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.     

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.     

Polymorphism and division of labour in a socially complex ant: neuromodulation of aggression in the Australian weaver ant,Oecophylla smaragdina

Complex social structure in eusocial insects can involve worker morphological and behavioural differentiation. Neuroanatomical variation may underscore worker division of labour, but the regulatory mechanisms of size-based task specialization in polymorphic species are unknown. The Australian weaver ant, Oecophylla smaragdina, exhibits worker polyphenism: larger major workers aggressively defend arboreal territories, whereas smaller minors nurse brood. Here, we demonstrate that octopamine (OA) modulates worker size-related aggression in O. smaragdina. We found that the brains of majors had significantly higher titres of OA than those of minors and that OA was positively and specifically correlated with the frequency of aggressive responses to non-nestmates, a key component of territorial defence. Pharmacological manipulations that effectively switched OA action in major and minor worker brains reversed levels of aggression characteristic of each worker size class. Results suggest that altering OA action is sufficient to produce differences in aggression characteristic of size-related social roles. Neuromodulators therefore may generate variation in responsiveness to task-related stimuli associated with worker size differentiation and collateral behavioural specializations, a significant component of division of labour in complex social systems.     

Behavioral responses of acacia ants correlate with age and location on the host plant

In social insects, worker specialization in location-related tasks could occur if they return to the same location over time. Location and task fidelity was tested in the acacia ants Pseudomyrmex spinicola, which nest inside the swollen spines of the tree, and all workers enter the spines at night and during rain. Workers were marked and followed at three locations: on the leaves, tree trunk, and ground near the plant. Behavioral tests were performed, testing the reactions of marked ants toward the food used to feed the larvae (Beltian Bodies, “BB”), and to brood outside the spines. Marked ants and ants of known age were tested for responses to disturbance of the spines. Ants were more likely to occur in the location where they were originally marked. Trunk-marked ants discarded the BB when it had a foliole fragment attached to it, while leaf-marked ants carried it to the spine. Trunk-marked ants left larvae and exited from disturbed spines more frequently than other ants. Leaf-marked ants carried larvae and pupae more often than trunk-marked ants. Spine-marked ants left pupae more often than trunk- or leaf-marked ants. When considering age, older ants reacted aggressively when threatened, whereas younger ants protected the brood. However, younger ants reacted more aggressively when older ants were absent, and older ants were more aggressive in the presence of larvae. In sum, the spatial segregation of the ants coincided with behavioral differences, and different behavioral responses are related to the age of the ant.     

Vitellogenin Underwent Subfunctionalization to Acquire Caste and Behavioral Specific Expression in the Harvester Ant Pogonomyrmex barbatus

The reproductive ground plan hypothesis (RGPH) proposes that the physiological pathways regulating reproduction were co-opted to regulate worker division of labor. Support for this hypothesis in honeybees is provided by studies demonstrating that the reproductive potential of workers, assessed by the levels of vitellogenin (Vg), is linked to task performance. Interestingly, contrary to honeybees that have a single Vg ortholog and potentially fertile nurses, the genome of the harvester ant Pogonomyrmex barbatus harbors two Vg genes (Pb_Vg1 and Pb_Vg2) and nurses produce infertile trophic eggs. P. barbatus, thus, provides a unique model to investigate whether Vg duplication in ants was followed by subfunctionalization to acquire reproductive and non-reproductive functions and whether Vg reproductive function was co-opted to regulate behavior in sterile workers. To investigate these questions, we compared the expression patterns of P. barbatus Vg genes and analyzed the phylogenetic relationships and molecular evolution of Vg genes in ants. qRT-PCRs revealed that Pb_Vg1 is more highly expressed in queens compared to workers and in nurses compared to foragers. By contrast, the level of expression of Pb_Vg2 was higher in foragers than in nurses and queens. Phylogenetic analyses show that a first duplication of the ancestral Vg gene occurred after the divergence between the poneroid and formicoid clades and subsequent duplications occurred in the lineages leading to Solenopsis invicta, Linepithema humile and Acromyrmex echinatior. The initial duplication resulted in two Vg gene subfamilies preferentially expressed in queens and nurses (subfamily A) or in foraging workers (subfamily B). Finally, molecular evolution analyses show that the subfamily A experienced positive selection, while the subfamily B showed overall relaxation of purifying selection. Our results suggest that in P. barbatus the Vg gene underwent subfunctionalization after duplication to acquire caste- and behavior- specific expression associated with reproductive and non-reproductive functions, supporting the validity of the RGPH in ants.     

