A morphologically specialized soldier caste improves colony defense in a neotropical eusocial bee

Small workers are more persistent when providing and requiring help in a monomorphic ant

The common sand-dwelling Formica cinerea ants possess monomorphic workers, yet with considerable and easily identified size variation. Considering the importance of body size in polymorphic ants and other animals, we test whether size-dependent differences in behaviour occur in this species. We focus on the behaviour of large and small foragers in the context of rescue occurring between nestmates when one of them is entrapped and requires help. We show that workers of different sizes are characterized by a similar frequency of rescue activity and time delay to the first act of rescue. However, small workers rescue for longer than large workers. These results indicate that, although there is no size-related rescue specialization in F. cinerea foragers, small rescuers behave differently than large ones in terms of rescue persistence. Additionally, we show that small workers are more active when trapped. We suggest that variation in behavioural persistence of differently-sized workers may increase the efficiency of rescue actions. This study is the first to find a connection between body size and rescue behaviour in ants and the first to quantify and analyze the behaviour of individuals in need of rescue. These findings add substantially to our understanding of social insects and, more generally, highlight the need to study among-individual behavioural variation in social animals, including those in which body size is judged minute and irrelevant.

A modeling framework for adaptive collective defense: crisis response in social-insect colonies

Living systems, from cells to superorganismic insect colonies, have an organizational boundary between inside and outside and allocate resources to defend it. Whereas the micro-scale dynamics of cell walls can be difficult to study, the adaptive allocation of workers to defense in social-insect colonies is more conspicuous. This is particularly the case for Tetragonisca angustula stingless bees, which combine different defensive mechanisms found across other colonial animals: (1) morphological specialization (distinct soldiers (majors) are produced over weeks); (2) age-based polyethism (young majors transition to guarding tasks over days); and (3) task switching (small workers (minors) replace soldiers within minutes under crisis). To better understand how these timescales of reproduction, development, and behavior integrate to balance defensive demands with other colony needs, we developed a demographic Filippov ODE system to study the effect of these processes on task allocation and colony size. Our results show that colony size peaks at low proportions of majors, but colonies die if minors are too plastic or defensive demands are too high or if there is a high proportion of quickly developing majors. For fast maturation, increasing major production may decrease defenses. This model elucidates the demographic factors constraining collective defense regulation in social insects while also suggesting new explanations for variation in defensive allocation at smaller scales where the mechanisms underlying defensive processes are not easily observable. Moreover, our work helps to establish social insects as model organisms for understanding other systems where the transaction costs for component turnover are nontrivial, as in manufacturing systems and just-in-time supply chains.

Differentiation of workers into soldiers is associated with a size reduction of higher-order brain centers in the neotropical termite Procornitermes araujoi

Comparing the size of functionally distinct brain regions across individuals with remarkable differences in sensory processing and cognitive demands provides important insights into the selective forces shaping animal nervous systems. We took advantage of the complex system of worker-to-soldier differentiation in the termitid Procornitermes araujoi, to investigate how a profound modification of body morphology followed by an irreversible shift in task performance are translated in terms of brain structure and size. This behavioural shift is characterised by a reduction of the once wide and complex behavioural repertoire of workers to one exclusively dedicated to nest defence (soldiers). In accordance with soldier's reduced cognitive and sensory demands, we show here that differentiation of workers into soldiers is associated with a size reduction of the mushroom body (MB) compartments, higher-order brain regions responsible for multimodal processing and integration of sensory information, as well as learning, memory, and decision-making. Moreover, in soldiers, we found an apparent fusion of the medial and lateral MB calyces likely associated with its volume reduction. These results illustrate a functional neuroplasticity of the MB associated with division of labour, supporting the link between MB size and behavioural flexibility in social insect workers.

Mechanisms and adaptations that shape division of labour in stingless bees

Stingless bees are a diverse and ecologically important group of pollinators in the tropics. Division of labour allows bee colonies to meet the various demands of their social life, but has been studied in only ∼3% of all described stingless bee species. The available data suggest that division of labour shows both parallels and striking differences compared with other social bees. Worker age is a reliable predictor of worker behaviour in many species, while morphological variation in body size or differences in brain structure are important for specific worker tasks in some species. Stingless bees provide opportunities to confirm general patterns of division of labour, but they also offer prospects to discover and study novel mechanisms underlying the different lifestyles found in eusocial bees.

Basic Structures of Gut Bacterial Communities in Eusocial Insects

Gut bacterial communities assist host animals with numerous functions such as food digestion, nutritional provision, or immunity. Some social mammals and insects are unique in that their gut microbial communities are stable among individuals. In this review, we focus on the gut bacterial communities of eusocial insects, including bees, ants, and termites, to provide an overview of their community structures and to gain insights into any general aspects of their structural basis. Pseudomonadota and Bacillota are prevalent bacterial phyla commonly detected in those three insect groups, but their compositions are distinct at lower taxonomic levels. Eusocial insects harbor unique gut bacterial communities that are shared within host species, while their stability varies depending on host physiology and ecology. Species with narrow dietary habits, such as eusocial bees, harbor highly stable and intraspecific microbial communities, while generalists, such as most ant species, exhibit relatively diverse community structures. Caste differences could influence the relative abundance of community members without significantly altering the taxonomic composition.

