Blending of heritable recognition cues among ant nestmates creates distinct colony gestalt odours but prevents within‐colony nepotism

The evolution of sociality is facilitated by the recognition of close kin, but if kin recognition is too accurate, nepotistic behaviour within societies can dissolve social cohesion. In social insects, cuticular hydrocarbons act as nestmate recognition cues and are usually mixed among colony members to create a Gestalt odour. Although earlier studies have established that hydrocarbon profiles are influenced by heritable factors, transfer among nestmates and additional environmental factors, no studies have quantified these relative contributions for separate compounds. Here, we use the ant Formica rufibarbis in a cross‐fostering design to test the degree to which hydrocarbons are heritably synthesized by young workers and transferred by their foster workers. Bioassays show that nestmate recognition has a significant heritable component. Multivariate quantitative analyses based on 38 hydrocarbons reveal that a subset of branched alkanes are heritably synthesized, but that these are also extensively transferred among nestmates. In contrast, especially linear alkanes are less heritable and little transferred; these are therefore unlikely to act as cues that allow within‐colony nepotistic discrimination or as nestmate recognition cues. These results indicate that heritable compounds are suitable for establishing a genetic Gestalt for efficient nestmate recognition, but that recognition cues within colonies are insufficiently distinct to allow nepotistic kin discrimination.     

Nest and colony-specific spectra in the weaver ant Oecophylla smaragdina

Animals in social groups need to differentiate between group members and others. In very large groups, such as those formed by many ant species, it is not possible to rely on individually specific cues to identify colonymates. Instead, recognition must be based on the colony-specific cues. Individual ant colonies tend to have a specific chemical gestalt that is maintained by the continual exchange of chemicals between workers. In very large polydomous colonies, the exchange of chemicals may be limited between nests within the colony, resulting in inter-nest variation in colony odour that might hinder identification of colonymates or conspecific intruders. We used near-infrared spectroscopy to explore variation in the chemical profile between and within colonies of the weaver ant Oecophylla smaragdina. We found that differences between colonies were reflected in the position, amplitude and width of spectral peaks, while differences between nests within colonies were reflected mainly in amplitude. Furthermore, in the context of colonymate recognition, the behaviour of the ants themselves was positively correlated with colony-specific spectral characteristics, rather than with nest-specific characteristics. Thus, colony spectra have features that are not obscured by intra-colonial variation and may potentially encode the chemical characteristics used by workers to identify colonymates.     

Disentangling environmental and heritable nestmate recognition cues in a carpenter ant

Discriminating between group members and strangers is a key feature of social life. Nestmate recognition is very effective in social insects and is manifested by aggression and rejection of alien individuals, which are prohibited to enter the nest. Nestmate recognition is based on the quantitative variation in cuticular hydrocarbons, which can include heritable cues from the workers, as well as acquired cues from the environment or queen-derived cues. We tracked the profile of six colonies of the ant Camponotus aethiops for a year under homogeneous laboratory conditions. We performed chemical and behavioral analyses. We show that nestmate recognition was not impaired by constant environment, even though cuticular hydrocarbon profiles changed over time and were slightly converging among colonies. Linear hydrocarbons increased over time, especially in queenless colonies, but appeared to have weak diagnostic power between colonies. The presence of a queen had little influence on nestmate discrimination abilities. Our results suggest that heritable cues of workers are the dominant factor influencing nestmate discrimination in these carpenter ants and highlight the importance of colony kin structure for the evolution of eusociality.     