Worker brain development and colony organization in ants: Does division of labor influence neuroplasticity?

Brain compartment size allometries may adaptively reflect cognitive needs associated with behavioral development and ecology. Ants provide an informative system to study the relationship of neural architecture and development because worker tasks and sensory inputs may change with age. Additionally, tasks may be divided among morphologically and behaviorally differentiated worker groups (subcastes), reducing repertoire size through specialization and aligning brain structure with task‐specific cognitive requirements. We hypothesized that division of labor may decrease developmental neuroplasticity in workers due to the apparently limited behavioral flexibility associated with task specialization. To test this hypothesis, we compared macroscopic and cellular neuroanatomy in two ant sister clades with striking contrasts in worker morphological differentiation and colony‐level social organization: Oecophylla smaragdina , a socially complex species with large colonies and behaviorally distinct dimorphic workers, and Formica subsericea , a socially basic species with small colonies containing monomorphic workers. We quantified volumes of functionally distinct brain compartments in newly eclosed and mature workers and measured the effects of visual experience on synaptic complex (microglomeruli) organization in the mushroom bodies—regions of higher‐order sensory integration—to determine the extent of experience‐dependent neuroplasticity. We demonstrate that, contrary to our hypothesis, O. smaragdina workers have significant age‐related volume increases and synaptic reorganization in the mushroom bodies, whereas F. subsericea workers have reduced age‐related neuroplasticity. We also found no visual experience‐dependent synaptic reorganization in either species. Our findings thus suggest that changes in the mushroom body with age are associated with division of labor, and therefore social complexity, in ants. © 2017 Wiley Periodicals, Inc. Develop Neurobiol 77: 1072–1085, 2017     

Foraging for Work and Age-Based Polyethism: The Roles of Age and Previous Experience on Task Choice in Ants

In social insects, colonies commonly show temporal polyethism in worker behavior, such that a worker follows a predictable pattern of changes between tasks as it ages. This pattern usually leads from workers first doing a safe task like brood care, to ending their lives doing the most dangerous tasks like foraging. Two mechanisms could potentially underlie this pattern: (1) age‐based task allocation, where the aging process itself predisposes workers to switch to more dangerous tasks; and (2) foraging for work, where ants switch to tasks that need doing from tasks which have too many associated workers. We tested the relative influence of these mechanisms by establishing nests of Camponotus floridanus with predetermined combinations of workers of known age and previous task specialization. The results supported both mechanisms. Nests composed of entirely brood‐tending workers had the oldest workers preferentially switching to foraging. However, in nests initially composed entirely of foragers, the final distribution of tenders and foragers was not different from random task‐switching and therefore supportive of foraging for work. Thus, it appears that in C. floridanus there is directionality to the mechanisms of task allocation. Switching to more dangerous tasks is age‐influenced, but switching to less dangerous tasks is age‐independent. The results also suggest that older workers are more flexible in their task choice behavior. Younger workers are more biased towards choosing within‐nest tasks. Finally, there are effects of previous experience that tend to keep ants in familiar tasks. Task allocation based on several mechanisms may balance between: (1) concentrating the most worn workers into the most dangerous tasks; (2) increasing task performance levels; and (3) maintaining behavioral flexibility to respond to demographic perturbations. The degree to which behavior is flexible may correlate to the frequency of such perturbations in a species.     

Tetracosane on the cuticle of the parasitic butterfly Phengaris (Maculinea) nausithous triggers the first contact in the adoption process by Myrmica rubra foragers

Phengaris (Maculinea) butterflies are social parasites of Myrmica ant colonies. Larvae of the parasite are adopted by the ant workers into the colonies. Apparently, chemical signals are used by Phengaris nausithous Bergsträsser larvae to mimic those of the host brood to be recognized by the ants. In the present study, chemical extracts of ant brood and butterfly larvae using four different solvents are tested in behavioural choice assays in search of compounds involved in the adoption process. Tetracosane is the main shared compound in all brood extracts of Myrmica rubra L. and in all larvae of P. nausithous. The attractiveness of tetracosane for M. rubra workers is confirmed by testing synthetic tetracosane in behavioural choice assays, suggesting that the adoption ritual may be initiated by tetracosane.     

