Sex allocation conflict in ants: when the queen rules

Insect societies are paramount examples of cooperation, yet they also harbor internal conflicts whose resolution depends on the power of the opponents. The male-haploid, female-diploid sex-determining system of ants causes workers to be more related to sisters than to brothers, whereas queens are equally related to daughters and sons. Workers should thus allocate more resources to females than to males, while queens should favor an equal investment in each sex. Female-biased sex allocation and manipulation of the sex ratio during brood development suggest that workers prevail in many ant species. Here, we show that queens of Formica selysi strongly influenced colony sex allocation by biasing the sex ratio of their eggs. Most colonies specialized in the production of a single sex. Queens in female-specialist colonies laid a high proportion of diploid eggs, whereas queens in male-specialist colonies laid almost exclusively haploid eggs, which constrains worker manipulation. However, the change in sex ratio between the egg and pupae stages suggests that workers eliminated some male brood, and the population sex-investment ratio was between the queens' and workers' equilibria. Altogether, these data provide evidence for an ongoing conflict between queens and workers, with a prominent influence of queens as a result of their control of egg sex ratio.     

Reproductive Caste Performs Intranidal Tasks Instead of Workers in the Ant Mystrium oberthueri

Division of labour improves the efficiency of animal societies. Efficiency is further improved in many social insects where morphologically specialized adults perform different tasks. In ants, the wingless worker caste performs non‐reproductive activities and sometimes exhibits multiple phenotypes when requirements between brood care and expert foraging diverge. Mystrium rogeri from Madagascar is a specialist predator on large centipedes, and the worker caste is highly polymorphic in size. In contrast, M. oberthueri has only large workers. The replacement of the queen caste by wingless intermorphs much smaller than workers explains this evolutionary shift in M. oberthueri. Many intermorphs occur in each colony but only a few mate and reproduce. In order to determine their contribution to non‐reproductive tasks, we performed multivariate analyses on behavioural data recorded by scan sampling from four M. oberthueri colonies in the laboratory. In unmanipulated colonies, workers and intermorphs exhibited two distinct behavioural profiles. Workers focused on guarding and foraging, while intermorphs performed brood care and nest cleaning, regardless of whether they reproduced or not. This pattern of polyethism where the reproductive caste completely takes charge of some non‐reproductive tasks is novel, as confirmed by our observations of one colony of M. rogeri where non‐reproductive tasks were restricted to workers, as in most ants. When isolated from one another, M. oberthueri workers and intermorphs developed less distinctive behavioural patterns. Some workers cared for the brood, but the intermorphs could not hunt because of their small mandibles. Such plasticity in polyethism at the colony level confers the ability to react to unexpected changes, including variable proportions of workers and intermorphs.     

Fungal hyphae as a source of nutrients for the leaf-cutting ant Atta sexdens

Abstract. The mutualistic fungus of leaf-cutting ants produces both ordinary hyphae and specialized ant rewards: the staphylae. Workers of Atta sexdens (L.) lived longer on diets which included nutritive staphylae than on those which provided only hyphae. However, hyphae were a better diet than sucrose solution or water alone. Small workers lived longer than large workers when receiving fungus garden, whether or not staphylae were available, probably because they are specialized to care for the garden and can exploit it. Large workers lived longer than small ones when only water was available. This may be a consequence of scale, larger workers containing proportionately more nutritional reserves in their bodies.
When starved workers were exposed to fungus for 3h, they gained weight. This weight gain represented the amount of food ingested during the test period. Workers gained 7.3 times more weight on natural fungus garden bearing staphylae than on garden with only hyphae. They gained only 1.5 times as much weight when the fungus was artificially cultured on agar. The results suggested that workers found hyphae attractive, but difficult to obtain in natural fungus gardens.
Material from hyphae and staphylae stained with a fluorescent dye was detected in worker crop contents using fluorescence microscopy. The crop contents of workers fed on staphylae fluoresced 1.14 times more than those of workers fed on hyphae.
Hyphae may provide a small source of food for workers, and the fungus as a whole may provide up to 9.0% of the respiratory energy requirements of workers, the remainder presumably being provided by plant sap.     

