Competition through ritualized aggressive interactions between sympatric colonies in solitary foraging neotropical ants.

Understanding the structure of food competition between conspecifics in their natural settings is paramount to addressing more complex questions in ecology, evolution, and conservation. While much research on ants focuses on aggressive food competition between large and foraging trail-using societies, we lack a thorough understanding of inter-colony competition in socially less derived, solitarily foraging species. To fill this gap, we explored the activity of ten neighbouring colonies of the giant ant Dinoponera quadriceps, monitoring 2513 foraging trips of hundreds of workers and all its inter-individual interactions. We found that, on encountering, workers from different colonies rarely engaged in aggressive fights but instead avoided each other or performed ritualised agonistic bouts. We discovered that during foraging trips, a few workers within each colony repeatedly rubbed their gaster on the substrate, a behaviour not observed in the field before. We propose that workers use this behaviour to mark the foraging area and mark more frequently in its periphery. Only 25% of the individuals specialised in this behaviour, and we hypothesise that the specialisation results from the history of interactions and experience of individual foragers. Our study suggests that workers of contiguous D. quadriceps colonies engage in low-risk conflict, mainly displaying ritualised behaviours. As these small societies mainly rely on tiny, unpredictably scattered, albeit abundant in the environment, arthropod prey, and not on persistent food sources, they do not aggressively defend exclusive foraging territories. On the other hand, colonies rely on large overlapping foraging areas to sustain their survival and growth, most often tolerating foragers from nearby colonies. We discuss whether this type of competitive interaction is expected in all solitary foraging species.

Nestmate recognition in the Amazonian stingless bee Melipona paraensis.

Meliponine bees use chemical-based nestmate discrimination to protect their colonies from unrelated intruders. However, species from the Amazon basin are relatively poorly known from this perspective. Here, we investigated Melipona paraensis nestmate discrimination in different contexts (nests vs. neutral arenas), testing aggression in bees facing other bees varying in age and origin (same/different colony or a different kleptoparasitic meliponine species) or experimentally treated with odors from unrelated colonies. As expected, M. paraensis did not discriminate against callow non-nestmate workers with weak/undifferentiated chemical signatures. Workers specialized in nest defense aggressed intruders more often than non-specialized workers, but were less aggressive in neutral arenas than in the nest. Our study provides novel behavioral information relevant for social insect research and meliponiculture.

Temporary prey storage along swarm columns of army ants: an adaptive strategy for successful raiding?

While pillaging the brood of other ant colonies, Eciton army ants accumulate prey in piles, or caches, along their foraging trails. Widely documented, these structures have historically been considered as by-products of heavy traffic or aborted relocations of the ants' temporary nest, or bivouac. However, we recently observed that caches of the hook-jawed army ant, Eciton hamatum, appeared independently from heavy traffic or bivouac relocations. In addition, the flow of prey through caches varied based on the quantity of prey items workers transported. As this suggested a potential adaptive function, we developed agent-based simulations to compare raids of caching and non-caching virtual army ants. We found that caches increased the amount of prey that relatively low numbers of raiders were able to retrieve. However, this advantage became less conspicuous-and generally disappeared-as the number of raiders increased. Based on these results, we hypothesize that caches maximize the amount of prey that limited amounts of raiders can retrieve, especially as prey colonies coordinately evacuate their brood. In principle, caches also allow workers to safely collect multiple prey items and efficiently transport them to the bivouac. Further field observations are needed to test this and other hypotheses emerging from our study.

Tandem running by foraging Pachycondyla striata workers in field conditions vary in response to food type, food distance, and environmental conditions.

Ants show collective and individual behavioral flexibility in their response to immediate context, choosing for example between different foraging strategies. In Pachycondyla striata, workers can forage solitarily or recruit and guide nestmates to larger food sources through tandem running. Although considered more ancestral and less efficient than pheromone trail-laying, this strategy is common especially in species with small colony size. What is not known is how the decision to recruit or follow varies according to the immediate context. That is, how fine adjustments in information transfer affect immediate foraging decisions at the colony level. Here, we studied individually marked workers and evaluated their foraging decisions when food items varied in nature (protein versus carbohydrate), size, and distance from the nest at different temperatures and humidity levels. Our results show that tandem run leaders and potential followers adjust their behavior according to a combination of external factors. While 84.2% of trips were solitary, most ants (81%) performed at least 1 tandem run. However, tandem runs were more frequent for nearby resources and at higher relative humidity. Interestingly, when food items were located far away, tandem runs were more successful when heading to protein sources (75%) compared with carbohydrate sources (42%). Our results suggest that the social information transfer between leaders and followers conveys more information than previously thought, and also relies on their experience and motivation.

