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.     

Recognition in a Social Symbiosis: Chemical Phenotypes and Nestmate Recognition Behaviors of Neotropical Parabiotic Ants

Social organisms rank among the most abundant and ecologically dominant species on Earth, in part due to exclusive recognition systems that allow cooperators to be distinguished from exploiters. Exploiters, such as social parasites, manipulate their hosts’ recognition systems, whereas cooperators are expected to minimize interference with their partner’s recognition abilities. Despite our wealth of knowledge about recognition in single-species social nests, less is known of the recognition systems in multi-species nests, particularly involving cooperators. One uncommon type of nesting symbiosis, called parabiosis, involves two species of ants sharing a nest and foraging trails in ostensible cooperation. Here, we investigated recognition cues (cuticular hydrocarbons) and recognition behaviors in the parabiotic mixed-species ant nests of Camponotus femoratus and Crematogaster levior in North-Eastern Amazonia. We found two sympatric, cryptic Cr. levior chemotypes in the population, with one type in each parabiotic colony. Although they share a nest, very few hydrocarbons were shared between Ca. femoratus and either Cr. levior chemotype. The Ca. femoratus hydrocarbons were also unusually long–chained branched alkenes and dienes, compounds not commonly found amongst ants. Despite minimal overlap in hydrocarbon profile, there was evidence of potential interspecific nestmate recognition –Cr. levior ants were more aggressive toward Ca. femoratus non-nestmates than Ca. femoratus nestmates. In contrast to the prediction that sharing a nest could weaken conspecific recognition, each parabiotic species also maintains its own aggressive recognition behaviors to exclude conspecific non-nestmates. This suggests that, despite cohabitation, parabiotic ants maintain their own species-specific colony odors and recognition mechanisms. It is possible that such social symbioses are enabled by the two species each using their own separate recognition cues, and that interspecific nestmate recognition may enable this multi-species cooperative nesting.     

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.     

Hydrocarbon-released nestmate aggression in the Argentine ant, Linepithema humile, following encounters with insect prey

Argentine ants, Linepithema humile, were attacked by their nestmates following contact with a particular prey item, the brown-banded cockroach, Supella longipalpa. Contact with prey, as brief as 2 min, provoked nestmate aggression. Argentine ants contaminated with hydrocarbons extracted from S. longipalpa also released nestmate aggression behavior similar to that released by the whole prey item, confirming the involvement of hydrocarbons. In contrast to S. longipalpa, little or no nestmate aggression was induced by other ant prey from diverse taxa. A comparison of prey hydrocarbon profiles revealed that all hydrocarbons of S. longipalpa were very long chain components with 33 or more carbons, while other prey had either less, or none, of the very long chain hydrocarbons of 33 carbons or greater. We identified the hydrocarbons of S. longipalpa and some new groups of long chain hydrocarbons of L. humile. The majority of S. longipalpa hydrocarbons were 35 and 37 carbons in length with one to three methyl branches, and closely resembled two previously unidentified groups of compounds from L. humile of similar chain length. The hydrocarbons of S. longipalpa and L. humile were compared and their role in the Argentine ant nestmate recognition is discussed.     

Chemical mimicry of the ant Oecophylla smaragdina by the myrmecophilous spider Cosmophasis bitaeniata: Is it colony-specific?

Workers of most social insects can distinguish between nestmates and non-nestmates, and actively attack the latter if they attempt to intrude into the nest or surrounding territory. Nevertheless, there are many records of heterospecific organisms living within the nests of social insects, and they are thought to gain access through chemical mimicry. The salticid spider Cosmophasis bitaeniata lives within the leaf nests of the ant Oecophylla smaragdina, where it preys on the ant larvae. We investigated, using behavioural bioassays and chemical analyses, whether the previously reported resemblance of the cuticular hydrocarbons of ant and spider was colony-specific. Behavioural experiments revealed that the spiders can distinguish between nestmate and non-nestmate major workers and are less inclined to escape when confined with ants that are nestmates. More significantly, C. bitaeniata were more likely to capture ant larvae from nestmate minor workers than non-nestmate minor workers. The chemical analyses revealed that the cuticular hydrocarbon profiles of the spiders and the major workers of the ant colonies were colony-specific. However, the hydrocarbon profiles of C. bitaeniata do not match those of the major workers of O. smaragdina from the same colony. Perhaps the colony-specific cuticular hydrocarbon profiles of C. bitaeniata function to obtain prey from the minor workers rather than avoid eliciting aggression from the major workers.     

Polygyny reduces rather than increases nestmate discrimination cue diversity in Formica exsecta ants

Although the majority of social insect colonies are headed by a single queen, some species possess nests that contain numerous reproductive queens (polygyny), a trait that is particularly widespread amongst the ants. Polygyny is often associated with a lack of conspecific inter-nest aggression between workers. This is hypothesised to result from increased nestmate cue diversity within nests, since polygynous nests are more genetically diverse than monogynous nests. Alternatively, it may reflect the common origin of polygynous nests that form polydomous networks. We exploit the recent discovery that the nestmate discrimination system in the ant Formica exsecta is based on cuticular hydrocarbons to investigate cue (Z9-alkenes) diversity in several monogynous and polygynous populations. Contrary to previous predictions, in all polygynous populations, the variation between nests in the Z9-alkene profiles was reduced relative to that found in monogynous populations. However, nest-specific Z9-alkene profiles with little variation amongst nestmate workers were still maintained irrespective of nest type or population. This suggests a very effective gestalt mechanism that homogenises the chemical discrimination cues, despite genetic diversity within colonies. Although the reduction in variation between nests was associated with reduced worker aggression on the population level, it cannot totally explain the weak aggression associated with polygynous populations.     

