Influence of queen phenotype, investment and maternity apportionment on the outcome of fights in cooperative foundations of the ant Lasius niger

Social buffer or avoidance depends on the similarity of stress between queen ants

The association of unrelated ant queens (pleometrosis) is supposed to improve nest foundation and competitiveness under environmental stress, but its evolutionary maintenance is difficult to explain because only one of the queens survives after nest foundation. My aim was to test the potential effect of queen association as a social buffer, i.e. as a mechanism reducing stress and improving fitness due to the benefits of social contact. I analysed the survival, fecundity, and behaviour of isolated and paired Lasius flavus queens exposed and not exposed to stressors (disturbing environmental conditions). I found no difference in survivorship between isolated and paired queens or between stressed and unstressed isolated queens. Groups in which one or two paired queens were stressed showed higher mortality. Unstressed queens died similarly to their stressed nestmates, suggesting stress transmission. A trend suggested that paired queens produced eggs more quickly, but eggs were produced similarly between isolated and paired queens. Social avoidance was observed in groups with one stressed and one unstressed queen. However, the groups with two stressed queens showed the expected behaviours according to social buffering: lower mobility and more interindividual inspection. My findings suggest the synergistic effect of pleometrosis and stress and the dependence of stress level similarity between nestmates on social buffering or rejection on ant queens.

Claustral colony founding does not prevent sensitivity to the detrimental effects of azole fungicides on the fecundity of ants

Azole fungicides (benzimidazoles, triazoles and imidazoles) are among the most widely used agrochemicals in the world. Unfortunately, azole fungicides are increasingly recognized for playing the role of endocrine disruptors in non-target organisms. Previously, the fecundity of ants with semi-claustral colony founding was found to be severely decreased in response to field-realistic concentrations of azole fungicides. However, during claustral colony founding, the ant queens do not feed and could therefore be protected against effects of agrochemicals applied during the colony founding. In the present study, we hypothesized that claustral colony founding is associated with a lower risk of oral exposure of ant queens to azole fungicides. We exposed queens of a common farmland ant species with claustral colony founding, Lasius niger, to four azole fungicides (epoxiconazole, flusilazole, prochloraz and thiophanate-methyl) that are commonly used in foliar applications and analyzed the differences in fecundity between fungicide-treated groups and the control water-treated group. We found that oral exposure to all four tested formulations of azole fungicides decreased the fecundity of L. niger queens. The decreases in fecundity ranged from 30.5% (epoxiconazole) to 40.3% (prochloraz), although the concentrations of fungicides used were several times lower than the minimum effective concentrations used to eliminate the target fungi by foliar applications of examined fungicides on various crops. Ants with both claustral and semi-claustral colony founding are highly vulnerable to field-realistic concentrations of azole fungicides that are sprayed in foliar applications. Azole fungicides substantially decrease the fitness of ant queens and may explain part of the recently observed decreases in farmland insect abundance and diversity.

Fecundity determines the outcome of founding queen associations in ants

Animal cooperation evolved because of its benefits to the cooperators. Pleometrosis in ants-the cooperation of queens to found a colony-benefits colony growth, but also incurs costs for some of the cooperators because only one queen usually survives the association. While several traits in queens influence queen survival, they tend to be confounded and it is unclear which factor specifically determines the outcome of pleometrosis. In this study, we used the ant Lasius niger to monitor offspring production in colonies founded by one or two queens. Then, we experimentally paired queens that differed in fecundity but not in size, and vice versa, to disentangle the effect of these factors on queen survival. Finally, we investigated how fecundity and size differed between queens depending on whether they were chosen as pleometrotic partners. Our results indicate that pleometrosis increased and accelerated worker production via a nutritional boost to the larvae. The most fecund queens more frequently survived the associations, even when controlling for size and worker parentage, and queens selected as pleometrotic partners were less fecund. Our results are consistent with fecundity being central to the onset and outcome of pleometrosis, a classic example of cooperation among unrelated animals.