Task-Related Variation of Cuticular Hydrocarbon Profiles Affect Nestmate Recognition in the Giant ant Dinoponera quadriceps

Recognition seems to be one of the more remarkable characteristics of social groups. In social insects, cuticular hydrocarbons are important for colonial recognition by providing a chemical signature for colony members. The acceptance threshold model predicts that colony members will accept conspecifics when the levels of nest mate cues dissimilarities are below the acceptance threshold. We tested the hypothesis that the encounter of a guard ant worker with a nurse may cause a delay in the process of recognition, because nurses from distinct colonies may share greater amount of chemical compounds. Dinoponera quadriceps guard workers decreased their effectiveness to recognize nurses rather than did to foragers. Alien foragers received significantly more bites and other stronger acts than non-nestmate nurses when they were experimentally introduced. In addition, guards took significantly more time to react against non-nestmate nurses than against alien foragers. Analysis of the cuticular hydrocarbon profiles corroborated our behavioural analysis that nurses from distinct colonies overlap greater amount of cuticular hydrocarbons.     

Experimentally increased group diversity improves disease resistance in an ant species

A leading hypothesis linking parasites to social evolution is that more genetically diverse social groups better resist parasites . Moreover, group diversity can encompass factors other than genetic variation that may also influence disease resistance. Here, we tested whether group diversity improved disease resistance in an ant species with natural variation in colony queen number. We formed experimental groups of workers and challenged them with the fungal parasite Metarhizium anisopliae . Workers originating from monogynous colonies (headed by a single queen and with low genetic diversity) had higher survival than workers originating from polygynous ones, both in uninfected groups and in groups challenged with M. anisopliae . However, an experimental increase of group diversity by mixing workers originating from monogynous colonies strongly increased the survival of workers challenged with M. anisopliae , whereas it tended to decrease their survival in absence of infection. This experiment suggests that group diversity, be it genetic or environmental, improves the mean resistance of group members to the fungal infection, probably through the sharing of physiological or behavioural defences.     

Recognition in ants: social origin matters

The ability of group members to discriminate against foreigners is a keystone in the evolution of sociality. In social insects, colony social structure (number of queens) is generally thought to influence abilities of resident workers to discriminate between nestmates and non-nestmates. However, whether social origin of introduced individuals has an effect on their acceptance in conspecific colonies remains poorly explored. Using egg-acceptance bioassays, we tested the influence of social origin of queen-laid eggs on their acceptance by foreign workers in the ant Formica selysi. We showed that workers from both single- and multiple-queen colonies discriminated against foreign eggs from single-queen colonies, whereas they surprisingly accepted foreign eggs from multiple-queen colonies. Chemical analyses then demonstrated that social origins of eggs and workers could be discriminated on the basis of their chemical profiles, a signal generally involved in nestmate discrimination. These findings provide the first evidence in social insects that social origins of eggs interfere with nestmate discrimination and are encoded by chemical signatures.     

Social context predicts recognition systems in ant queens

Recognition of group‐members is a key feature of sociality. Ants use chemical communication to discriminate nestmates from intruders, enhancing kin cooperation and preventing parasitism. The recognition code is embedded in their cuticular chemical profile, which typically varies between colonies. We predicted that ants might be capable of accurate recognition in unusual situations when few individuals interact repeatedly, as new colonies started by two to three queens. Individual recognition would be favoured by selection when queens establish dominance hierarchies, because repeated fights for dominance are costly; but it would not evolve in absence of hierarchies. We previously showed that Pachycondyla co‐founding queens, which form dominance hierarchies, have accurate individual recognition based on chemical cues. Here, we used the ant Lasius niger to test the null hypothesis that individual recognition does not occur when co‐founding queens do not establish dominance hierarchies. Indeed, L. niger queens show a similar level of aggression towards both co‐foundresses and intruders, indicating that they are unable of individual recognition, contrary to Pachycondyla. Additionally, the variation in chemical profiles of Lasius and Pachycondyla queens is comparable, thus informational constraints are unlikely to apply. We conclude that selection pressure from the social context is of crucial significance for the sophistication of recognition systems.     