Positive geographic correlation between soldiers' weapon size and defensive prowess in a eusocial aphid, Ceratovacuna japonica

Some aphid species produce a soldier caste with enlarged forelegs and horns (weapons). It has been hypothesised that the evolution of morphological specialization by soldiers in social aphids is accelerated by high predation pressure, but this possibility has not been tested. Here, we investigated the relationship between local predator abundance and soldiers’ weapon size and aggressiveness in a prey–predator system comprising a eusocial aphid, Ceratovacuna japonica, and its predators (larvae of the butterfly Taraka hamada and of the moth Atkinsonia ignipicta) in two populations with different predator abundances. We found that the soldiers in the predator-abundant population had larger weapons and were more aggressive than those in the population with lower predator abundance. Furthermore, the soldiers’ defensive prowess (evaluated as the survival of aphids in the presence of predators) was greater in the predator-abundant population. These results provide the first evidence that a population of eusocial aphids experiencing high predation pressure has soldiers with pronounced defensive traits and defensive prowess.

Termite Valkyries: Soldier-Like Alate Termites From the Cretaceous and Task Specialization in the Early Evolution of Isoptera

In several insect eusocial lineages, e.g., some aphids, thrips, ants, some stingless bees, and termites, task specialization is brought to its climax with a sterile soldier caste solely devoted to colony defense. In Isoptera, while the reproductives are defenseless, the soldiers have unique morpho-physiological specializations whose origin and evolution remain unresolved. Here we report on two instances of Cretaceous fossil termite reproductives belonging to different families († Valkyritermes inopinatus gen. et sp. nov. and an unpublished specimen from the Crato Formation), with intriguing phragmotic soldier-like heads and functional wings. These individuals, herein called Valkyries, are the first termite reproductives known with defensive features and suggest that phragmosis arose at least in the Early Cretaceous. Valkyries resemble modern neotenic soldiers except for their complete wings. Their discovery supports the hypothesis that the division between reproductive (indicated by the winged condition of Valkyries) and defensive tasks (indicated by the phragmotic head) has not always been complete in termite history. We explore two alternative scenarios regarding the origin of Valkyries (i.e., relatively recent and convergent origins vs. plesiomorphic condition) and discuss how they might relate to the development of soldiers. We argue that, in both cases, Valkyries likely evolved to face external threats, a selective pressure that could also have favored the origin of soldiers from helpers. Valkyries highlight the developmental flexibility of termites and illustrate the tortuous paths that evolution may follow.

Brain size and behavioral specialization in the jataí stingless bee (Tetragonisca angustula)

Social insects are instructive models for understanding the association between investment in brain size and behavioral variability because they show a relatively simple nervous system associated with a large set of complex behaviors. In the jataí stingless bee (Tetragonisca angustula), division of labor relies both on age and body size differences among workers. When young, both minors and soldiers engage in intranidal tasks and move to extranidal tasks as they age. Minors switch to foraging activities, while soldiers take over defensive roles. Nest defense performed by soldiers includes two different tasks: (1) hovering around the nest entrance for the detection and interception of heterospecific bees (a task relying mostly on vision) and (2) standing at the nest entrance tube for inspection of returning foragers and discrimination against conspecific non-nestmates based on olfactory cues. Here, using different-sized individuals (minors and soldiers) as well as same-sized individuals (hovering and standing soldiers) performing distinct tasks, we investigated the effects of both morphological and behavioral variability on brain size. We found a negative allometric growth between brain size and body size across jataí workers, meaning that minors had relatively larger brains than soldiers. Between soldier types, we found that hovering soldiers had larger brain compartments related to visual processing (the optic lobes) and learning (the mushroom bodies). Brain size differences between jataí soldiers thus correspond to behavioral specialization in defense (i.e., vision for hovering soldiers) and illustrate a functional neuroplasticity underpinning division of labor.

Fighting ability and the toxicity of raiding pheromone in an obligate kleptoparasite, the stingless bee Lestrimelitta niitkib

Abstract

The evolution of obligate kleptoparasitism, the theft of food, has led to remarkable innovations, including physical weapons and chemical signals that can evolve into chemical weapons. Stingless bees in the genus Lestrimelitta are excellent examples of this phenomenon because they are obligate kleptoparasites that no longer collect floral resources and instead steal brood resources from other bees. Their ability to raid successfully is thus essential to their fitness even when they fight species that are physically bigger, have larger defense forces, or both. We conducted morphometric analyses, quantified Lestrimelitta niitkib mandibular gland pheromone (MGP) components, and carried out individual fighting trials between L. niitkib and the stingless bee Scaptotrigona mexicana, a common victim species, to shed light on the detailed reasons for their success at robbing. Measurements showed that L. niitkib mandibles have thicker exoskeleton cuticles and overall greater width, particularly in the medial and proximal sections, than S. mexicana, which is quite similar in body size. In all fights, L. niitkib bit victims and released MGP, as it does during raids. Scaptotrigona mexicana victims exhibited significantly increased uncoordinated behaviors and showed partial or complete paralysis. We analyzed and quantified the major components of MGP, which consisted of large quantities of geranial (mean of 253 μg) and neral (48 μg) per bee. Microinjections of 1 bee equivalent (BE) of natural or synthetic MGP and ≥ 0.1 BE of geranial significantly increased deleterious behaviors and paralysis as compared to control injections. We suggest that the large quantities of MGP used during raiding have led to an unexpected outcome, a semiochemical evolving the additional function of a toxin, and contribute to the ability of Lestrimelitta to rob its victims.