Colony co-founding in ants is an active process by queens

Cooperative breeding may be selected for in animals when, on average, it confers greater benefits than solitary breeding. In a number of eusocial insects (i.e., ants, bees, wasps, and termites), queens join together to co-create new nests, a phenomenon known as colony co-founding. It has been hypothesised that co-founding evolved because queens obtain several fitness benefits. However, in ants, previous work has suggested that co-founding is a random process that results from high queen density and low nest-site availability. We experimentally examined nest-founding behaviour in the black garden ant, Lasius niger. We gave newly mated queens the choice between two empty nesting chambers, and compared their distribution across the two chambers with that expected under random allocation. We found that queens formed associations of various sizes; in most instances, queens group together in a single chamber. Across all experiments, the frequency of larger groups of queens was significantly higher than expected given random assortment. These results indicate colony co-founding in ants may actually be an active process resulting from mutual attraction among queens. That said, under natural conditions, ecological constraints may limit encounters among newly mated queens.

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.

Superorganismal anisogamy: queen-male dimorphism in eusocial insects

Colonies of insects such as ants and honeybees are commonly viewed as 'superorganisms', with division of labour between reproductive 'germline-like' queens and males and 'somatic' workers. On this view, properties of the superorganismal colony are comparable with those of solitary organisms to such an extent that the colony itself can be viewed as a unit analogous to an organism. Thus, the concept of a superorganism can be useful as a guide to thinking about life history and allocation traits of colonies as a whole. A pattern that seems to reoccur in insects with superorganismal societies is size dimorphism between queens and males, where queens tend to be larger than males. It has been proposed that this is analogous to the phenomenon of anisogamy at the level of gametes in organisms with separate sexes; more specifically, it is suggested that this caste dimorphism may have evolved via similar selection pressures as gamete dimorphism arises in the 'gamete competition' theory for the evolution of anisogamy. In this analogy, queens are analogous to female gametes, males are analogous to male gametes, and colony survival is analogous to zygote survival in gamete competition theory. Here, we explore if this question can be taken beyond an analogy, and whether a mathematical model at the superorganism level, analogous to gamete competition at the organism level, may explain the caste dimorphism seen in superorganismal insects. We find that the central theoretical idea holds, but that there are also significant differences between the way this generalized 'propagule competition' theory operates at the levels of solitary organisms and superorganisms. In particular, we find that the theory can explain superorganismal caste dimorphism, but compared with anisogamy evolution, a central coevolutionary link is broken, making the requirements for the theory to work less stringent than those found for the evolution of anisogamy.

Body size variation of the ant Lasius niger along a metal pollution gradient

The phenotypic diversity of ant workers plays a fundamental role in their biology. In this study, we asked if the body size variation of monomorphic workers of the ant Lasius niger (Formicidae) responds adaptively to metal pollution in a post-mining metal-polluted area. Nest samples of workers were collected along a pollution gradient to calculate the within-colony variance in body size (expressed as maximum head width, HW). The results showed that the body size variation of L. niger was unrelated to the pollution index but demonstrated considerable variation between colonies even within the same study site. We suggest that the differences in morphological diversity between the colonies of L. niger could be shaped by colony personality traits, i.e., by colony-specific foraging and/or the feeding efficiency of nursing workers. The study supports previous findings, showing that morphological traits in Lasius ants are weakly related to environmental metal pollution.

The Role of Brood in Eusocial Hymenoptera

Study of social traits in offspring traditionally reflects on interactions in simple family groups, with famous examples including parent-offspring conflict and sibling rivalry in birds and mammals. In contrast, studies of complex social groups such as the societies of ants, bees, and wasps focus mainly on adults and, in particular, on traits and interests of queens and workers. The social role of developing individuals in complex societies remains poorly understood. We attempt to fill this gap by illustrating that development in social Hymenoptera constitutes a crucial life stage with important consequences for the individual as well as the colony. We begin by describing the complex social regulatory network that modulates development in Hymenoptera societies. By highlighting the inclusive fitness interests of developing individuals, we show that they may differ from those of other colony members. We then demonstrate that offspring have evolved specialized traits that allow them to play a functional, cooperative role within colonies and give them the potential power to act toward increasing their inclusive fitness. We conclude by providing testable predictions for investigating the role of brood in colony interactions and giving a general outlook on what can be learned from studying offspring traits in hymenopteran societies.

No evidence for social immunity in co-founding queen associations

Ant queens often associate to found new colonies, yet the benefits of this behaviour remain unclear. A major hypothesis is that queens founding in groups are protected by social immunity and can better resist disease than solitary queens, due to mutual grooming, sharing of antimicrobials, or higher genetic diversity among their workers. We tested this hypothesis by manipulating the number of queens in incipient colonies of Lasius niger and measuring their resistance to the fungal entomopathogen Metarhizium brunneum. We found no evidence for social immunity in associations of founding queens. First, co-founding queens engaged in self-grooming, but performed very little allo-grooming or trophallaxis. Second, co-founding queens did not exhibit higher pathogen resistance than solitary queens, and their respective workers did not differ in disease resistance. Finally, queens founding in groups increased their investment in a component of individual immunity, as expected if they do not benefit from social immunity but respond to a higher risk of disease. Overall, our results provide no evidence that joint colony founding by L. niger queens increases their ability to resist fungal pathogens.