Is there a bias in spatial maze judgment bias tests? Individual differences in subjects' novelty response can affect test results.

Judgment bias tests have become an important tool in the assessment of animals' affective states. Subjects are first trained to discriminate between two cues associated with a positive and a less-positive outcome. After successful training, they are confronted with an ambiguous cue, and responses are used for judgment bias assessment. In spatial settings, ambiguous cue presentation is typically linked with novelty, i.e. to yet unexplored areas or areas to which the animal has a low degree of habituation. We hypothesized that in such settings, responses to ambiguity might be biased by the animals' perception of novelty. We conducted judgment bias tests in mound-building mice phenotyped for their exploration tendency. After subjects had learned to distinguish between the positively and less-positively rewarded arms of a maze, a new ambiguous middle-arm was introduced. During the first test trial, more exploratory, less neophobic individuals displayed higher bidirectional locomotion in the ambiguous arm, indicating intensive exploration. Although this resulted in longer latencies to the reward in more exploratory animals, we conclude that this did not reflect a 'more pessimistic judgment of ambiguity'. Indeed, during the following two trials, with increasing habituation to the ambiguous arm, the direction of the association was inversed compared to the first trial, as more exploratory individuals showed relatively shorter approach latencies. We suggest that in spatial test settings associating the ambiguous cue to novel areas, results can be confounded by subjects' personality-dependent motivational conflict between exploration and reaching the reward. Findings obtained under such conditions should be interpreted with care.

Time till death affects spider mobility and web-building behavior during web construction in an orb-web spider.

It is well known that age influences organism mobility. This was demonstrated in vertebrates (such as mammals and birds) but has been less studied in invertebrates with the exception of Drosophila and the nematode Caenorhabditis elegans. Here we studied the influence of age on the mobility of the orb-weaving spider Zygiella x-notata during web construction. The orb-web is a good model because it has a characteristic geometrical structure and video tracking can be used to easily follow the spider's movements during web building. We investigated the influence of age (specifically chronological age, life span, and time till death) on different parameters of spider mobility during the construction of the capture spiral (distance traveled, duration of construction, spider velocity, spider movement, and spider inactivity) with a generalized linear model (GLM) procedure adjusted for the spider mass. The results showed that neither chronological age, nor life span affected the mobility parameters. However, when the time till death decreased, there was a decrease in the distance traveled, the duration of the construction of the capture spiral, and the spider movement. The spider velocity and the time of inactivity were not affected. These results could be correlated with a decrease in the length of the silky thread deposited for the construction of the capture spiral. Spiders with a shorter time till death built smaller web using less silk. Thus, our study suggests strongly that time till death affects spider mobility during web construction but not the chronological age and thus may be a good indicator of senescence.

Fertility Signaling and Partitioning of Reproduction in the Ant Neoponera apicalis.

All individuals in social insect colonies benefit from being informed about the presence and fertility state of reproducers. This allows the established reproductive individuals to maintain their reproductive monopoly without the need for physical control, and the non-reproductive individuals to make appropriate reproductive choices. Here, we studied whether fertility signaling is responsible for the partitioning of reproduction in the ant Neoponera apicalis. This species forms small colonies from one single-mated queen, with workers establishing reproductive hierarchies when hopelessly queenless. Previous studies identified putative fertility signals, particularly the hydrocarbon 13-methylpentacosane (13-MeC25), and have shown that precise status discrimination based on these signals could be involved in the regulation of reproductive activities. Here, we extend these findings and reveal that all individuals, be they queens or workers, differ in their cuticular hydrocarbon profile according to fertility state. Proportions of 13-MeC25 were a strong predictor of an individual's ovarian activity, and could, thus, advertise the established reproducer(s) in both queenright and queenless conditions. Furthermore, this compound might play a key role in the establishment of the reproductive hierarchy, since workers with low fertility at the onset of hierarchy formation already have relatively high amounts of 13-MeC25. Dyadic encounters showed that individuals with experimentally increased amounts of 13-MeC25 triggered less agonistic interactions from top rankers, in accord with them "advertising" higher status. Thus, these bioassays supported the use of 13-MeC25 by competing ants. This simple recognition system potentially allows permanent regulation of partitioning of reproduction in this species.