Significance statement

Kleptoparasites, organisms that steal food resources, employ multiple physical and chemical tools to survive. The success of kleptoparasitism requires a balance between honesty and coercion in interspecific communication. The genus Lestrimellita consists of a group of kleptoparasitic stingless bee species that raid other bee colonies for food and therefore depend upon winning these raids. However, why they succeed remains not fully understood. We studied differences in morphology between L. niitkib and its victims, the pheromones they release during raids, and ran individual fight trials between L. niitkib and a common victim to identify why they are successful. We suggest that the release of pheromones at the beginning of raids, in concert with the pheromone’s toxicity, has been combined to improve L. niitkib’s ability to successfully rob.

Side effects of a fungus-based biopesticide on stingless bee guarding behaviour

Pathogenic fungi have been used worldwide to control crop pests and are assumed to pose negligible threats to the survival of pollinators. Although eusocial stingless bees provide essential pollination services and might be exposed to these biopesticides in tropical agroecosystems, there is a substantial knowledge gap regarding the side effects of fungal pathogens on behavioural traits that are crucial for colony functioning, such as guarding behaviour. Here, we evaluated the effect of Beauveria bassiana on the sophisticated kin recognition system of Tetragonisca angustula, a bee with morphologically specialized entrance guards. By combining behavioural assays and chemical analyses, we show that guards detect pathogen-exposed nestmates, preventing them from accessing nests. Furthermore, cuticular profiles of pathogen-exposed foragers contained significantly lower amounts of linear alkanes than the unexposed ones. Such chemical cues associated with fungal conidia may potentially trigger aggression towards pathogen-exposed bees, preventing pathogen spread into and among colonies. This is the first demonstration that this highly abundant native bee seems to respond in a much more adaptive way to a potentially infectious threat, outweighing the costs of losing foraging workforce when reducing the chances of fungal pathogen outbreaks within their colonies, than honeybees do.

Soldier neural architecture is temporarily modality-specialized but poorly predicted by repertoire size in the stingless bee Tetragonisca angustula

Individual heterogeneity within societies provides opportunities to test hypotheses about adaptive neural investment in the context of group cooperation. Here we explore neural investment in defense specialist soldiers of the eusocial stingless bee (Tetragonisca angustula) which are age sub-specialized on distinct defense tasks and have an overall higher lifetime task repertoire than other sterile workers within the colony. Consistent with predicted behavioral demands, soldiers had higher relative visual (optic lobe) investment than non-soldiers but only during the period when they were performing the most visually demanding defense task (hovering guarding). As soldiers aged into the less visually demanding task of standing guarding this difference disappeared. Neural investment was otherwise similar across all colony members. Despite having larger task repertoires, soldiers had similar absolute brain size and smaller relative brain size compared to other workers, meaning that lifetime task repertoire size was a poor predictor of brain size. Both high behavioral specialization in stable environmental conditions and reassignment across task groups during a crisis occur in T. angustula. The differences in neurobiology we report here are consistent with these specialized but flexible defense strategies. This work broadens our understanding of how neurobiology mediates age and morphological task specialization in highly cooperative societies. This article is protected by copyright. All rights reserved.

Data Reliability in a Citizen Science Protocol for Monitoring Stingless Bees Flight Activity

Simple Summary

This work aims to validate a citizen science protocol for monitoring the flight activity of stingless bees. The count of flight activity (entrance, exit, and entrance carrying pollen) filmed in 30 s videos was compared among three different groups: “original” citizen scientists (group that filmed and performed the count in their own videos), “replicator” citizen scientists (group of citizen scientists who performed flight activity counts on videos shot by other citizen scientists), and experts (researchers who work with bees and who performed the counts on videos shot by citizen scientists). The analysis was divided into two levels: perception (detection of activity in videos) and counting. The results of this analysis revealed that citizen scientists and experts have similar perception and count of bee entrance and exit activity, as no statistical differences were found in these two items. However, replicator citizen scientists noticed more bees carrying pollen than original citizen scientists and experts. Despite this, considering only the videos in which the groups agreed on the presence of pollen, the count was similar for both. These results enabled the validation of the protocol and indicated high quality of data produced by individuals who participate in scientific practices following a citizen science approach.

Abstract

Although the quality of citizen science (CS) data is often a concern, evidence for high-quality CS data increases in the scientific literature. This study aimed to assess the data reliability of a structured CS protocol for monitoring stingless bees’ flight activity. We tested (1) data accuracy for replication among volunteers and for expert validation and (2) precision, comparing dispersion between citizen scientists and expert data. Two distinct activity dimensions were considered: (a) perception of flight activity and (b) flight activity counts (entrances, exits, and pollen load). No significant differences were found among groups regarding entrances and exits. However, replicator citizen scientists presented a higher chance of perceiving pollen than original data collectors and experts, likely a false positive. For those videos in which there was an agreement about pollen presence, the effective pollen counts were similar (with higher dispersion for citizen scientists), indicating the reliability of CS-collected data. The quality of the videos, a potential source of variance, did not influence the results. Increasing practical training could be an alternative to improve pollen data quality. Our study shows that CS provides reliable data for monitoring bee activity and highlights the relevance of a multi-dimensional approach for assessing CS data quality.