Lack of inbreeding avoidance in the Argentine ant Linepithema humile

Although workers might increase their inclusive fitness by favoringcloser over more distant kin, evidence suggest that nepotismgenerally does not occur within colonies of social insects.It has been suggested that this may be due to the cost of recognitionerrors. We tested whether recognition occurs in a system wherea better than random ability to recognize kin should be selected for. Using DNA microsatellites, we show that sexuals of theArgentine ant Linepithema humile fail to use genetic cues toavoid sib-mating. When offspring of two queens were allowedto mate, the percentage of matings among siblings was not significantlylower than expected under the hypothesis of random mating.The finding that sexuals fail to use genetic cues to avoid sib-matings cannot be attributed to the cost of recognitionerrors because any recognition system that would lead to abetter than random ability to avoid sib-mating should be selectedfor when there are costs to inbreeding. These data are thusconsistent with the view that kin recognition mediated solelyby genetic cues might be intrinsically error prone within coloniesof social insects.     

Are you my mother? Kin recognition in the ant Formica fusca

In social insects, workers trade personal reproduction for indirect fitness returns from helping their mother rear collateral kin. Colony membership is generally used as a proxy for kin discrimination, but the question remains whether recognition allows workers to discriminate between kin and nonkin regardless of colony affiliation. We investigated whether workers of the ant Formica fusca can identify their mother when fostered with their mother, their sisters, a hetero‐colonial queen or hetero‐colonial workers. We found that workers always displayed less aggression towards both their mother and their foster queen, as compared to an unfamiliar hetero‐colonial queen. In support of this finding, workers maintain their colony hydrocarbon profile regardless of foster regime, yet show modifications when exposed to different environments. This indicates that recognition entails environmental and genetic components, which allow both discrimination of kin in the absence of prior contact and learning of recognition cues based on group membership.     

Sources of nestmate recognition cues in the imported fire ant Solenopsis invicta buren (Hymenoptera: Formicidae)

Aggression bioassays were conducted to investigate nestmate (worker-worker) recognition in monogyne colonies of the imported fire ant. Environmentally correlated cues dominated the recognition cue hierarchy of laboratory and field colonies, anddiet alone significantly modified recognition labels and templates of laboratory-reared workers. ‘Discriminators’ associated with worker genotype also affected recognition, but ‘queen discriminators’ did not significantly affect either labels or templates of laboratoryreared workers exposed as adults to alien queens for 28 days. Factors contributing to the importance of environmentally derived recognition cues in this species and the potential implications of such cues for the formation of polygynous fire ant populations are discussed.     

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.     

Cleptoparasites, social parasites and a common host: Chemical insignificance for visiting host nests, chemical mimicry for living in

Social insect colonies contain attractive resources for many organisms. Cleptoparasites sneak into their nests and steal food resources. Social parasites sneak into their social organisations and exploit them for reproduction. Both cleptoparasites and social parasites overcome the ability of social insects to detect intruders, which is mainly based on chemoreception. Here we compared the chemical strategies of social parasites and cleptoparasites that target the same host and analyse the implication of the results for the understanding of nestmate recognition mechanisms. The social parasitic wasp Polistes atrimandibularis (Hymenoptera: Vespidae), and the cleptoparasitic velvet ant Mutilla europaea (Hymenoptera: Mutillidae), both target the colonies of the paper wasp Polistes biglumis (Hymenoptera: Vespidae). There is no chemical mimicry with hosts in the cuticular chemical profiles of velvet ants and pre-invasion social parasites, but both have lower concentrations of recognition cues (chemical insignificance) and lower proportions of branched alkanes than their hosts. Additionally, they both have larger proportions of alkenes than their hosts. In contrast, post-invasion obligate social parasites have proportions of branched hydrocarbons as large as those of their hosts and their overall cuticular profiles resemble those of their hosts. These results suggest that the chemical strategies for evading host detection vary according to the lifestyles of the parasites. Cleptoparasites and pre-invasion social parasites that sneak into host colonies limit host overaggression by having few recognition cues, whereas post-invasion social parasites that sneak into their host social structure facilitate social integration by chemical mimicry with colony members.     