Genetic distance and age affect the cuticular chemical profiles of the clonal ant Cerapachys biroi.

Although cuticular hydrocarbons (CHCs) have received much attention from biologists because of their important role in insect communication, few studies have addressed the chemical ecology of clonal species of eusocial insects. In this study we investigated whether and how differences in CHCs relate to the genetics and reproductive dynamics of the parthenogenetic ant Cerapachys biroi. We collected individuals of different ages and subcastes from several colonies belonging to four clonal lineages, and analyzed their cuticular chemical signature. CHCs varied according to colonies and clonal lineages in two independent data sets, and correlations were found between genetic and chemical distances between colonies. This supports the results of previous research showing that C. biroi workers discriminate between nestmates and non-nestmates, especially when they belong to different clonal lineages. In C. biroi, the production of individuals of a morphological subcaste specialized in reproduction is inversely proportional to colony-level fertility. As chemical signatures usually correlate with fertility and reproductive activity in social Hymenoptera, we asked whether CHCs could function as fertility-signaling primer pheromones determining larval subcaste fate in C. biroi. Interestingly, and contrary to findings for several other ant species, fertility and reproductive activity showed no correlation with chemical signatures, suggesting the absence of fertility related CHCs. This implies that other cues are responsible for subcaste differentiation in this species.

Epistasis between adults and larvae underlies caste fate and fitness in a clonal ant.

In social species, the phenotype and fitness of an individual depend in part on the genotype of its social partners. However, how these indirect genetic effects affect genotype fitness in competitive situations is poorly understood in animal societies. We therefore studied phenotypic plasticity and fitness of two clones of the ant Cerapachys biroi in monoclonal and chimeric colonies. Here we show that, while clone B has lower fitness in isolation, surprisingly, it consistently outcompetes clone A in chimeras. The reason is that, in chimeras, clone B produces more individuals specializing in reproduction rather than cooperative tasks, behaving like a facultative social parasite. A cross-fostering experiment shows that the proportion of these individuals depends on intergenomic epistasis between larvae and nursing adults, explaining the flexible allocation strategy of clone B. Our results suggest that intergenomic epistasis can be the proximate mechanism for social parasitism in ants, revealing striking analogies between social insects and social microbes.

Specific recognition of reproductive parasite workers by nest-entrance guards in the bumble bee Bombus terrestris.

BACKGROUND: The impact of social parasites on their hosts' fitness is a strong selective pressure that can lead to the evolution of adapted defence strategies. Guarding the nest to prevent the intrusion of parasites is a widespread response of host species. If absolute rejection of strangers provides the best protection against parasites, more fine-tuned strategies can prove more adaptive. Guarding is indeed costly and not all strangers constitute a real threat. That is particularly true for worker reproductive parasitism in social insects since only a fraction of non-nestmate visitors, the fertile ones, can readily engage in parasitic reproduction. Guards should thus be more restrictive towards fertile than sterile non-nestmate workers. We here tested this hypothesis by examining the reaction of nest-entrance guards towards nestmate and non-nestmate workers with varying fertility levels in the bumble bee Bombus terrestris. Because social recognition in social insects mainly relies on cuticular lipids (CLs), chemical analysis was also conducted to examine whether workers' CLs could convey the relevant information upon which guards could base their decision. We thus aimed to determine whether an adapted defensive strategy to worker reproductive parasitism has evolved in B. terrestris colonies. RESULTS: Chemical analysis revealed that the cuticular chemical profiles of workers encode information about both their colony membership and their current fertility, therefore providing potential recognition cues for a suitable adjustment of the guards' defensive decisions. We found that guards were similarly tolerant towards sterile non-nestmate workers than towards nestmate workers. However, as predicted, guards responded more aggressively towards fertile non-nestmates. CONCLUSION: Our results show that B. terrestris guards discriminate non-nestmates that differ in their reproductive potential and respond more strongly to the individuals that are a greatest threat for the colony. Cuticular hydrocarbons are the probable cues underlying the specific recognition of reproductive parasites, with the specific profile of highly fertile bees eliciting the agonistic response when combined with non-colony membership information. Our study therefore provides a first piece of empirical evidence supporting the hypothesis that an adapted defensive strategy against worker reproductive parasitism exists in B. terrestris colonies.