The role of epigenetics, particularly DNA methylation, in the evolution of caste in insect societies

Eusocial insects can be defined as those that live in colonies and have distinct queens and workers. For most species, queens and workers arise from a common genome, and so caste-specific developmental trajectories must arise from epigenetic processes. In this review, we examine the epigenetic mechanisms that may be involved in the regulation of caste dimorphism. Early work on honeybees suggested that DNA methylation plays a causal role in the divergent development of queen and worker castes. This view has now been challenged by studies that did not find consistent associations between methylation and caste in honeybees and other species. Evidence for the involvement of methylation in modulating behaviour of adult workers is also inconsistent. Thus, the functional significance of DNA methylation in social insects remains equivocal. This article is part of the theme issue 'How does epigenetics influence the course of evolution?'

Large body size variation is associated with low communication success in tandem running ants

Abstract

Diversity in animal groups is often assumed to increase group performance. In insect colonies, genetic, behavioural and morphological variation among workers can improve colony functioning and resilience. However, it has been hypothesized that during communication processes, differences between workers, e.g. in body size, could also have negative effects. Tandem running is a common recruitment strategy in ants and allows a leader to guide a nestmate follower to resources. A substantial proportion of tandem runs fail because leader and follower lose contact. Using the ant Temnothorax nylanderi as a model system, we tested the hypothesis that tandem running success is impaired if leader and follower differ in size. Indeed, we found that the success rate of tandem pairs drops considerably as size variation increases: tandem runs were unsuccessful when the leader–follower size difference exceeded 10%, whereas ~ 80% of tandem runs were successful when ants differed less than 5% in body length. Possible explanations are that size differences are linked to differences in walking speed or sensory perception. Ants did not choose partners of similar size, but extranidal workers were larger than intranidal workers, which could reduce recruitment mistakes because it reduced the chance that very large and very small ants perform tandem runs together. Our results suggest that phenotypic differences between interacting workers can have negative effects on the efficiency of communication processes. Whether phenotypic variation has positive or negative effects is likely to depend on the task and the phenotypic trait that shows variation.

Significance statement

Diversity is often assumed to increase colony performance in social insects. However, phenotypic differences among workers could also have negative effects, e.g. during communication. Tandem running is a common recruitment strategy in ants, but tandem runs often fail when ants lose contact. We used the ant Temnothorax nylanderi to test the hypothesis that body size differences between tandem leader and follower impair tandem communication. We show that the success rate of tandem pairs drops considerably as size variation increases, possibly because ants of varying size also differ in walking speed. Our study supports the hypothesis that phenotypic variation among workers might not always be beneficial and can negatively impact the efficiency of communication processes.

Experimental predator intrusions in a cooperative breeder reveal threat-dependent task partitioning

In cooperatively breeding species, nonbreeding individuals provide alloparental care and help in territory maintenance and defense. Antipredator behaviors of subordinates can enhance offspring survival, which may provide direct and indirect fitness benefits to all group members. Helping abilities and involved costs and benefits, risks, and outside options (e.g., breeding independently) usually diverge between group members, which calls for status-specific differentiated behavioral responses. Such role differentiation within groups may generate task-specific division of labor, as exemplified by eusocial animals. In vertebrates, little is known about such task differentiation among group members. We show how breeders and helpers of the cooperatively breeding cichlid Neolamprologus savoryi partition predator defense depending on intruder type and the presence of dependent young. In the field, we experimentally simulated intrusions by different fish species posing a risk either specifically to eggs, young, or adults. We used intrusions by harmless algae eaters as a control. Breeders defended most when dependent young were present, while helper investment hinged mainly on their body size and on the potential threat posed by the respective intruders. Breeders and helpers partitioned defense tasks primarily when dependent young were exposed to immediate risk, with breeders investing most in antipredator defense, while helpers increased guarding and care in the breeding chamber. Breeders’ defense likely benefits helpers as well, as it was especially enhanced in the treatment where helpers were also at risk. These findings illustrate that in a highly social fish different group members exhibit fine-tuned behavioral responses in dependence of ecological and reproductive parameter variation.

Soldiers of the termite Nasutitermes corniger (Termitidae: Nasutitermitinae) increase the ability to exploit food resources

The performance of eusocial insect colonies is optimized by the division of labor among castes. Throughout the evolution of termites, there was an evident increase in the proportion of soldiers in the colonies. In derived termite species, the soldiers have a crucial role in defense and the initial phases of foraging. Here, we evaluated whether the soldiers of the Neotropical termite Nasutitermes corniger improve the foraging decisions. We tested the hypotheses that groups with soldiers (i) are more efficient searching for food, (ii) more efficiently recruit individuals, and (iii) a higher proportion of soldiers results in a more efficient balance between exploitation and exploration of food resources. Using behavioral bioassays under laboratory conditions, we show that the presence of soldiers in termite groups promotes a faster exploitation of the environment that allows them to encounter more food sources simultaneously. However, the presence of soldiers did not significantly increase the attraction of termite groups. Despite that, termite groups with higher soldier proportions were more able to perceive changes in the resource offerings by redirecting individuals to explore more profitable sources. Our results show that the role of soldiers in N. corniger increases the efficiency of termite groups in the exploration and exploitation of food resources.