Specialization Does Not Predict Individual Efficiency in an Ant

The ecological success of social insects is often attributed to an increase in efficiency achieved through division of labor between workers in a colony. Much research has therefore focused on the mechanism by which a division of labor is implemented, i.e., on how tasks are allocated to workers. However, the important assumption that specialists are indeed more efficient at their work than generalist individuals—the “Jack-of-all-trades is master of none” hypothesis—has rarely been tested. Here, I quantify worker efficiency, measured as work completed per time, in four different tasks in the ant Temnothorax albipennis: honey and protein foraging, collection of nest-building material, and brood transports in a colony emigration. I show that individual efficiency is not predicted by how specialized workers were on the respective task. Worker efficiency is also not consistently predicted by that worker''s overall activity or delay to begin the task. Even when only the worker''s rank relative to nestmates in the same colony was used, specialization did not predict efficiency in three out of the four tasks, and more specialized workers actually performed worse than others in the fourth task (collection of sand grains). I also show that the above relationships, as well as median individual efficiency, do not change with colony size. My results demonstrate that in an ant species without morphologically differentiated worker castes, workers may nevertheless differ in their ability to perform different tasks. Surprisingly, this variation is not utilized by the colony—worker allocation to tasks is unrelated to their ability to perform them. What, then, are the adaptive benefits of behavioral specialization, and why do workers choose tasks without regard for whether they can perform them well? We are still far from an understanding of the adaptive benefits of division of labor in social insects.     

Sources of spores for the possible horizontal transmission of Thelohania solenopsae (Microspora: Thelohaniidae) in the red imported fire ants, Solenopsis invicta

We screened adult and larval secretions and midden piles for the presence of Thelohania solenopsae spores to decipher potential sources for the horizontal transmission of the pathogen in fire ants. Hemolymph samples from both adult and larvae were also screened to rule out hemolymph contamination of samples. In adults, Thelohania spores were found in the crop and the fecal fluids, although only free spores were found in the fecal fluids of adults. In fourth instar larvae, both free and octospores were seen in midgut and the meconium samples. All of the midden pile samples had T. solenopsae spores of both types. Based on these results, we theorize that the pathogen may be horizontally transmitted within a colony by the removal and sharing of meconium of prepupating fourth instar larvae by adult workers and by the adult fecal droppings, and intercolonially by contamination of midden piles or brood raiding.     

Brood adoption in the leaf-cutting ant <Emphasis Type="Italic">Acromyrmex echinatior</Emphasis>: adaptation or recognition noise?

The ability to discriminate between nestmates and non-nestmates is an important prerequisite for the evolution of eusociality. Indeed, social insect workers are typically able to discriminate between nestmate and non-nestmate workers. Adult non-nestmate workers are readily detected and rejected from the colony. Whether social insects can discriminate between nestmate and non-nestmate brood, however, is less clear. Here, we show that workers of the leaf-cutting ant Acromyrmex echinatior discriminate between nestmate and non-nestmate brood, and among brood of different stages. Initially, non-nestmate brood is attacked, but it is adopted after a delay. Adoption could occur due to inefficiency of the recognition system, or it could be adaptive because it is an inexpensive way to increase the workforce. Our results suggest that brood adoption may occur accidentally. We also report how workers replace fungal hyphae on the brood’s surface before transporting the brood into their fungus garden.     

