Ant Colonies Prefer Infected over Uninfected Nest Sites

During colony relocation, the selection of a new nest involves exploration and assessment of potential sites followed by colony movement on the basis of a collective decision making process. Hygiene and pathogen load of the potential nest sites are factors worker scouts might evaluate, given the high risk of epidemics in group-living animals. Choosing nest sites free of pathogens is hypothesized to be highly efficient in invasive ants as each of their introduced populations is often an open network of nests exchanging individuals (unicolonial) with frequent relocation into new nest sites and low genetic diversity, likely making these species particularly vulnerable to parasites and diseases. We investigated the nest site preference of the invasive pharaoh ant, Monomorium pharaonis, through binary choice tests between three nest types: nests containing dead nestmates overgrown with sporulating mycelium of the entomopathogenic fungus Metarhizium brunneum (infected nests), nests containing nestmates killed by freezing (uninfected nests), and empty nests. In contrast to the expectation pharaoh ant colonies preferentially (84%) moved into the infected nest when presented with the choice of an infected and an uninfected nest. The ants had an intermediate preference for empty nests. Pharaoh ants display an overall preference for infected nests during colony relocation. While we cannot rule out that the ants are actually manipulated by the pathogen, we propose that this preference might be an adaptive strategy by the host to “immunize” the colony against future exposure to the same pathogenic fungus.     

Diverse societies are more productive: a lesson from ants

The fitness consequences of animal personalities (also known as behavioural syndromes) have recently been studied in several solitary species. However, the adaptive significance of collective personalities in social insects and especially of behavioural variation among group members remains largely unexplored. Although intracolonial behavioural variation is an important component of division of labour, and as such a key feature for the success of societies, empirical links between behavioural variation and fitness are scarce. We investigated aggression, exploration and brood care behaviour in Temnothorax longispinosus ant colonies. We focused on two distinct aspects: intercolonial variability and its consistency across time and contexts, and intracolonial variability and its influence on productivity. Aggressiveness was consistent over four to five months with a new generation of workers emerging in between trial series. Other behaviours were not consistent over time. Exploration of novel environments responded to the sequence of assays: colonies were faster in discovering when workers previously encountered opponents in aggression experiments. Suites of correlated behaviours (e.g. aggression-exploration syndrome) present in the first series did not persist over time. Finally, colonies with more intracolonial behavioural variation in brood care and exploration of novel objects were more productive under standardized conditions than colonies with less variation.     

Defending the nest: variation in the alarm aggression response and nest mound damage in the harvester ant Pogonomyrmex occidentalis

A number of studies have found that ant colonies vary in many colony-level phenotypes, including the level of aggression towards non-nestmates. The extent of a colony’s aggression and defense of the nest in response to attacks by predators is likely to affect its survival and reproduction, but the degree to which colonies vary in their defensive response is poorly known. We documented considerable variation in damage to the external nest mound of Pogonomyrmex occidentalis (Cresson) at our long-term study site in 2012. Heavily damaged colonies formed several spatial clusters, consistent with predation within a home range. We tested two competing hypotheses for the variation in nest damage: (1) colonies vary in their level of aggression, those with stronger defensive responses are better protected, and suffer less damage, versus (2) colonies have similar levels of aggression, but those that suffer predator-induced damage subsequently display a stronger defense response. We measured the alarm/aggression response in colonies exposed to a standardized stimulus and determined whether the level of aggression was correlated to the level of damage. Colonies with strong defensive responses were significantly less likely to exhibit damage than those with weak responses, suggesting that aggression level is a colony phenotype.     

Individual and Group Personalities Characterise Consensus Decision‐Making in an Ant

Non‐human animals can exhibit idiosyncratic behaviour across individuals in much in the same way as humans. Animals with specific personalities may have advantages in some environments, and this idiosyncrasy may thus be of considerable ecological and evolutionary importance. In group‐living organisms, personality can occur at the level of the group as well as that of the individual. However, at present, we have very little understanding of the possible benefits of group‐level personality, and how this is linked with individual personality. In this study, I examine the influence of individual and group personality during the process of colony migration in the Japanese ant, Myrmecina nipponica. These ants use a consensus decision process to decide among alternatives when searching for a new home. Individuals contribute to this process by scouting for new nest sites, recruiting nestmates by laying pheromone trails, and carrying brood to the new site, although whether these roles are consistent among individuals and how roles are distributed within and between colonies remain unclear. Individual contributions to the nest‐site selection process were quantified over five repeated relocations in five colonies. Results demonstrate that contributions to the relocation effort were highly skewed within the colonies and that individuals were consistent in their contributions over repeated relocation events. Furthermore, the distribution of effort differed between colonies, indicating that intercolony differences in composition of behavioural types resulted in colony‐level personality. While these differences did not lead to any detectable difference in relocation performance between colonies in the simple experimental arrangement used, colony personality could influence decision outcomes in more complex environments.     