Can morphological and behavioral traits predict the foraging and feeding dynamics of social arachnids?

Complex social insect species exhibit task specialization mediated by morphological and behavioral traits. However, evidence of such traits is scarce for other social arthropods. We investigated whether the social pseudoscorpion Paratemnoides nidificator exhibits morphologically and behaviorally specialized individuals in prey capture. We measured body and chela sizes of adult pseudoscorpions and analyzed predation processes. Larger individuals spent more time moving through the colony and foraging than smaller pseudoscorpions. Individuals that captured prey had increased body and absolute chelae sizes. Although larger individuals had relatively small chelae size, they showed a higher probability of prey capture. Larger individuals manipulated prey often, although they fed less than smaller pseudoscorpions. Individuals that initiated captures fed more frequently and for more time than the others. Natural selection might be favoring individuals specialized in foraging and colony protection, allowing smaller and less efficient adults to avoid contact with dangerous prey. To our knowledge, there is incipient information regarding specialized individuals in arachnids, and our results might indicate the emergence of a morphologically specialized group in this species.

Environmental cue affects the hearing-related behaviors of Drosophila melanogaster by targeting the redox pathways

Environmental cues like noise, pressure, and circadian rhythm can affect the hearing ability of human beings. Nevertheless, the complex physiology of the human being does not allow us to understand how these factors can affect hearing and hearing-related behaviors. Conversely, these effects can be easily checked using the hearing organ of Drosophila melanogaster, the Johnston organ. In the current study, the Drosophila was exposed to challenging environments like noise, low pressure, and altered circadian rhythm. The hearing organ of larvae, as well as adults, was analyzed for hearing-related defects. In the third instar larva, the cell deaths were detected in the antenna imaginal disc, the precursor of Johnston's organ. Elevated levels of reactive oxygen species and antioxidant enzymes were also detected in the adult antennae of environmentally challenged flies. The ultrastructure of the antennae suggests the presence of abundant mitochondria in the scolopidia of control. Fewer amounts of mitochondria are found in the environmentally challenged adult antennae. In adults, various hearing-related behaviors were analyzed as a readout of functionality of the hearing organ. Analysis of climbing, aggressive, and courtship behaviors suggests abnormal behavior in environmentally challenged flies than the control. The current study suggests that the environmental cues can alter hearing-related behaviors in Drosophila. The methods used in this study can be used to monitor the environmental pollution or to study the effect of alteration of noise, pressure, and circadian rhythm on hearing-related behaviors taking Drosophila melanogaster as a model organism. Graphical abstract.

Chemosensory reception in the stingless bee Tetragonisca angustula

In stingless bees, unlike honey bees, the relationship between chemosensory abilities and colony labor division has been poorly studied. Here we examined odor reception and gustatory responsiveness of the stingless bee Tetragonisca angustula focusing on workers, whose are involved in different tasks. Using the proboscis extension response, we studied sucrose response thresholds (SRTs) of foragers and guards. Peripheral responses to odors at the antennae were recorded by electroantennography (EAG). Additionally, we quantified and described the number and type of sensilla present on the antennae using scanning electron microscopy. Foragers' SRTs changed according to the resource collected: nonpollen foragers showed higher SRTs than pollen foragers and guards, that showed similar sucrose responsiveness. EAG signal strength of both foragers and guards increased with increasing odor concentration. Interestingly, guard bees showed the highest response to citral, an odor that triggers defensive behavior in T. angustula. Type and number of sensilla present in the antennae of guards and foragers were similar. Our results suggest that differences found in chemosensory responses among worker subcastes are task dependent.

Body Size Differences between Foraging and Intranidal Workers of the Monomorphic Ant Lasius niger

The association between the division of labour and worker body size of ants is typical for species that maintain physical castes. Some studies showed that this phenomenon can be also observed in the absence of distinct morphological subcastes among workers. However, the general and consistent patterns in the size-based division of labour in monomorphic ants are largely unidentified. In this study, we performed a field experiment to investigate the link between worker body size and the division of labour of the ant Lasius niger (Linnaeus, 1758), which displays limited worker size variation. We demonstrated that the body size of workers exploring tuna baits is slightly but significantly smaller than the size of workers located in the upper parts of the nest. Comparing the present results with existing studies, large workers do not seem to be dedicated to work outside the nest. We suggest that monomorphic workers of certain body sizes are flexible in the choice of task they perform, and food type may be the important determinant of this choice.

Body size variation in bees: regulation, mechanisms, and relationship to social organization

Size polymorphism is common in bees, and is determined by environmental factors such as temperature, brood cell size, and the diet provided to developing larvae. In social bees, these factors are further influenced by intricate interactions between the queen, workers, and the developing brood which eventually determine the final size and caste of developing larvae. Environmental and social factors act in part on juvenile hormone and ecdysteroids, which are key hormonal regulators of body size and caste determination. In some social bees, body size variation is central for social organization because it structures reproductive division of labor, task allocation among workers, or both. At ecological scales, body size also impacts bee-mediated pollination services in solitary and social species by influencing floral visitation and pollination efficacy.