Age polyethism in the leaf-cutting ant Acromyrmex subterraneus brunneus Forel, 1911 (Hym., Formicidae)

Abstract  This study describes and quantifies the behavioural acts of two laboratory colonies of Acromyrmex subterraneus brunneus by investigating worker age polyethism. Twenty-nine behavioural acts were recorded during the 19-week observation period. Young individuals performed tasks inside the nest related to brood care and care for the fungus garden, whereas older individuals performed activities outside the nest such as foraging and activities in the waste chamber. The average longevity (±SD) was 108.21 ± 3.30, 109.15 ± 1.92 and 122.71 ± 1.55 days for large, medium and small workers, respectively. The small-sized workers presented a higher probability of reaching older age than large- and medium-sized workers. This study describes task switching according to age polyethism and the relationship of physical and temporal subcastes.     

Cuticular lipids correlate with age and insemination status in queen honeybees

Eusocial insects exhibit reproductive division of labour, in which one or a few queens perform almost all of the reproduction, while the workers are largely sterile and assist in rearing their siblings. Consequently, many of the colony’s tasks (e.g. nest construction and brood rearing) should be modulated depending on whether the queen is fertile. Here, we tested whether queens’ cuticular lipids could provide reliable signals of fertility in the honeybee Apis mellifera, as they do in other social Hymenoptera. Specifically, we tested whether cuticular lipids differ between virgin queens of different ages, and between queens exposed to different artificial insemination treatments being semen (sperm and seminal fluid), seminal fluid only or saline control. Using gas chromatography–mass spectrometry, we found 27 lipids: 21 different hydrocarbons, namely alkanes, alkenes and dienes, as well as six wax esters. The lipid profile changed dramatically in the first 10 days after eclosion, and there were differences in lipid composition between virgin and artificially inseminated queens. Insemination with semen, seminal fluid or saline did not result in distinct chemical profiles. Our findings indicate that the physical stimulus of insemination was responsible for the observed changes in the cuticular profile in honeybee queens. Our results demonstrate that cuticular lipid profiles encode information on queen age, fertility and mating status, which could in principle be utilised by workers and rival queens.     

Division of labor inPonera pennsylvannica (Formicidae: Ponerinae)

Summary We examined division of labor and colony demography in the antPonera pennsylvannica. Observation of three colonies with individually marked workers revealed a high degree of interindividual behavioral variation and a rough but consistent division of labor between brood tenders and foragers. This division was present both in colonies consisting entirely of workers produced in the previous summer and in colonies containing freshly eclosed ants. Two colonies showed typical age-based polyethism, with young ants focusing on brood care and overwintered ants on foraging. No such age basis was detected in the third colony. This difference may relate to variability in brood production schedules. Colonies showing temporal polyethism had two peaks of brood production and thus had relatively large brood populations when the first young workers eclosed, while the third colony had only one peak and little brood for the young workers to tend. Even if young ants have a lower threshold for brood care, it may have been concealed in the latter situation. Demographic data indicate that natural colonies produce one brood per year and that workers typically eclose into colonies with relatively low brood care demands. This suggests that overwintered workers do most of a colony's work and that the division of labor among overwintered ants is the more important one under natural conditions. The basis of this division is as yet unknown. These results also suggest that small colony size, univoltine brood schedule and a close association between foraging and brood care do not preclude division of labor among specialized castes, as has been suggested for another ponerine species (Traniello 1978).     

Hygropreference and brood care in the honeybee (Apis mellifera)

Terrestrial organisms need to limit evaporation from their bodies in order to maintain a homeostatic water balance. Owing to a large surface to volume ratio, arthropods are particularly susceptible to desiccation and have evolved behavioural and physiological mechanisms to conserve water. In social insects, water balance is also affected by the interactions between nestmates and by the architecture of the nest. For honeybees, humidity is particularly important for the brood because it affects the hatching success of eggs and because, unlike ants, honeybees cannot relocate their brood to parts of the nest with more favourable humidity. To advance the understanding of the water economy in honeybee nests, we investigated whether workers exhibit a hygropreference when exposed to a gradient of 24-90% relative humidity (RH) and whether the expression of this preference and their behaviour is affected by the presence of brood. The results show that young honeybee workers in the absence of brood exhibit a weak hygropreference for approximately 75% RH. When brood is present the expression of this preference is further weakened, suggesting that workers tend to the brood by distributing evenly in the gradient. In addition, fanning behaviour is shown to be triggered by an increase in humidity above the preferred level but not by a decrease. Our results suggest that humidity in honeybee colonies is actively controlled by workers.     