Frequent nest relocation in the ant Aphaenogaster araneoides: resources, competition, and natural enemies

Sessile and vagile organisms differ from one another in some fundamental ways, including methods of resource acquisition and competition. Ant colonies are typically studied as sessile entities, even though a large fraction of ant species frequently relocate their nests in the course of their life history. Little is known about the causes and consequences of nest relocation, but it is likely that the costs and benefits of relocation are driven by nest quality, neighborhood competition, or resource availability. In this paper, we document several cycles of nest relocation in a population of the Central American ant Aphaenogaster araneoides . In our first experiment, we tracked the pattern of relocation, testing whether environmental characteristics and colony demography were associated with relocation behavior. In our second experiment, we manipulated resource availability by adding or subtracting leaf litter, which is known to predict colony growth. We found that colonies relocated their nests once per week on average and colonies often reoccupied nests from which they had once emigrated. Larger colonies relocated more frequently than smaller colonies, and quickly growing colonies utilized a greater number of nests within their home range compared to slowly growing colonies. Relocation events were most likely to occur in periods when vapor pressure deficits were greatest. Nearest neighbor distance and other measures of environmental conditions were not associated with relocation behavior and there was no significant effect of litter removal or supplementation. We found evidence that multiple natural enemies attacked A. araneoides colonies. Based on the demographic correlates of relocation and our rejection of other plausible hypotheses, we propose that nest relocation is driven by the escape from natural enemies.     

Colony defense in Damaraland mole-rats, Cryptomys damarensis

I examined defense against conspecific intruders in coloniesof the Damaraland mole-rat. I introduced foreign animals tocaptive colonies and examined the effect of the sex and breedingstatus of the intruder on the sex and breeding status of thosethat defended. Intruding mole-rats were almost always attacked.Colony defense against intruders was carried out almost exclusivelyby dominant, reproductive individuals and was directed primarily toward individuals of the same sex. Nonbreeding, subordinatemole-rats participated only rarely. These results contrastsharply with colony defense in the related naked mole-rat andsuggest that investment in defense activities in Damaralandmole-rats primarily reflects repulsion of potential competitorsfor breeding status. Subordinate mole-rats may lack incentiveto participate in potentially costly defense, as they are unlikelyto gain reproductive opportunities within the colony and probablygain fitness primarily through dispersing to breed.     

Fitness in invasive social wasps: the role of variation in viral load,immune response and paternity in predicting nest size and reproductive output

Within any one habitat, the relative fitness of organisms in a population can vary substantially. Social insects like the common wasp are among the most successful invasive animals, but show enormous variation in nest size and other fitness‐related traits. Some of this variation may be caused by pathogens such as viruses that can have serious consequences in social insects, which range from reduced productivity to colony death. Both individual immune responses and colony‐level traits such as genetic diversity are likely to influence effects of pathogen infections on colony fitness. Here we investigate how infections with Kashmir Bee Virus (KBV), immune response and intracolony genetic diversity (due to queen polyandry) affect nest size in the invasive common wasp Vespula vulgaris. We show that KBV is highly prevalent in wasps and expression of antiviral immune genes is significantly increased with higher viral loads across individuals. Patriline membership within a nest did not influence KBV susceptibility or immune response. A permutational MANCOVA revealed that polyandry, viral load and expression of the immune gene Dicer were significant predictors of variation in nest size. High intracolony genetic diversity due to polyandry has previously been hypothesized to improve colony‐level resistance to parasites and pathogens. Consistent with this hypothesis, we observed genetically diverse colonies to be significantly larger and to produce more queens, although this effect was not driven by the pathogen we investigated. Invasive wasps clearly suffer from pathogens and expend resources, as indicated here by elevated immune gene expression, toward reducing pathogen‐impact on colony fitness.     

The police are not the army: context-dependent aggressiveness in a clonal ant

Animals often exhibit particular ‘personalities’, i.e. their behaviour is correlated across different situations. Recent studies suggest that this limitation of behavioural plasticity may be adaptive, since continuous adjustment of one''s behaviour may be time-consuming and costly. In social insects, particularly aggressive workers might efficiently take over fighting in the contexts of both nest defence and ‘policing’, i.e. the regulation of kin conflict in the society. Here, we examine whether workers who engage in aggressive policing in the ant Platythyrea punctata play a prominent role also in nest defence against intruders. The participation of individuals in policing and nest defence was highly skewed and a minority of workers exhibited most of the aggression. Workers who attacked reproductives after experimental colony fusion were less active during nest defence and vice versa. This suggests that workers show situation-dependent behavioural plasticity rather than consistently aggressive personalities.     