Social trait definitions influence evolutionary inferences: a phylogenetic approach to improving social terminology for bees

The comparative method relies not only on a good understanding of the phylogenetic relationships among taxa, but also on consistent terminology for describing phenotypes. Clear and consistent terminology allows similar phenotypes to be described and phylogenetically analyzed in different organisms, whereas inconsistent terminology is a major impediment to comparisons, even for taxonomically restricted groups such as bees. Here, I propose that the usefulness of social terminology can be judged by its value in phylogenetic trait-mapping aimed at uncovering evolutionary transitions between solitary and social behavior. I propose a four-step approach to evaluate and update social terminology, in which definitions are first updated based on recent behavioral studies (step 1), mapped onto a phylogeny (step 2), evaluated for their utility in the trait-mapping exercise (step 3), and then, if necessary, revised (step 4). To demonstrate the approach, I define four terms important for understanding social evolution in bees (solitary, social, eusocial, and hypersocial) and map them onto a very recent phylogeny of Apidae. This not only illustrates an objective method for evaluating social terminology, but also provides novel inferences about social evolution in Apidae, including support for a parasocial origin of eusociality and at least two Major Evolutionary Transitions to hypersociality.

Changing of the guard: mixed specialization and flexibility in nest defense (Tetragonisca angustula)

Task allocation is a central challenge of collective behavior in a variety of group-living species, and this is particularly the case for the allocation of social insect workers for group defense. In social insects, both benefits and considerable costs are associated with the production of specialized soldiers. We asked whether colonies mitigate costs of production of specialized soldiers by simultaneously employing behavioral flexibility in nonspecialist workers that can augment defense capabilities at short time scales. We studied colonies of the stingless bee Tetragonisca angustula, a species that has 2 discrete nest-guarding tasks typically performed by majors: hovering guarding and standing guarding. Majors showed age polyethism across nest-guarding tasks, first hovering and then changing to the task of standing guarding after 1 week. Colonies were also able to reassign minors to guarding tasks when majors were experimentally removed. Replacement guards persisted in nest defense tasks until colonies produced enough majors to return to their initial state. Tetragonisca angustula colonies thus employed a coordinated set of specialization strategies in nest defense: morphologically specialized soldiers, age polyethism among soldiers within specific guarding tasks, and rapid flexible reallocation of nonspecialists to guarding during soldier loss. This mixed strategy achieves the benefits of a highly specialized defensive force while maintaining the potential for rapid reinforcement when soldiers are lost or colonies face unexpectedly intense attack.

Temporal Response of Foragers and Guards of Two Stingless Bee Species to Cephalic Compounds of the Robber Bee Lestrimelitta niitkib (Ayala) (Hymenoptera, Apidae)

Lestrimelitta spp. are stingless bees that steal food and nesting materials from other highly social bees to survive. Though most of their victim species respond, either aggressively or submissively, to cephalic components of Lestrimelitta, little is known about if such response changes at some point during extended periods of exposure. Moreover, potential synergistic effects due to a mixture of victim's alarm/defense pheromones and Lestrimelitta mandibular pheromones, like in an actual attack, have not been examined so far. In this paper, we investigated the response of two species of non-robber stingless bees, Scaptotrigona mexicana (Guérin) and Tetragonisca angustula (Latreille), to (a) cephalic compounds from crushed heads of nestmates, (b) cephalic compounds of Lestrimelitta niitkib (Ayala), and (c) a mixture of (a) and (b). We found that even though T. angustula did not react to nestmates' crushed head, its response towards L. niitkib cephalic compounds was stronger and lasted longer than that of S. mexicana. Interestingly, the addition of crushed heads of the non-robber species to L. niitkib crushed heads caused no significant increase in the alarm response of both species. It may be that the absence of an alarm pheromone in T. angustula made this species more receptive to extraneous odors, which is not the case for S. mexicana; however, more species must be studied to elucidate any pattern regarding the absence/presence of alarm pheromones and the corresponding response to intruders' pheromones.

Organization enhances collective vigilance in the hovering guards of Tetragonisca angustula bees

One benefit of group living is vigilance against predators. Previous studies have investigated the group size effect, where individual vigilance decreases as group size increases without reducing the overall ability of the group to detect predators. However, there has been comparatively little research on whether the positioning of individuals can improve the collective vigilance of the group. We studied the coordination of vigilance and its effect on predator detection in the eusocial bee Tetragonisca angustula. Nests are defended by hovering guards that detect and intercept intruders before they reach the nest entrance, in addition to those that stand upon it. We show that hovering guards are positioned nonrandomly, with a strong tendency for equal numbers on both sides of the entrance. This organization increases the collective vigilance of the guard group, as groups distributed in an even ratio, either side of the entrance, have a greater collective field of view than groups that deviate from an even ratio. Finally, we use a bioassay to show that when guards are on both sides of the entrance, their ability to detect intruders before they reach the entrance increases. Overall, our results provide strong evidence that vigilance is coordinated and that this improves nest defense. Although other group-living animals are often selfish in their individual vigilance behaviors and face competing time constraints such as foraging, the altruistic nature of eusocial insect workers has probably facilitated the evolution of coordinated vigilance, as documented here in T. angustula.