Drifting behaviour as an alternative reproductive strategy for social insect workers.

Restricted reproduction is traditionally posited as the defining feature of eusocial insect workers. The discovery of worker reproduction in foreign colonies challenges this view and suggests that workers' potential to pursue selfish interests may be higher than previously believed. However, whether such reproductive behaviour truly relies on a reproductive decision is still unknown. Workers' reproductive decisions thus need to be investigated to assess the extent of workers' reproductive options. Here, we show in the bumblebee Bombus terrestris that drifting is a distinct strategy by which fertile workers circumvent competition in their nest and reproduce in foreign colonies. By monitoring workers' movements between colonies, we show that drifting is a remarkably dynamic behaviour, widely expressed by both fertile and infertile workers. We demonstrate that a high fertility is, however, central in determining the propensity of workers to enter foreign colonies as well as their subsequent reproduction in host colonies. Moreover, our study shows that the drifting of fertile workers reflects complex decision-making processes associated with in-nest reproductive competition. This novel finding therefore adds to our modern conception of cooperation by showing the previously overlooked importance of alternative strategies which enable workers to assert their reproductive interests.

Enforcement of reproductive synchrony via policing in a clonal ant.

In insect societies, worker policing controls genetic conflicts between individuals and increases colony efficiency. However, disentangling relatedness from colony-level effects is usually impossible. We studied policing in the parthenogenetic ant Cerapachys biroi, where genetic conflicts are absent due to clonality and reproduction is synchronized through stereotyped colony cycles. We show that larval cues regulate the cycles by suppressing ovarian activity and that individuals that fail to respond to these cues are policed and executed by their nestmates. These individuals are genetically identical to other colony members, confirming the absence of intracolonial genetic conflicts. At the same time, they bear distinct cuticular hydrocarbon profiles, which could serve as proximate recognition cues for policing. Policing in C. biroi keeps uncontrolled reproduction at bay and thereby maintains the colony-level phenotype. This study shows that policing can enforce adaptive colony-level phenotypes in societies with minimal or no potential genetic conflicts. In analogy to immunosurveillance on cancer cells in genetically homogeneous multicellular organisms, colony efficiency is improved via the control of individuals that do not respond properly to regulatory signals and compromise the functioning of the higher-level unit.

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.

Do host species evolve a specific response to slave-making ants?

BACKGROUND: Social parasitism is an important selective pressure for social insect species. It is particularly the case for the hosts of dulotic (so called slave-making) ants, which pillage the brood of host colonies to increase the worker force of their own colony. Such raids can have an important impact on the fitness of the host nest. An arms race which can lead to geographic variation in host defenses is thus expected between hosts and parasites. In this study we tested whether the presence of a social parasite (the dulotic ant Myrmoxenus ravouxi) within an ant community correlated with a specific behavioral defense strategy of local host or non-host populations of Temnothorax ants. Social recognition often leads to more or less pronounced agonistic interactions between non-nestmates ants. Here, we monitored agonistic behaviors to assess whether ants discriminate social parasites from other ants. It is now well-known that ants essentially rely on cuticular hydrocarbons to discriminate nestmates from aliens. If host species have evolved a specific recognition mechanism for their parasite, we hypothesize that the differences in behavioral responses would not be fully explained simply by quantitative dissimilarity in cuticular hydrocarbon profiles, but should also involve a qualitative response due to the detection of particular compounds. We scaled the behavioral results according to the quantitative chemical distance between host and parasite colonies to test this hypothesis. RESULTS: Cuticular hydrocarbon profiles were distinct between species, but host species did not show a clearly higher aggression rate towards the parasite than toward non-parasite intruders, unless the degree of response was scaled by the chemical distance between intruders and recipient colonies. By doing so, we show that workers of the host and of a non-host species in the parasitized site displayed more agonistic behaviors (bites and ejections) towards parasite than toward non-parasite intruders. CONCLUSIONS: We used two different analyses of our behavioral data (standardized with the chemical distance between colonies or not) to test our hypothesis. Standardized data show behavioral differences which could indicate qualitative and specific parasite recognition. We finally stress the importance of considering the whole set of potentially interacting species to understand the coevolution between social parasites and their hosts.