Divide and conquer: Multicolonial structure,nestmate recognition,and antagonistic behaviors in dense populations of the invasive ant Brachymyrmex patagonicus

The ecological success of ants has made them abundant in most environments, yet inter‐ and intraspecific competition usually limit nest density for a given population. Most invasive ant populations circumvent this limitation through a supercolonial structure, eliminating intraspecific competition through a loss of nestmate recognition and lack of aggression toward non‐nestmates. Native to South America, Brachymyrmex patagonicus has recently invaded many locations worldwide, with invasive populations described as extremely large and dense. Yet, in contrast with most invasive ants, this species exhibits a multicolonial structure, whereby each colony occupies a single nest. Here, we investigated the interplay between genetic diversity, chemical recognition, and aggressive behaviors in an invasive population of B. patagonicus. We found that, in its invasive range, this species reaches a high nest density with individual colonies located every 2.5 m and that colony boundaries are maintained through aggression toward non‐nestmates. This recognition and antagonism toward non‐nestmates is mediated by chemical differentiation between colonies, as different colonies exhibit distinct chemical profiles. We highlighted that the level of aggression between colonies is correlated with their degree of genetic difference, but not their overall chemical differentiation. This may suggest that only a few chemical compounds influence nestmate recognition in this species or that weak chemical differences are sufficient to elicit aggression. Overall, this study demonstrates that invasive ant populations can reach high densities despite a multicolonial structure with strong aggression between colonies, raising questions about the factors underlying their ecological success and mitigating negative consequences of competitive interactions.     

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.

     

Nest relocation in the slavemaking ants Formica subintegra and Formica pergandei: a response to host nest availability that increases raiding success

Social insects typically occupy spatially fixed nests which may thus constrain their mobility. Nevertheless, colony movements are a frequent component of the life cycle of many social insects, particularly ants. Nest relocation in ants may be driven by a variety of factors, including nest deterioration, seasonality, disturbances, changes in microclimate, and local depletion of resources. The colony movements of slavemaking ants have been noted anecdotally, and in recent studies such relocations were primarily attributed to nest deterioration or shifts to overwintering locations. In this study we explore nest relocations in large colonies of formicine slavemakers which occupy stable and persistent earthen nest mounds. We investigate the hypothesis that colony relocations of these slavemakers are best explained by efforts to improve raiding success by seeking areas of higher host availability. Five summers of monitoring the raiding behavior of 11–14 colonies of the slavemakers Formica subintegra and Formica pergandei revealed relatively frequent nest relocations: of 14 colonies that have been tracked for at least three of 5 years, all but one moved at least once by invading existing host nests. Movements tended to occur in the middle of the raiding season and were typically followed by continued raiding of nearby host colonies. Spatial patterns of movements suggest that their purpose is to gain access to more host colonies to raid: the distance moved is typically farther than the mean raiding distance before the move, which may indicate an effort to escape their local neighborhood. Furthermore, the mean distance of raids after relocation is shorter than the distance before relocation. For many slavemaking ant colonies, particularly those on the verge of relocating, raiding distance increased as the raiding season progressed. In addition, movements tended to be toward areas of higher local host density. Nest relocation is likely an important component of the ecology of slavemaking ants that contributes to the dynamic nature of their interaction with the host ant population.     

Differential Response of Ant Colonies to Intruders: Attack Strategies Correlate With Potential Threat

Animals are often threatened by predators, parasites, or competitors, and attacks against these enemies are a common response, which can help to remove the danger. The costs of defense are complex and involve the risk of injury, the loss of energy/time, and the erroneous identification of a friend as a foe. Our goal was to study the specificity of defense strategies. We analyzed the aggressive responses of ant colonies by confronting them with workers of an unfamiliar congeneric species, a non‐nestmate conspecific, a co‐occurring congeneric competitor species, and a social parasite—a slave‐making ant. As expected, the latter species, which can inflict dramatic fitness losses to the colony, was treated with most aggression. A co‐occurring competitor was also attacked, but the ants used different behaviors in their responses to both enemies. While the slavemaker was attacked by biting and stinging and was approached with spread mandibles, the competitor was dragged, a behavioral strategy only possible if the defending ant is similar in size and strength to the opponent. Non‐nestmate conspecifics were treated aggressively as well, but less than the slavemaker and the co‐occurring competitor, presumably because they are less easily recognized as enemies. An unfamiliar congeneric species was rarely attacked. This first detailed study comparing the aggressive responses of ant colonies toward slave‐making ants to other species posing different threats indicates that the responses of ant colonies are adjusted to the risk each opponent poses to the colony.     