Enemy recognition is linked to soldier size in a polymorphic stingless bee

Many ant and termite colonies are defended by soldiers with powerful mandibles or chemical weaponry. Recently, it was reported that several stingless bee species also have soldiers for colony defence. These soldiers are larger than foragers, but otherwise lack obvious morphological adaptations for defence. Thus, how these soldiers improve colony fitness is not well understood. Robbing is common in stingless bees and we hypothesized that increased body size improves the ability to recognize intruders based on chemosensory cues. We studied the Neotropical species Tetragonisca angustula and found that large soldiers were better than small soldiers at recognizing potential intruders. Larger soldiers also had more olfactory pore plates on their antennae, which is likely to increase their chemosensory sensitivity. Our results suggest that improved enemy recognition might select for increased guard size in stingless bees.

Why war is a man's game

Interest in the evolutionary origins and drivers of warfare in ancient and contemporary small-scale human societies has greatly increased in the last decade, and has been particularly spurred by exciting archaeological discoveries that suggest our ancestors led more violent lives than previously documented. However, the striking observation that warfare is an almost-exclusively male activity remains unexplained. Three general hypotheses have been proposed, concerning greater male effectiveness in warfare, lower male costs, and patrilocality. But while each of these factors might explain why warfare is more common in men, they do not convincingly explain why women almost never participate. Here, we develop a mathematical model to formally assess these hypotheses. Surprisingly, we find that exclusively male warfare may evolve even in the absence of any such sex differences, though sex biases in these parameters can make this evolutionary outcome more likely. The qualitative observation that participation in warfare is almost exclusive to one sex is ultimately explained by the fundamentally sex-specific nature of Darwinian competition—in fitness terms, men compete with men and women with women. These results reveal a potentially key role for ancestral conditions in shaping our species' patterns of sexual division of labour and violence-related adaptations and behavioural disorders.

Insect societies fight back: the evolution of defensive traits against social parasites

Insect societies face many social parasites that exploit their altruistic behaviours or their resources. Due to the fitness costs these social parasites incur, hosts have evolved various behavioural, chemical, architectural and morphological defence traits. Similar to bacteria infecting multicellular hosts, social parasites have to successfully go through several steps to exploit their hosts. Here, we review how social insects try to interrupt this sequence of events. They can avoid parasite contact by choosing to nest in parasite-free locales or evade attacks by adapting their colony structure. Once social parasites attack, hosts attempt to detect them, which can be facilitated by adjustments in colony odour. If social parasites enter the nest, hosts can either aggressively defend their colony or take their young and flee. Nest structures are often shaped to prevent social parasite invasion or to safeguard host resources. Finally, if social parasites successfully establish themselves in host nests, hosts can rebel by killing the parasite brood or by reproducing in the parasites' presence. Hosts of social parasites can therefore develop multiple traits, leading to the evolution of complex defence portfolios of co-dependent traits. Social parasites can respond to these multi-level defences with counter-adaptations, potentially leading to geographical mosaics of coevolution.This article is part of the Theo Murphy meeting issue 'Evolution of pathogen and parasite avoidance behaviours'.

Colony-level non-associative plasticity of alarm responses in the stingless honey bee, Tetragonisca angustula

In ants, bees, and other social Hymenoptera alarm pheromones are widely employed to coordinate colony nest defense. In that context, alarm pheromones elicit innate species-specific defensive behaviors. Therefore, in terms of classical conditioning, an alarm pheromone could act as an unconditioned stimulus (US). Here we test this hypothesis by establishing whether repeated exposure to alarm pheromone in different testing contexts modifies the alarm response. We evaluate colony level alarm responses in the stingless bee, Tetragonisca angustula, which has a morphologically distinct guard caste. First, we describe the overall topology of defense behaviors in the presence of an alarm pheromone. Second, we show that repeated, regular exposure to synthetic alarm pheromone reduces different components of the alarm response, and memory of that exposure decays over time. This observed decrease followed by recovery occurs over different time frames and is consistent with behavioral habituation. We further tested whether the alarm pheromone can act as a US to classically condition guards to modify their defense behaviors in the presence of a novel (conditioned) stimulus (CS). We found no consistent changes in the response to the CS. Our study demonstrates the possibility that colony-level alarm responses can be adaptively modified by experience in response to changing environmental threats. Further studies are now needed to reveal the extent of these habituation-like responses in regard to other pheromones, the potential mechanisms that underlie this phenomenon, and the range of adaptive contexts in which they function at the colony level.

Genetics and Evolution of Social Behavior in Insects

The study of insect social behavior has offered tremendous insight into the molecular mechanisms mediating behavioral and phenotypic plasticity. Genomic applications to the study of eusocial insect species, in particular, have led to several hypotheses for the processes underlying the molecular evolution of behavior. Advances in understanding the genetic control of social organization have also been made, suggesting an important role for supergenes in the evolution of divergent behavioral phenotypes. Intensive study of social phenotypes across species has revealed that behavior and caste are controlled by an interaction between genetic and environmentally mediated effects and, further, that gene expression and regulation mediate plastic responses to environmental signals. However, several key methodological flaws that are hindering progress in the study of insect social behavior remain. After reviewing the current state of knowledge, we outline ongoing challenges in experimental design that remain to be overcome in order to advance the field.