Contact between supercolonies elevates aggression in Argentine ants

Complex recognition systems underlie the social organization of many organisms. In social insects the acceptance of other individuals as nestmates can involve a variety of different cues, but the relative importance of these cues can change in relation to the fitness costs of accepting or rejecting other individuals. In this study we investigate the mechanisms that underlie recognition behaviour in Argentine ants (Linepithema humile). Introduced populations of Argentine ants are characterized by a social structure known as unicoloniality where intraspecific aggression is absent over large distances resulting in the formation of expansive supercolonies. Recent research has identified sites where multiple, mutually aggressive supercolonies co-occur allowing an examination of Argentine ant behaviour at territorial boundaries. We found that workers from different supercolonies always interact aggressively with one another, but that neighbours from different colonies (i.e., workers from nests located in the immediate vicinity of territory borders) consistently exhibited higher levels of aggression compared to those displayed by non-neighbours from different colonies (i.e., workers from nests located far enough away from a territory border so that interactions are unlikely). This difference in the level of aggression displayed between neighbours and between non-neighbours from different supercolonies cannot be explained by differences in relatedness or genetic similarity. Instead our findings suggest that direct contact between mutually antagonistic colonies is sufficient to elevate aggression. A laboratory experiment in which we manipulated the extent to which colonies with no prior history of contact could interact with one another, revealed that aggression increased after colonies were permitted to interact, but dropped after connections between colonies were severed. Moreover, the mere presence of an aggressive supercolony was sufficient to elicit elevated aggression. Overall these patterns are opposite to the “dear enemy” phenomenon and could be the result of the intense territorial aggression exhibited by established supercolonies of this species. Received 8 January 2007; revised 27 March 2007; accepted 28 March 2007.     

Heterogeneous activity causes a nonlinear increase in the group energy use of ant workers isolated from queen and brood

Increasing evidence has shown that the energy use of ant colonies increases sublinearly with colony size so that large colonies consume less per capita energy than small colonies. It has been postulated that social environment (e.g., in the presence of queen and brood) is critical for the sublinear group energetics, and a few studies of ant workers isolated from queens and brood observed linear relationships between group energetics and size. In this paper, we hypothesize that the sublinear energetics arise from the heterogeneity of activity in ant groups, that is, large groups have relatively more inactive members than small groups. We further hypothesize that the energy use of ant worker groups that are allowed to move freely increases more slowly than the group size even if they are isolated from queen and brood. Previous studies only provided indirect evidence for these hypotheses due to technical difficulties. In this study, we applied the automated behavioral monitoring and respirometry simultaneously on isolated worker groups for long time periods, and analyzed the image with the state‐of‐the‐art algorithms. Our results show that when activity was not confined, large groups had lower per capita energy use, a lower percentage of active members, and lower average walking speed than small groups; while locomotion was confined, however, the per capita energy use was a constant regardless of the group size. The quantitative analysis shows a direct link between variation in group energy use and the activity level of ant workers when isolated from queen and brood.     

Low intraspecific aggression level in the polydomous and facultative polygynous ant Ectatomma tuberculatum

Nestmate recognition is a key feature of social insects, as it preserves colony integrity. However, discrimination of non‐nestmates and nestmate recognition mechanisms are highly variable according to species and social systems. Here, we investigated the intraspecific level of aggression in the facultative polygynous and polydomous ant, Ectatomma tuberculatum Olivier (Hymenoptera: Formicidae: Ectatomminae), in a population with a strong genetic structure. We found that the intraspecific level of aggression was generally low in this population of E. tuberculatum. However, the level of aggression was significantly correlated with the geographical distance, suggesting that both genetic and environmental cues could be involved in nestmate recognition and discrimination mechanisms. Moreover, polydomy was confirmed by the absence of aggression between workers from nests at a distance of 3 m, while the level of aggression was significantly higher between workers from nests separated by a distance of 10 or 1300 m. Field experiments showed that the low level of aggression between neighbouring colonies was associated with shared foraging areas, which could suggest that familiarization processes may occur in this species. We propose that the particular social organization of this species, with secondary polygyny, polydomy, and budding, may have favoured a high acceptance threshold, because of the low probability of interactions with unrelated conspecifics, the high cost of erroneously rejecting nestmates, and the low cost of accepting non‐nestmate workers. The resulting open recognition system can thus allow privileged relationships between neighbouring colonies and promote the ecological dominance of E. tuberculatum in the mosaic of arboreal ants.     

