occurs, or in queenless colonies, all demonstrated similar, low rates of release of JH. These results suggest that older workers may inhibit the corpora allata of younger workers in queenless colonies.     

Gene expression patterns associated with caste and reproductive status in ants: worker‐specific genes are more derived than queen‐specific ones

Variation in gene expression leads to phenotypic diversity and plays a central role in caste differentiation of eusocial insect species. In social Hymenoptera, females with the same genetic background can develop into queens or workers, which are characterized by divergent morphologies, behaviours and lifespan. Moreover, many social insects exhibit behaviourally distinct worker castes, such as brood‐tenders and foragers. Researchers have just started to explore which genes are differentially expressed to achieve this remarkable phenotypic plasticity. Although the queen is normally the only reproductive individual in the nest, following her removal, young brood‐tending workers often develop ovaries and start to reproduce. Here, we make use of this ability in the ant Temnothorax longispinosus and compare gene expression patterns in the queens and three worker castes along a reproductive gradient. We found the largest expression differences between the queen and the worker castes (~2500 genes) and the smallest differences between infertile brood‐tenders and foragers (~300 genes). The expression profile of fertile workers is more worker‐like, but to a certain extent intermediate between the queen and the infertile worker castes. In contrast to the queen, a high number of differentially expressed genes in the worker castes are of unknown function, pointing to the derived status of hymenopteran workers within insects.     

Ants under crowded conditions consume more energy

Social insects live in colonies consisting of many workers, where worker interactions play an important role in regulating colony activities. Workers interact within the social space of the nest; therefore, constraints on nest space may alter worker behaviour and affect colony activities and energetics. Here we show in the ant Temnothorax rugatulus that changes in nest space have a significant effect on colony energetics. Colonies with restricted nest space showed a 14.2 per cent increase in metabolic rate when compared with the same colonies in large uncrowded nests. Our study highlights the importance of social space and shows that constraints on social space can significantly affect colony behaviour and energy use in ants. We discuss the implications of our findings regarding social insects in general.     

Variation in social structure alters queen body mass and productivity in the desert seed-harvester ant Messor pergandei

Parent–offspring conflict theory predicts conflict between parents and their offspring over per-offspring resource investment. Across the range of the desert seed-harvester ant, Messor pergandei, daughter queens use three different social strategies during colony founding that are expected to alter the optimal level of parental investment. To test whether social strategy variation is associated with shifts in body mass, we surveyed queen live mass over 3 years at 25 sites that spanned the range of behavioral strategies and founding group sizes. To test whether reduction in parental investment into individual offspring negatively impacts their productivity, queens were individually isolated and allowed to produce a single worker cohort under common garden conditions. Queen live mass was highly variable, from 24 mg on average at the site with the lightest queens to 1.5 times that size, 37 mg, at the site with the heaviest queens. As predicted by parent–offspring conflict over investment, solitary colony founding sites contained the heaviest queens, followed by secondary monogyny. Polygynous queens were lightest, with a strong negative relationship between group size and live mass. Reductions in body mass had a negative effect on queen productivity across all queen social types; however, queens from sites where queen–queen aggression is typical were significantly more efficient at brood rearing, resulting in lower mass loss during founding per unit offspring biomass. This may represent an adaptation to queen competition to gain a strength advantage over potential rivals.     

Extraordinary starvation resistance in Temnothorax rugatulus (Hymenoptera, Formicidae) colonies: Demography and adaptive behavior

Summary. Ant colony mortality has not been sufficiently studied, even though it is crucial for understanding social insect population biology and can serve as an important model for general aging and mortality processes. Particularly, studies on proximate mechanisms on mortality and stress resistance of ant colonies are lacking. This study explores the long-term colony starvation resistance of the small myrmecine ant Temnothorax rugatulus. We report extraordinary starvation resistance in the 21 colonies investigated, as most survived the eight months of total starvation. Furthermore, we studied demographic and behavioral changes over the experimental period. Brood decline began first (after two months) and mortality was highest, worker decline was intermediate, and queen mortality started latest and remained lowest. We found brood (its relative change during the first four months and the level of brood relative to colony size) to be the only significant predictor of colony starvation resistance, but not the degree of polygyny. As expected, rates of trophallaxis increased during the starvation period while colony activity bouts occurred more frequently but were much shorter, leading to an overall decrease in activity levels. This study is the first to comprehensively study mechanisms of starvation resistance in ant colonies, linking demography and behavior.Received 10 January 2005; revised 16 February 2005; accepted 23 February 2005.