Task-specific expression of the foraging gene in harvester ants

In social insects, groups of workers perform various tasks such as brood care and foraging. Transitions in workers from one task to another are important in the organization and ecological success of colonies. Regulation of genetic pathways can lead to plasticity in social insect task behaviour. The colony organization of advanced eusocial insects evolved independently in ants, bees, and wasps and it is not known whether the genetic mechanisms that influence behavioural plasticity are conserved across species. Here we show that a gene associated with foraging behaviour is conserved across social insect species, but the expression patterns of this gene are not. We cloned the red harvester ant (Pogonomyrmex barbatus) ortholog (Pbfor) to foraging, one of few genes implicated in social organization, and found that foraging behaviour in harvester ants is associated with the expression of this gene; young (callow) worker brains have significantly higher levels of Pbfor mRNA than foragers. Levels of Pbfor mRNA in other worker task groups vary among harvester ant colonies. However, foragers always have the lowest expression levels compared to other task groups. The association between foraging behaviour and the foraging gene is conserved across social insects but ants and bees have an inverse relationship between foraging expression and behaviour.     

Species‐level predation network uncovers high prey specificity in a Neotropical army ant community

Army ants are among the top arthropod predators and considered keystone species in tropical ecosystems. During daily mass raids with many thousand workers, army ants hunt live prey, likely exerting strong top‐down control on prey species. Many tropical sites exhibit a high army ant species diversity (>20 species), suggesting that sympatric species partition the available prey niches. However, whether and to what extent this is achieved has not been intensively studied yet. We therefore conducted a large‐scale diet survey of a community of surface‐raiding army ants at La Selva Biological Station in Costa Rica. We systematically collected 3,262 prey items from eleven army ant species (genera Eciton, Nomamyrmex and Neivamyrmex). Prey items were classified as ant prey or non‐ant prey. The prey nearly exclusively consisted of other ants (98%), and most booty was ant brood (87%). Using morphological characters and DNA barcoding, we identified a total of 1,103 ant prey specimens to the species level. One hundred twenty‐nine ant species were detected among the army ant prey, representing about 30% of the known local ant diversity. Using weighted bipartite network analyses, we show that prey specialization in army ants is unexpectedly high and prey niche overlap very small. Besides food niche differentiation, we uncovered a spatiotemporal niche differentiation in army ant raid activity. We discuss competition‐driven multidimensional niche differentiation and predator–prey arms races as possible mechanisms underlying prey specialization in army ants. By combining systematic prey sampling with species‐level prey identification and network analyses, our integrative approach can guide future research by portraying how predator–prey interactions in complex communities can be reliably studied, even in cases where morphological prey identification is infeasible.     

Alarm-recruitment behaviour in Pheidole militicida (Hymenoptera: Formicidae)

ABSTRACT.

• 1 Pheidole militicida Wheeler, a seed-harvesting species of the southwestern United States, possesses a major worker caste (soldiers) with unusually large heads. Previous work suggested that these large major workers are specialized defenders against large seed-harvesting species in the genus Pogonomyrmex.
• 2 Experimental introductions of Pogonomyrmex maricopa Wheeler and the omnivorous fire ant, Solenopsis xyloni McCook, demonstrate that P. militicida colonies alarm-recruit major workers against S.xyloni workers but not against the larger Pogonomyrmex workers.
• 3 P.militicida is the second species in which enemy-specific defence against Solenopsis fire ants has been detected. I suggest that the special role of Solenopsis as both competitor and predator in ant communities frequently favours major worker specialization against this genus.
• 4 Major workers of P.militicida and Pheidole dentata Mayr, the other species with enemy-specific defence against Solenopsis, are morphologically very different from one another. It is suggested that interspecific variation in major worker morphology is not necessarily associated with variation in behavioural specialization.

     

Social organization in some primitive Australian ants. I.Nothomyrmecia macrops Clark

Summary Results of laboratory-based ethological studies on twoNothomyrmecia macrops colonies with individually marked workers are reported. Interactive behavioural acts constituted less than 1% of all those recorded, revealing a strong tendency by the ants not to engage in social contact. Very few workers performed queen-directed acts. They stayed near the queen, though seldom in direct contact. Division of labour was otherwise barely apparent, except that some individuals showed a propensity to guard the nest entrance. No exchange of food was observed between workers, workers and queen, or adults and larvae (apart from worker placement of prey items with larvae). A queen fed from aDrosophila carcass retrieved from the nest floor, without assistance from workers. Systematic scanned observations confirmed levels of inactivity higher than previously observed in ants (comprising almost 2/3 of recorded behavioural acts). The time budget for activities directed toward the immature stages was the same in both colonies, and fluctuated during the circadian period. Non-nestmate larvae added to worker groups were more frequently licked than nestmate larvae, but this might not involve the particular recognition of nestmateversus non-nestmate brood. These observations support the hypothesis thatNothomyrmecia is primitively eusocial, and of special significance in myrmecology.     