The role of cuticular hydrocarbons as chemical cues for nestmate recognition in the invasive Argentine ant (<Emphasis Type="Italic">Linepithema humile</Emphasis>)

Argentine ants (Linepithema humile) in their native South American range, like most other ant species, form spatially restricted colonies that display high levels of aggression toward other such colonies. In their introduced range, Argentine ants are unicolonial and form massive supercolonies composed of numerous nests among which territorial boundaries are absent. Here we examine the role of cuticular hydrocarbons (CHCs) in nestmate recognition of this highly damaging invasive ant using three supercolonies from its introduced range. We conducted behavioral assays to test the response of Argentine ants to workers treated with colonymate or non-colonymate CHCs. Additionally, we quantified the amount of hydrocarbons transferred to individual ants and performed gas chromatography-mass spectrometry (GC/MS) to qualitatively characterize our manipulation of CHC profiles. The GC/MS data revealed marked differences in the hydrocarbon profiles across supercolonies and indicated that our treatment effectively masked the original chemical profile of the treated ants with the profile belonging to the foreign individuals. We found that individual workers treated with foreign CHCs were aggressively rejected by their colonymates and this behavior appears to be concentration-dependent: larger quantities of foreign CHCs triggered higher levels of aggression. Moreover, this response was not simply due to an increase in the amount of CHCs applied to the cuticle since treatment with high concentrations of nestmate CHCs did not trigger aggression.The results of this study bolster the findings of previous studies on social insects that have implicated CHCs as nestmate recognition cues and provide insight into the mechanisms of nestmate recognition in the invasive Argentine ant. Received 6 February 2007; revised 31 May and 27 July 2007; accepted 16 August 2007.     

Food resources, chemical signaling, and nest mate recognition in the ant Formica aquilonia

Animals such as social insects that live in colonies can recognizeintruders from other colonies of the same or different speciesusing colony-specific odors. Such colony odors usually haveboth a genetic and an environmental origin. When within-colonyrelatedness is high (i.e., one or very few reproductive queens),colonies comprise genetically distinct entities, and recognitionbased on genetic cues is reliable. However, when nests containmultiple queens and colonies comprise multiple nests (polydomy),the use of purely genetically determined recognition labelsmay become impractical. This is due to high within-colony geneticheterogeneity and low between-colony genetic heterogeneity.This may favor the use of environmentally determined recognitionlabels. However, because nests within polydomous colonies maydiffer in their microenvironment, the use of environmental labelsmay also be impractical unless they are actively mixed amongthe nests. Using a laboratory experiment, we found that bothisolation per se and diet composition influenced the cuticularchemical profiles in workers of Formica aquilonia. In addition,the level of aggression increased when both the proportionsof dietary ingredients and the availability of food were altered.This suggests that increased aggression was mediated by changesin the chemical profile and that environmental cues can mediaterecognition between colonies. These results also suggest thatthe underlying recognition cues are mutable in response to extrinsicfactors such as the amount and the composition of food.     