Spatial Aspects of Corpse Removal in the Western Harvester Ant, <Emphasis Type="Italic">Pogonomyrmex occidentalis</Emphasis>

Eusocial insects actively combat pathogen proliferation with a myriad of tactics, one of which is the removal of corpses from the nest, a behavior known as necrophoresis. Spatial patterns of corpse depositions by colonies of the western harvester ant, Pogonomyrmex occidentalis, were examined. Colonies were presented with nestmate and non-nestmate corpses to discern if each type of waste was handled differently. Specialized areas for corpse disposal were not observed. Non-nestmate corpses were carried farther from the nest than were nestmate corpses, perhaps reducing the chance of introduction of pathogens new to the colony that may be harbored by a non-nestmate. Factors external to the nest mound, such as slope and neighboring colonies, had no perceptible effect on these depositions and did not change the uniformity of dispersal of this particular waste. These findings add to our knowledge of the intricacies of corpse removal in eusocial insects and suggest that this is a more dynamic activity than previously thought.     

The parasite's long arm: a tapeworm parasite induces behavioural changes in uninfected group members of its social host

Parasites can induce alterations in host phenotypes in order to enhance their own survival and transmission. Parasites of social insects might not only benefit from altering their individual hosts, but also from inducing changes in uninfected group members. Temnothorax nylanderi ant workers infected with the tapeworm Anomotaenia brevis are known to be chemically distinct from nest-mates and do not contribute to colony fitness, but are tolerated in their colonies and well cared for. Here, we investigated how tapeworm- infected workers affect colony aggression by manipulating their presence in ant colonies and analysing whether their absence or presence resulted in behavioural alterations in their nest-mates. We report a parasite-induced shift in colony aggression, shown by lower aggression of uninfected nest-mates from parasitized colonies towards conspecifics, potentially explaining the tolerance towards infected ants. We also demonstrate that tapeworm-infected workers showed a reduced flight response and higher survival, while their presence caused a decrease in survival of uninfected nest-mates. This anomalous behaviour of infected ants, coupled with their increased survival, could facilitate the parasites'' transmission to its definitive hosts, woodpeckers. We conclude that parasites exploiting individuals that are part of a society not only induce phenotypic changes within their individual hosts, but in uninfected group members as well.     

Social life-history response to individual immune challenge of workers of Bombus terrestris L.: a possible new cooperative phenomenon

Parasites typically reduce host survival or fecundity. To minimize fitness loss, hosts can make temporal adjustments of their reproductive effort. To date such plastic shifts of life‐history traits in response to parasitism are only known from solitary organisms where infected individuals can react by themselves. In the case of social insects, where brood care and reproductive effort is shared between reproductive individuals (typically the queen) and workers, adjustments of the reproductive effort would depend on collective decision‐making. We tested for this possibility by experimentally activating the immune response of individual workers in colonies of the bumblebee, Bombus terrestris L. This induction resulted, in combination with environmental conditions, in a reduction of fitness of the social unity (i.e. colony success, measured by number and biomass of offspring) and a collective response towards earlier reproduction. As both phenomena are expressed at the level of the colony, the result suggests that key elements of the use of immune defence have been maintained through the evolutionary transition to sociality.     

Dynamics of colony emigration in the ant Aphaenogaster senilis

In many social insect species, colonies frequently emigrate to a new nest. This requires the coordination of many individuals, and it puts the queen at risks of being lost or predated. We experimentally studied colony emigration in the ant Aphaenogaster senilis, who emigrates frequently and obligatorily reproduces by colony fission. As in other species, colony emigration was characterised by a synchronised relocation of workers. Foragers found the new nest site and triggered the relocation of the “inside” workers, which built up following a sigmoid curve. Unlike in Temnothorax, where workers are transported to the new nest, most individuals relocated by walking. The brood was transported around the middle of colony relocation, mostly by “inside” workers because they represent most of the workforce. The queen walked to the new nest at the middle of colony relocation, when the flow of ants to the new nest was maximal. Overall, this temporal dynamic of colony emigration is similar to that observed in other species. However, we argue that species-specific traits, such as whether workers are transported to the new nest or relocate by themselves, may affect parts of the process of colony emigration.     