Alarm Pheromone Composition and Behavioral Activity in Fungus-Growing Ants

Chemical communication is a dominant method of communication throughout the animal kingdom and can be especially important in group-living animals in which communicating threats, either from predation or other dangers, can have large impacts on group survival. Social insects, in particular, have evolved a number of pheromonal compounds specifically to signal alarm. There is predicted to be little selection for interspecific variation in alarm cues because individuals may benefit from recognizing interspecific as well as conspecific cues and, consequently, alarm cues are not normally thought to be used for species or nestmate recognition. Here, we examine the composition of the alarm pheromones of seven species of fungus-growing ants (Attini), including both basal and derived species and examine the behavioral responses to alarm pheromone of Acromyrmex leaf-cutting ants, the sister genus to the highly studied Atta leaf-cutting ants. We find surprisingly high interspecific variation in alarm pheromone composition across the attine phylogeny. Interestingly, the active component of the alarm pheromone was different between the two leaf-cutting ant genera. Furthermore, in contrast to previous studies on Atta, we found no differences among morphological castes in their responses to alarm pheromone in Acromyrmex but we did find differences in responses among putative age classes. The results suggest that the evolution of alarm communication and signaling within social insect clades can be unexpectedly complex and that further work is warranted to understand whether the evolution of different alarm pheromone compounds is adaptive.

Repeated evolution of soldier sub-castes suggests parasitism drives social complexity in stingless bees

The differentiation of workers into morphological castes represents an important evolutionary innovation that is thought to improve division of labor in insect societies. Given the potential benefits of task-related worker differentiation, it is puzzling that physical worker castes, such as soldiers, are extremely rare in social bees and absent in wasps. Following the recent discovery of soldiers in a stingless bee, we studied the occurrence of worker differentiation in 28 stingless bee species from Brazil and found that several species have specialized soldiers for colony defence. Our results reveal that worker differentiation evolved repeatedly during the last ~ 25 million years and coincided with the emergence of parasitic robber bees, a major threat to many stingless bee species. Furthermore, our data suggest that these robbers are a driving force behind the evolution of worker differentiation as targets of robber bees are four times more likely to have nest guards of increased size than non-targets. These findings reveal unexpected diversity in the social organization of stingless bees.Although common in ants and termites, worker differentiation into physical castes is rare in social bees and unknown in wasps. Here, Grüter and colleagues find a guard caste in ten species of stingless bees and show that the evolution of the guard caste is associated with parasitization by robber bees.

The behavioral ecology of variation in social insects

Understanding the ecological relevance of variation within and between colonies has been an important and recurring theme in social insect research. Recent research addresses the genomic and physiological factors and fitness effects associated with behavioral variation, within and among colonies, in regulation of activity, cognitive abilities, and aggression. Behavioral variation among colonies has consequences for survival and reproductive success that are the basis for evolutionary change.

Local differences in parasitism and competition shape defensive investment in a polymorphic eusocial bee

Many colonial animals rely for their defense on a soldier caste. Adaptive colony demography theory predicts that colonies should flexibly adjust the investment in different worker castes depending on the colony needs. For example, colonies should invest more in defensive workers (e.g., soldiers) in dangerous environments. However, evidence for this prediction has been mixed. We combined descriptive and experimental approaches to examine whether defensive investment and worker size are adjusted to local ecology in the only known bee with polymorphic workers, Tetragonisca angustula. Colonies of this species are defended by a morphologically specialized soldier caste. Our study included three populations that differed in the density of food competition and the occurrence of a parasitic robber bee. We found that colonies coexisting with robber bees had on average 43% more soldiers defending the nest entrance, while colonies facing stronger foraging competition had soldiers that were -6-7% smaller. We then experimentally relocated colonies to areas with different levels of competition. When released from intense food competition, body sizes of guards and foragers increased. After introducing chemical robber bee cues at nest entrances, we found both a short-term and a long-term up-regulation of the number of soldiers defending the colony. Active soldier numbers remained high after the experiment for a duration equivalent to 2-3 worker life spans. How information about past parasite threat is stored in the colony is currently unknown. In summary, T. angustula adjusts both the number and the body size of active soldiers to local ecological conditions. Competitor density also affects forager (or minor) size, an important colony trait with potential community ecological consequences. Our study supports adaptive colony demography theory in a eusocial bee and highlights the importance of colony threats and competition as selective forces shaping colony phenotype.

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.

Inter-caste communication in social insects

Social insect colonies function as highly integrated units despite consisting of many individuals. This requires the different functional parts of the colony (e.g. different castes) to exchange information that aid in colony functioning and ontogeny. Here we discuss inter-caste communication in three contexts, firstly, the communication between males and females during courtship, secondly, the communication between queens and workers that regulate reproduction and thirdly, the communication between worker castes that allows colonies to balance the number of different worker types. Some signals show surprising complexity in both their chemistry and function, whereas others are simple compounds that were probably already used as pheromones in the solitary ancestors of several social insect lineages.