From speciation to introgressive hybridization: the phylogeographic structure of an island subspecies of termite, Reticulitermes lucifugus corsicus.

BACKGROUND: Although much research has been carried out into European Reticulitermes taxonomy in recent years, there is still much discussion about phylogenetic relationships. This study investigated the evolution from intra- to interspecific phylogeny in the island subspecies Reticulitermes lucifugus corsicus and threw new light on this phenomenon. An integrative approach based on microsatellites and mitochondrial and nuclear DNA sequences was used to analyze samples taken from a wide area around the Tyrrhenian sea and showed how the subspecies evolved from its origins to its most recent form on continental coasts. RESULTS: According to mitochondrial phylogeny and molecular clock calculations, island and continental taxa diverged significantly by vicariance in the Pleistocene glacial period. However, more recently, numerous migrations, certainly human-mediated, affected the structure of the populations. This study provided evidence of direct hybridization and multiple introgressions which occurred in several hybrid areas. Analysis using STRUCTURE based on microsatellite data identified a population in Provence (France) which differed considerably (Fst = 0.477) from populations on the island of Corsica and in Tuscany in the Italian peninsula. This new population, principally distributed in urban areas, is highly heterogeneous especially within the ITS2 regions where homogenization by concerted evolution does not appear to have been completed. CONCLUSION: This study provides an unusual picture of genetic interaction between termite populations in the Tyrrhenian area and suggests that more attention should be paid to the role of introgression and human impact on the recent evolution of European termites.

Individual experience alone can generate lasting division of labor in ants.

Division of labor, the specialization of workers on different tasks, largely contributes to the ecological success of social insects [1, 2]. Morphological, genotypic, and age variations among workers, as well as their social interactions, all shape division of labor [1-12]. In addition, individual experience has been suggested to influence workers in their decision to execute a task [13-18], but its potential impact on the organization of insect societies has yet to be demonstrated [19, 20]. Here we show that, all else being equal, ant workers engaged in distinct functions in accordance with their previous experience. When individuals were experimentally led to discover prey at each of their foraging attempts, they showed a high propensity for food exploration. Conversely, foraging activity progressively decreased for individuals who always failed in the same situation. One month later, workers that previously found prey kept on exploring for food, whereas those who always failed specialized in brood care. It thus appears that individual experience can strongly channel the behavioral ontogeny of ants to generate a lasting division of labor. This self-organized task-attribution system, based on an individual learning process, is particularly robust and might play an important role in colony efficiency.

Learning and discrimination of individual cuticular hydrocarbons by honeybees (Apis mellifera).

In social insect colonies, recognition of nestmates, kinship, caste and reproductive status is crucial both for individuals and for the colony. The recognition cues used are thought to be chemical, with the hydrocarbons found on the cuticle of insects often cited as being particularly important. However, in honeybees (Apis mellifera) the role of cuticular hydrocarbons in nestmate recognition is controversial. Here we use the proboscis extension response (PER) conditioning paradigm to determine how well honeybees learn long-chain linear alkanes and (Z)-alkenes present on the cuticle of worker bees, and also how well they can discriminate between them. We found large differences both in learning and discrimination abilities with the different cuticular hydrocarbons. Thus, the tested hydrocarbons could be classified into those which the bees learnt and discriminated well (mostly alkenes) and those which they did not (alkanes and some alkenes). These well-learnt alkenes may constitute important compounds used as cues in the social recognition processes.

Reassessing the role of the honeybee (Apis mellifera) Dufour's gland in egg marking.

Dufour's gland secretion may allow worker honeybees to discriminate between queen-laid and worker-laid eggs. To investigate this, we combined the chemical analysis of individually treated eggs with an egg removal bioassay. We partitioned queen Dufour's gland into hydrocarbon and ester fractions. The bioassay showed that worker-laid eggs treated with either whole gland extract, ester fraction or synthetic gland esters were removed more slowly than untreated worker-laid eggs. However, the effect only lasted up to 20 h. Worker-laid eggs treated with the hydrocarbon fraction were removed at the same rate as untreated eggs. The amount of ester which reduced the egg removal rate was far higher than that naturally found on queen-laid or worker-laid eggs, and at natural ester levels no effect was found. Our results indicate that esters or hydrocarbons probably do not function as the signal by which eggs can be discriminated.