Measuring inotocin receptor gene expression in chronological order in ant queens

In vertebrates and invertebrates, oxytocin/vasopressin-like peptides modulate a variety of behaviors. The recent discovery of the gene and receptor sequences of inotocin, the insect ortholog of oxytocin/vasopressin, opens new opportunities for understanding the role of this peptide family in regulating behaviors in the most populated class of living animals. Ants live in highly organized colonies. Once a year, they produce future queens that soon leave the nest to mate and found new colonies. During the first months of their lives, ant queens display a sequence of behaviors ranging from copulation and social interactions to violent fighting. In order to investigate the potential roles of inotocin in shaping queen behavior, we measured gene expression of the inotocin receptor in the heads of Lasius niger ant queens at different points in time. The highest levels of expression occurred early in queen life when they experience crowded conditions in their mother nests and soon thereafter set out to mate. Inotocin could thus be involved in regulating social and reproductive behaviors as reported in other animals. While oxytocin and vasopressin are also involved in aggression in mammals, we found no direct link between these behaviors and inotocin receptor expression in L. niger. Our study provides a first glimpse into the roles the inotocin receptor might play in regulating important processes in ant physiology and behavior. Further studies are needed to understand the molecular function of this complex signaling system in more detail.

Co-founding ant queens prevent disease by performing prophylactic undertaking behaviour

BACKGROUND

Social insects form densely crowded societies in environments with high pathogen loads, but have evolved collective defences that mitigate the impact of disease. However, colony-founding queens lack this protection and suffer high rates of mortality. The impact of pathogens may be exacerbated in species where queens found colonies together, as healthy individuals may contract pathogens from infectious co-founders. Therefore, we tested whether ant queens avoid founding colonies with pathogen-exposed conspecifics and how they might limit disease transmission from infectious individuals.

RESULTS

Using Lasius niger queens and a naturally infecting fungal pathogen Metarhizium brunneum, we observed that queens were equally likely to found colonies with another pathogen-exposed or sham-treated queen. However, when one queen died, the surviving individual performed biting, burial and removal of the corpse. These undertaking behaviours were performed prophylactically, i.e. targeted equally towards non-infected and infected corpses, as well as carried out before infected corpses became infectious. Biting and burial reduced the risk of the queens contracting and dying from disease from an infectious corpse of a dead co-foundress.

CONCLUSIONS

We show that co-founding ant queens express undertaking behaviours that, in mature colonies, are performed exclusively by workers. Such infection avoidance behaviours act before the queens can contract the disease and will therefore improve the overall chance of colony founding success in ant queens.

Onset of fights and mutual assessment in ant founding queens

In animals, the progress and outcome of contests can be influenced by an individual's own condition, their opponent's condition or a combination of the two. The use of chemical information to assess the quality of rivals has been underestimated despite its central role in the regulation of social interactions in many taxa. Here, we studied pairwise contests between founding queens of the ant Lasius niger to investigate whether the decision to engage in agonistic interactions relies on self-assessment or mutual assessment. Queens modulated their aggressive behaviours depending on both their own status and their opponent's status. We found no influence of lipid stores or size on the onset of fights. However, differences in cuticular chemical signatures linked to fertility status accurately predicted the probability of behaving aggressively in pairs. Our study thus suggests that ant queens could rely on mutual assessment via chemical cues to make informed decisions about fight initiation.