Do Maculinea rebeli caterpillars provide vestigial mutualistic benefits to ants when living as social parasites inside Myrmica ant nests?

Caterpillars of the lycaenid butterfly Maculinea rebeli Hirschke (Lepidoptera: Lycaenidae) live for 11–23 months as social parasites in Myrmica (Hymenoptera: Formicidae) red ant nests, a trait that is believed to have evolved from mutualistic myrmecophilous ancestry. Although Maculinea rebeli caterpillars harm Myrmica larvae, they simultaneously produce copious secretions which the adult worker ants imbibe, perhaps representing a vestige of the ancestral mutualism. We report the results of laboratory experiments designed to test alternative hypotheses: (i) Maculinea rebeli caterpillars provide a beneficial source of sugar in return for being tended by Myrmicaworkers; (ii) Maculinea rebeli harms its host by stressing the workers by competing for available sugar. Comparisons were made of Myrmica worker fitness after 90–450 days under all possible combinations of three experimental treatments: ± M. rebeli caterpillars, ± sucrose and ± ant brood. Caterpillars always reduced the survival of both ant workers and their larvae, even when sugar was not provided, suggesting that M. rebeli is wholly parasitic on all stages in its host colony. The results also confirmed the importance of sucrose in the diet of Myrmica, and showed that M. rebeli caterpillars which eat ant brood to supplement their normal trophallactic feeding by workers develop more quickly - but have the same survival and pupal weights – as caterpillars that are fed solely by worker ants.     

Effect of past and present behavioural specialization on brain levels of biogenic amines in workers of the red wood ant Formica polyctena

Social insect workers usually participate first in intranidal tasks (i.e. act as nurses within the nest) and then switch to extranidal tasks and become foragers. However, foragers sometimes switch back again to brood care and become reverted nurses. Behavioural and physiological correlates of the transition nurse–forager (behavioural maturation) and forager–reverted nurse (behavioural reversion) are relatively well known in the honeybee, although they are less explored in ants. To understand better the role of biogenic amines in ant behavioural maturation and behavioural reversion, the levels of octopamine (OA), dopamine (DA) and serotonin (5‐HT) are examined in the brains of nurses, foragers and reverted nurses of the red wood ant Formica polyctena Först. (Hymenoptera: Formicidae). Brain OA levels and the ratios OA : DA and OA : 5‐HT are higher in nurses than in foragers and reverted nurses. Reverted nurses and foragers do not differ significantly with respect to brain biogenic amine levels and amine ratios. Biogenic amine levels in brains of workers of F. polyctena are thus maturation‐related rather than task‐related. This is one of the first studies of neurochemical correlates of ant behavioural maturation and the first attempt to identify neurobiological correlates of ant behavioural reversion. The data obtained provide further evidence that neurobiological processes underlying honeybee and ant behavioural maturation and behavioural reversion reveal important differences.     

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.     

Physiology as a caste-defining feature

Division of labour is a key factor in the ecological success of social insects. Groups of individuals specializing on a particular behaviour are known as castes and are usually distinguished by morphology or age. Physiology plays a key role in both these types of caste, in either the developmental physiology which determines morphology, or the temporal changes in physiology over an insect’s life. Physiological correlates of morphological or temporal caste include differences in gland structure, secretory products, leanness, neuroanatomy and neurochemistry. However, purely physiological castes could also occur. Physiological castes are discrete groups of same-age same-size individuals with particular physiological competencies, or groups of individuals with similar physiology crossing age or size groups. A stable physiological caste occurs in the monomorphic Pharaoh’s ant, where some ants can detect old pheromone trails and retain this specialization over time. These ants differ physiologically from other workers, and the differences arise before eclosion. More temporary physiological castes occur in the ant Ectatomma where brood care specialists have more developed ovarioles than other same-aged workers, and in the honeybee where nurses, wax-workers and soldiers all differ physiologically from same-aged nestmates. Physiology is an important aspect of caste, not only in its contribution to age-related and morphological castes, but also in its own right as a caste grouping factor. While age and morphological differences make caste structures accessible for study, more cryptic physiological castes may play just as important a role in division of labour. Received 19 December 2007; revised 24 July and 18 September 2008; accepted 19 September 2008.