Nest defense behavior against ant attacks in post-emergent colonies of wasp Mischocyttarus cerberus (Hymenoptera, Vespidae)

The primitive eusocial wasp Mischocyttarus cerberus forms colonies of independent foundation, without morphological differentiation among castes. Ants are natural enemies of the social wasps and defending the wasps’ nests involves chemical and active defense strategies. The aims of this work were to verify the kind of defense the wasps use most frequently in post-emergent colonies of M. cerberus. We also observed whether the nest was abandoned during the ant attack and whether any relationship existed between the forms that colony defense took and the number of adults, the number of cells, and the number of immature wasps. The study was carried out on the campus of Universidade Estadual Paulista of Rio Claro, São Paulo, Brazil. The 23 nests under study were mapped weekly, and 68 bioassays were performed by simulating ant attacks against the nests. The results showed that wasps used both active and chemical strategies for nest defense, and the PCA analysis showed that the aggressive behaviors of biting the ant, wings vibrating, gaster hitting, and abdomen pumping were the dominant terms; the PCA correlation values were 2.70, 2.54, 1.64, and 1.63, respectively. The colonies in pre- and post-male substages with few immature wasps and the nests in post-male substage with one adult were more correlated with the nonaggressive behaviors of hiding, staying immobile, and flying; their PCA correlation values were 3.12, 2.56, and 1.77, respectively. These results show that the number of immature wasps is an important factor in the kind and in the intensity of the defense behavior against ant attacks.     

Defending the Weak: Parental Defense Peaks When Chick Vulnerability is Greatest in the Herring Gull (Larus argentatus)

Successful reproduction often depends upon parents providing offspring with resources and protection. In birds, reproductive success can often be enhanced by parents engaging in antipredator behaviors, but these behaviors can be costly. Theoretically, individuals should temporally modify the intensity of nest defense behavior to balance the costs and benefits of current and future reproductive success. More specifically, nest defense should vary throughout a nesting attempt to maximize fitness of the adults. Here, we consider the relationship between nest defense behavior and chick vulnerability in the herring gull (Larus argentatus), where chicks are under high predation risk. We estimated chick vulnerability by quantifying survival probabilities at different periods of the nestling stage. Simultaneously, we quantified changes in parental aggression throughout the nesting cycle by simulating predation attempts using a human predator model. We found that chick survival probabilities were lowest (i.e., vulnerability was highest) and parental aggression in nest defense was greatest during the first 10 days after hatching. Thus, we show that parents are most defensive when chicks are most vulnerable and that adults optimize nest defense behaviors in a way that maximizes their fitness.     

Colony variation in the collective regulation of foraging by harvester ants

This study investigates variation in collective behavior in a natural population of colonies of the harvester ant, Pogonomyrmex barbatus. Harvester ant colonies regulate foraging activity to adjust to current food availability; the rate at which inactive foragers leave the nest on the next trip depends on the rate at which successful foragers return with food. This study investigates differences among colonies in foraging activity and how these differences are associated with variation among colonies in the regulation of foraging. Colonies differ in the baseline rate at which patrollers leave the nest, without stimulation from returning ants. This baseline rate predicts a colony's foraging activity, suggesting there is a colony-specific activity level that influences how quickly any ant leaves the nest. When a colony's foraging activity is high, the colony is more likely to regulate foraging. Moreover, colonies differ in the propensity to adjust the rate of outgoing foragers to the rate of forager return. Naturally occurring variation in the regulation of foraging may lead to variation in colony survival and reproductive success.     

Collective exploration and area marking in the ant Lasius niger

Summary: The aphid tending ant species, Lasius niger collectively explore new areas. Scouts are mobilised within the first five minutes of exploration with activity thereafter decreasing and stabilising after 40 minutes. Collective path choices of colonies underline the existence of chemical recruitment and area marking during exploration. To identify the territorial or home-range role of exploratory marking, we examined its influence on agonistic behaviour during intraspecific encounters. Ten nestmates and ten alien conspecifics were confronted: (i) on a paper marked by their own colony, (ii) on a paper marked by the alien colony or (iii) on a paper not chemically marked by ants. Residents and intruders showed a weak level of aggression and behaved similarly whatever the type of area marking. As no colony-specificity was observed in the odds of eliciting aggression, exploratory area marking in Lasius niger seems to be rather a home-range than a true territorial marking. We discuss the role of collective exploration, home-range marking and intraspecific interactions in the structuring and use of foraging space by neighbour Lasius niger colonies.