Worker laying in leafcutter ant Acromyrmex subterraneus brunneus （Formicidae, Attini）

We studied the process of offspring production in queenless colonies of Acromyrmex subterraneus brunneus, and particularly evaluated the ovary development of workers as a function of their age. For this, subcolonies were set up and evaluated at different periods of isolation from the queen （2, 4 and 6 months）, besides individually labeled age groups. The subcolonies were assessed according to offspring production and ovaries containing oocytes or not. The evaluations showed worker oviposition and development of males originating from worker-laid eggs. At 2 months＇ absence of the queen, eggs and larvae were found, with eggs in a higher proportion than larvae. After 4 months, the proportion of eggs had reduced while larvae had increased, and a pupa was found in one subcolony. At 6 months, besides a higher share of larvae, one pupa and one adult male were found. Dissection of workers revealed ovaries containing oocytes during the periods of evaluation. Only a group of medium-sized and large workers, 23.3%, 20.9% and 37.5% of the population from each period assessed in queenless subcolonies respectively, presented developed oocytes in the ovary. The same was observed in colonies with a queen, with 17.6%, 19.6% and 7.8% of the group of dissected workers from each time period, respectively. With respect to worker age, we observed by dissection of the ovary, that the greatest percentage of individuals with ovarioles containing oocytes occurred at 45 days （6 weeks） up to 90 days （12 weeks）. These results probably are associated with the workers reproduction and the laying of trophic and reproductive eggs in colonies with and without a queen; these eggs have distinct functions in each situation.     

Direct assessment of queen quality and lack of worker suppression in a paper wasp

Assessing a conspecific's potential is often crucial to increase one's fitness, e.g. in female choice, contests with rivals or reproductive conflicts in animal societies. In the latter, helpers benefit from accurately assessing the fertility of the breeder as an indication of inclusive fitness. There is evidence that this can be achieved using chemical correlates of reproductive activity. Here, we show that queen quality can be assessed by directly monitoring her reproductive output. In the paper wasp Polistes dominulus, we mimicked a decrease in queen fertility by regularly removing brood. This triggered ovarian development and egg-laying by many workers, which strongly suggests that brood abundance is a reliable cue of queen quality. Brood abundance can be monitored when workers perform regular brood care in small size societies where each brood element is kept in a separate cell. Our results also show that although the queen was not manipulated, and thus remained healthy and fully fertile, she did not control worker egg-laying. Nevertheless, when workers laid eggs, the queen secured a near reproductive monopoly by selectively destroying these eggs, a mechanism known as 'queen policing'. By contrast, workers destroyed comparatively few queen-laid eggs, but did destroy each other's eggs.     

Regulation of Brood-Care Behavior in the Dimorphic Castes of the Ant Pheidole morrisi (Hymenoptera: Formicidae): Effects of Caste Ratio, Colony Size, and Colony Needs

This study examines the social stimuli that regulate brood-care behavior in the two physical castes of the ant Pheidole morrisi. By increasing the proportion of major workers in a colony, brood-care behavior could be induced in individuals of this caste, which do not normally care for brood. Minor and major worker brood-care rates increased with the proportion of majors in the colony or as the need for brood care increased. Changes in colony size did not significantly affect the rate of either minor or major brood care. Major workers began to care for brood when the caste ratio reached a critical threshold but did not appear to be as efficient at rearing larvae as minor workers, which normally perform this task. After a perturbation that skewed the caste ratio toward majors, minor workers increased their rate of brood care, apparently to compensate for the inefficiency of brood care provided by majors. These results suggest that caste plasticity involves a social mechanism of ldquo;coupled compensation that maintains the efficiency of labor by ensuring that tasks are completed.     