Queen execution increases relatedness among workers of the invasive Argentine ant, Linepithema humile

Polygyny in social insects can greatly reduce within‐nest genetic relatedness. In polygynous ant species, potential rival queens in colonies with multiple queens are often executed by other queens, workers, or both. The Argentine ant, Linepithema humile, native to South America, forms a “supercolony” that is composed of a large number of nests and is considered to contribute to the ant's invasion success. Currently, four mutually antagonistic supercolonies are contiguously distributed within a small area of Japan. Here, we analyzed the genetic structure and relatedness within and among the four supercolonies using microsatellite markers to clarify how L. humile maintains its supercoloniality. The results of AMOVA and BASP, the F_ST values, and the existence of several private alleles indicated that the L. humile population in the Kobe area had a characteristic genetic structure. Within a given supercolony, there was significant genetic differentiation (F_ST) among workers collected in May and those collected in September. The significant deviation from Hardy–Weinberg equilibrium increased, and the relatedness among workers significantly increased from May to September in all supercolonies. This result suggested that the supercolonies replaced old queens with new ones during the reproductive season, thus supporting the plausibility of queen execution. From the perspective of kin selection, workers collectively eliminate queens, thereby increasing their own inclusive fitness. Restricted gene flow among supercolonies, together with mating with sib and queen execution, could help to maintain the unique social structure of L. humile, the distribution of which is expanding worldwide.

Monomorphic ants undergo within-colony morphological changes along the metal-pollution gradient

In ants, intra and inter-colony variation in body size can be considerable, even in monomorphic species. It has been previously shown that size-related parameters can be environmentally sensitive. The shape of the body size distribution curve is, however, rarely investigated. In this study, we measured head widthes of the black garden ant Lasius niger workers using digital methods. The ants were sampled from 51 colonies originating from 19 sites located along a metal pollution gradient, established in a former mining area in Poland. Total zinc concentrations in random samples of small invertebrates were used as a measure of site pollution levels. We found that the skewness of head size distribution grows significantly in line with the pollution level of the site, ranging from values slightly below zero (about -0.5) in the least polluted site up to a positive value (about 1.5) in the most polluted site. This result indicates that the frequency of small ants grows as pollution levels increase. The coefficient of variation, as well as the measures of central tendency, was not related to the pollution level. Four hypotheses explaining the obtained results are proposed. The bias towards the higher frequency of small workers may result from energy limitation and/or metal toxicity, but may also have an adaptive function.

The differential response of workers and queens of the ant Lasius niger to an environment marked by workers: ants dislike the unknown

It is well known that ants can use cuticular hydrocarbons (CHCs) as a specific recognition cue. Most previous studies addressed the perception of CHCs occurring on the cuticle. However, the presence of CHCs in the environment (e.g., on the substrate) and the role of these compounds as a signal cue are less clear. In this study of the ant L. niger, CHCs were extracted from corpses of workers, from the legs of workers and from filter paper marked by workers. Behavioural choice tests were then performed to study the reactions of workers and queens to CHC passively deposited on the substrate by walking workers. A GC-MS analysis of the CHCs revealed a complex mixture of 28 peaks composed of linear alkanes, alkenes and mono-, di- or trimethylalkane. This mixture was consistently present in the three sources tested (corpses of workers, the tarsus of workers and paper marked by walking workers). Nevertheless, discriminant analyses distinguished clearly among the three types of profiles. The results of our behavioural experiments showed that the workers were able to distinguish between marked and unmarked papers but that they exhibited no preferences for different types of marked papers (papers marked by workers from their own colony or papers marked by workers from a different colony). In queens, the perception of marked papers depended on age or physiological state. Just-mated queens did not discriminate between unmarked papers and papers marked by an alien colony. In contrast, the egg-laying queens 3 days after mating significantly favoured the marked papers. We discuss the ecological relevance of the differences in decision-making between castes.

Selfish strategies and honest signalling: reproductive conflicts in ant queen associations

Social insects offer unique opportunities to test predictions regarding the evolution of cooperation, life histories and communication. Colony founding by groups of unrelated queens, some of which are later killed, may select for selfish reproductive strategies, honest signalling and punishment. Here, we use a brood transfer experiment to test whether cofounding queens of the ant Lasius niger 'selfishly' adjust their productivity when sharing the nest with future competitors. We simultaneously analysed queen cuticular hydrocarbon (CHC) profiles to investigate whether queens honestly signal their reproductive output or produce dishonest, manipulative signals, providing a novel test of the evolutionary significance of queen pheromones. Queens produced fewer workers when their colony contained ample brood, but only in the presence of competitors, suggesting selfish conservation of resources. Several CHCs correlated with reproductive maturation, and to a lesser extent with productivity; the same hydrocarbons were more abundant on queens that were not killed, suggesting that workers select productive queens using these chemical cues. Our results highlight the role of honest signalling in the evolution of cooperation: whenever cheaters can be reliably identified, they may incur sanctions that reduce the incentive to be selfish.