Nest mate recognition in ants with complex colonies: within- and between-population variation

The ability to recognize kin is widespread, and especially importantin highly social organisms. We studied kin recognition by assessingpatterns of aggression within and between nests of the antLeptothorax longispinosus. Colonies of this species can befractionated into subunits, a condition called polydomy. Theproblem of recognizing relatives is therefore more complexwhen those relatives can live in two or more different places.We hypothesized that spatial subdivision may have resulted ina stronger genetic component to kin recognition than in caseswhere colonies live in a single location. To test our hypothesiswe assessed recognition capabilities for two populations ofthis ant that differ in the complexity of their colonies. Ina New York, USA, population, polydomy is very common, and coloniesalso can have multiple queens. By contrast, a population inWest Virginia, USA, has colonies that typically are monogynousand rarely are polydomous. We conducted introductions of antsbetween different nests collected in the same neighborhood,with self-introductions and alien introductions as controls.Nests from the two populations showed corresponding differencesin their aggression towards intruders. For New York nests, the extent of genetic similarity was the single best predictor ofaggression, whereas for West Virginia nests aggression wasjointly influenced by genetic similarity and spatial distance.In both populations, we found nest pairs for which aggressionwas nonreciprocal; these probably reflect recognition errors by one of the nests. After the ants were maintained in the laboratoryfor 3 months, their aggression scores rose and fewer recognitionerrors were made. Thus nest-mate and colony-mate recognitionin this species are mediated primarily by endogenous cues (geneticsimilarity); the importance of exogenous cues for nest materecognition depends on the population's social system.     

Parametric and non-parametric masking of randomness in sequence alignments can be improved and leads to better resolved trees

Background

Ants typically distinguish nestmates from non-nestmates based on the perception of colony-specific chemicals, particularly cuticular hydrocarbons present on the surface of the ants' exoskeleton. These recognition cues are believed to play an important role in the formation of vast so-called supercolonies that have been described for some invasive ant species, but general conclusions about the role of these cues are hampered by only few species being studied. Here we use data on cuticular hydrocarbons, aggression and microsatellite genetic markers to investigate the interdependence of chemical recognition cues, genetic distance and nestmate discrimination in the pharaoh ant (Monomorium pharaonis), a widespread pest species, and ask whether introduced populations of this species are genetically differentiated and exhibit intraspecific aggression.

Results

Microsatellite analyses of a total of 35 colonies from four continents revealed extremely high levels of genetic differentiation between almost all colonies (F _ST = 0.751 ± 0.006 SE) and very low within-colony diversity. This implies that at least 34 and likely hundreds more independent lineages of this ant have spread worldwide. Aggression tests involving workers from 14 different colonies showed only low levels of aggression, even between colonies that were geographically and/or genetically very distant. Chemical analyses of groups of worker ants showed that all colonies had the same cuticular compounds, which varied only quantitatively among colonies. There was a positive correlation between geographical and genetic distance, but no other significant relationships were detected between aggression, chemical profile, genetic distance and geographical distance.

Conclusions

The pharaoh ant has a global invasion history of numerous independent introductions resulting in genetically highly differentiated colonies typically displaying surprisingly low levels of intraspecific aggression, a behaviour that may have evolved in the native range or by lineage selection in the introduced range.     

High recombination frequency creates genotypic diversity in colonies of the leaf-cutting ant Acromyrmex echinatior

Honeybees are known to have genetically diverse colonies because queens mate with many males and the recombination rate is extremely high. Genetic diversity among social insect workers has been hypothesized to improve general performance of large and complex colonies, but this idea has not been tested in other social insects. Here, we present a linkage map and an estimate of the recombination rate for Acromyrmex echinatior, a leaf-cutting ant that resembles the honeybee in having multiple mating of queens and colonies of approximately the same size. A map of 145 AFLP markers in 22 linkage groups yielded a total recombinational size of 2076 cM and an inferred recombination rate of 161 kb cM(-1) (or 6.2 cM Mb(-1)). This estimate is lower than in the honeybee but, as far as the mapping criteria can be compared, higher than in any other insect mapped so far. Earlier studies on A. echinatior have demonstrated that variation in division of labour and pathogen resistance has a genetic component and that genotypic diversity among workers may thus give colonies of this leaf-cutting ant a functional advantage. The present result is therefore consistent with the hypothesis that complex social life can select for an increased recombination rate through effects on genotypic diversity and colony performance.