Sex-ratio regulation: the economics of fratricide in ants

In many insect societies, workers can manipulate the reproductive output of their colony by killing kin of lesser value to them. For instance, workers of the mound-building Formica exsecta eliminate male brood in colonies headed by a single-mated queen. By combining an inclusive fitness model and empirical data, we investigated the selective causes underlying these fratricides. Our model examines until which threshold stage in male brood development do the workers benefit from eliminating males to rear extra females instead. We then determined the minimal developmental stage reached by male larvae before elimination in F. exsecta field colonies. Surprisingly, many male larvae were kept until they were close to pupation, and only then eliminated. According to our model, part of the eliminated males were so large that workers would not benefit from replacing them with new females. Moreover, males were eliminated late in the season, so that new females could no longer be initiated, because matings take place synchronously during a short period. Together, these results indicate that workers did not replace male brood with new females, but rather reduced total brood size during late larval development. Male destruction was probably triggered by resource limitation, and the timing of brood elimination suggests that males may have been fed to females when these start to grow exponentially during the final larval stage. Hence, the evolution of fratricides in ants is best explained by a combination of ecological, demographic and genetic parameters.     

Occurrence and morphometry of the hypopharyngeal glands inScaptotrigona postica Lat. (Hymenoptera, Apidae, Meliponinae)

As compared toApis mellifera where only workers have hypopharyngeal glands, inScaptotrigona postica, these glands occur in workers, queens and males. They are composed of two long axial ducts with many unicellular secretory alveoli interconnected by secretory canaliculi. The axial ducts are longer in males than in workers, but the alveolar areas of queens and males are generally smaller. In workers the alveoli have their greatest size in the nurses or middle-aged individuals while in queens and males they are larger in newly emerged individuals. The results indicate that the glands in workers may produce food for the brood as inA. mellifera, since they are well developed in the nurse workers. However, the function of the glands in queens and males remains to be clarified since these individuals have no part in brood care.     

Effect of queen excluders on ovary activation in workers of the Eastern honeybee Apis cerana

Honeybee workers generally refrain from personal reproduction when a queen is present. Workers discern the presence and fecundity of the queen via volatile pheromones that permeate throughout the colony. Pheromones are emitted both by the queen herself and by the brood that she produces. If pheromone production is disrupted, some workers initiate egg laying. The Eastern honeybee Apis cerana is unusual in that workers have high levels of ovary activation even in the presence of a queen. To investigate the effect of disruption to pheromone dispersal, we fitted three A. cerana colonies with vertical queen excluders, thus splitting the colonies into a half containing a queen and a half without a queen. We regularly sampled adult workers from both sides of the excluder for 3 weeks. We also sampled workers from three control colonies that did not contain excluders. We found a significant increase in worker ovary activation 3 days after addition of excluders, suggesting that the reduced dispersal of pheromones allowed some workers to become reproductively active. Workers attempted to rear queen cells on the queenless halves of all three colonies. Queen-rearing ceased on day 9, at which time no queen-laid brood remained on the queenless halves of the colonies. Ovary activation rates continued to climb until day 9 and then gradually began to decline. With the exception of one egg, we did not observe worker-laid brood on the queenless side of the colonies, suggesting that workers continued policing eggs laid by workers. We conclude that if the distribution of brood pheromone is impeded, workers prepare to supersede their queen, accompanied by high levels of worker ovary activation. However, because workers continue to police each other, high ovary activation does not result in worker-produced drones.     

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.     

Nest Enlargement in Leaf-Cutting Ants: Relocated Brood and Fungus Trigger the Excavation of New Chambers

During colony growth, leaf-cutting ants enlarge their nests by excavating tunnels and chambers housing their fungus gardens and brood. Workers are expected to excavate new nest chambers at locations across the soil profile that offer suitable environmental conditions for brood and fungus rearing. It is an open question whether new chambers are excavated in advance, or will emerge around brood or fungus initially relocated to a suitable site in a previously-excavated tunnel. In the laboratory, we investigated the mechanisms underlying the excavation of new nest chambers in the leaf-cutting ant Acromyrmex lundi. Specifically, we asked whether workers relocate brood and fungus to suitable nest locations, and to what extent the relocated items trigger the excavation of a nest chamber and influence its shape. When brood and fungus were exposed to unfavorable environmental conditions, either low temperatures or low humidity, both were relocated, but ants clearly preferred to relocate the brood first. Workers relocated fungus to places containing brood, demonstrating that subsequent fungus relocation spatially follows the brood deposition. In addition, more ants aggregated at sites containing brood. When presented with a choice between two otherwise identical digging sites, but one containing brood, ants'' excavation activity was higher at this site, and the shape of the excavated cavity was more rounded and chamber-like. The presence of fungus also led to the excavation of rounder shapes, with higher excavation activity at the site that also contained brood. We argue that during colony growth, workers preferentially relocate brood to suitable locations along a tunnel, and that relocated brood spatially guides fungus relocation and leads to increased digging activity around them. We suggest that nest chambers are not excavated in advance, but emerge through a self-organized process resulting from the aggregation of workers and their density-dependent digging behavior around the relocated brood and fungus.     

The ant Lasius niger shows no relationship between task efficiency and body size variation among workers

In ants, workers of different sizes may perform various tasks, even in so-called monomorphic species with relatively low body size variation. However, it is unclear if the body size diversity of monomorphic workers correlates with task efficiency, especially in stressful contingencies. Here we tested if the body size variation of workers corresponds with its efficiency in transferring pupae. Transferring brood is a pre-set behavioral response to stress, e.g. suboptimal temperature. Here we applied a laboratory experiment simulating nest damage. The study was performed on the common garden ant (Lasius niger (Linnaeus, 1758)) – a species with no distinct worker subcastes. The efficiency of workers was measured as the latency of transferring pupae from a lit part of the experimental colony to a darkened part, while the body size diversity was expressed as the within-colony coefficient of variation in head width. We did not find any significant correlation between efficiency and body size variation. Summarizing the existing studies and the present results, we propose the hypothesis that the body size diversity of L. niger may have implications for workers’ division of labor but not for their task efficiency in a stressful contingency.     

Social Context and Reproductive Potential Affect Worker Reproductive Decisions in a Eusocial Insect

Context-dependent decision-making conditions individual plasticity and is an integrant part of alternative reproductive strategies. In eusocial Hymenoptera (ants, bees and wasps), the discovery of worker reproductive parasitism recently challenged the view of workers as a homogeneous collective entity and stressed the need to consider them as autonomous units capable of elaborate choices which influence their fitness returns. The reproductive decisions of individual workers thus need to be investigated and taken into account to understand the regulation of reproduction in insect societies. However, we know virtually nothing about the proximate mechanisms at the basis of worker reproductive decisions. Here, we test the hypothesis that the capacity of workers to reproduce in foreign colonies lies in their ability to react differently according to the colonial context and whether this reaction is influenced by a particular internal state. Using the bumble bee Bombus terrestris, we show that workers exhibit an extremely high reproductive plasticity which is conditioned by the social context they experience. Fertile workers reintroduced into their mother colony reverted to sterility, as expected. On the contrary, a high level of ovary activity persisted in fertile workers introduced into a foreign nest, and this despite more frequent direct contacts with the queen and the brood than control workers. Foreign workers'' reproductive decisions were not affected by the resident queen, their level of fertility being similar whether or not the queen was removed from the host colony. Workers'' physiological state at the time of introduction is also of crucial importance, since infertile workers failed to develop a reproductive phenotype in a foreign nest. Therefore, both internal and environmental factors appear to condition individual reproductive strategies in this species, suggesting that more complex decision-making mechanisms are involved in the regulation of worker reproduction than previously thought.