Nest-area marking with faeces: a chemical signature that allows colony-level recognition in seed harvesting ants (Hymenoptera, Formicidae)

Summary. Nestmate recognition systems in ants are largely based on chemical signals. The hydrocarbon fraction of the lipid layer which covers the insect cuticle plays a determinant role in this context. Here we report a novel extension of nestmate and alien recognition – nest area marking with faeces containing the same hydrocarbons as the cuticle of workers – in a harvesting ant, Messor capitatus. Workers of M. capitatus deposit large quantities of brown-yellow material from the hindgut (termed spots) in the vicinity of the nest. Behavioural investigation showed that such spotting behaviour has a communicative value in the context of nest area identification. Anal fluids deposited in the nest surroundings contain colony-specific cues which the ants use to recognize their own nest areas, and distinguish them from foreign areas even in the absence of nestmate or alien ants. Chemical analyses by gas chromatography-mass spectrometry (GC-MS) of the contents of anal spots, rectal sacs, and cuticular extracts revealed that all contain the same long-chained linear and branched hydrocarbons in varying proportions. Importantly, multivariate analyses showed that the relative proportions of these compounds on the cuticle and in spots are colony-specific. This provides a mechanism by which spot marking could be used by workers to define and recognize their colony area, and would represent a simple extension of the existing nestmate recognition template based on colonial cuticular signatures. The ecological and sociobiological implications of these findings are discussed.Received 3 February 2004; revised 10 June 2004; accepted 14 June 2004.     

Chemical Discrimination and Aggressiveness via Cuticular Hydrocarbons in a Supercolony-Forming Ant,Formica yessensis

Background

Territorial boundaries between conspecific social insect colonies are maintained through nestmate recognition systems. However, in supercolony-forming ants, which have developed an extraordinary social organization style known as unicoloniality, a single supercolony extends across large geographic distance. The underlying mechanism is considered to involve less frequent occurrence of intraspecific aggressive behaviors, while maintaining interspecific competition. Thus, we examined whether the supercolony-forming species, Formica yessensis has a nestmate recognition system similar to that of the multicolonial species, Camponotus japonicus with respect to the cuticular hydrocarbon-sensitive sensillum (CHC sensillum), which responds only to non-nestmate CHCs. We further investigated whether the sensory system reflects on the apparent reduced aggression between non-nestmates typical to unicolonial species.

Methodology/Principal Findings

F. yessensis constructs supercolonies comprising numerous nests and constitutes the largest supercolonies in Japan. We compared the within-colony or between-colonies’ (1) similarity in CHC profiles, the nestmate recognition cues, (2) levels of the CHC sensillar response, (3) levels of aggression between workers, as correlated with geographic distances between nests, and (4) their genetic relatedness. Workers from nests within the supercolony revealed a greater similarity of CHC profiles compared to workers from colonies outside it. Total response of the active CHC sensilla stimulated with conspecific alien CHCs did not increase as much as in case of C. japonicus, suggesting that discrimination of conspecific workers at the peripheral system is limited. It was particularly limited among workers within a supercolony, but was fully expressed for allospecific workers.

Conclusions/Significance

We demonstrate that chemical discrimination between nestmates and non-nestmates in F. yessensis was not clear cut, probably because this species has only subtle intraspecific differences in the CHC pattern that typify within a supercolony. Such an incomplete chemical discrimination via the CHC sensilla is thus an important factor contributing to decreased occurrence of intraspecific aggressive behavior especially within a supercolony.     

A new colony structure of the invasive Argentine ant (<Emphasis Type="Italic">Linepithema humile</Emphasis>) in Southern Europe

The Argentine ant (Linepithema humile, Mayr) is a highly invasive species that has successfully spread from its native range in South America across many zones of the globe. In Southern Europe, two continental supercolonies have been identified, the Catalonian supercolony and the main European supercolony spreading over 6,000 km. In Corsica, a French Mediterranean island, the Argentine ant has been present for 60 years. Here we compare patterns of intraspecific aggression and cuticular hydrocarbon profiles of Argentine ants in Corsica to three mainland European colonies. Chemical analyses reveal the existence of cuticular signature variations among the six study sites relative to a gradient of aggression. We find two distinct colony groups not belonging to the Catalonian supercolony, suggesting that the new population originates either (1) from an independent introduction event from the native range resulting in a third European supercolony, or (2), given the chemical proximity and the moderate level of aggression between the two groups, from an existing European population followed by a drift producing a division within the main European supercolony.     

Four mutually incompatible Argentine ant supercolonies in Japan: inferring invasion history of introduced Argentine ants from their social structure

In recent years, highly invasive ant species successively invaded warm regions of Asia. In Japan, the Argentine ant, Linepithema humile, has become established in several coastal regions. This species forms unusual social organizations called supercolonies consisting of numerous mutually non-aggressive nests. We studied the behavioral relationships, similarity of cuticular hydrocarbon profiles (nestmate recognition cue), and genetic relationships among the introduced Argentine ant populations of Japan. The Japanese populations were divided into four behaviorally, chemically, and genetically distinct supercolonies, which may have derived from independent source populations. The result represents the recent trend of increasing invasions of invasive ants to Asia. The discontinuous distribution of one supercolony throughout most of the Japanese range suggests rapid expansion of the supercolony via human-mediated jump dispersal. Meanwhile, localization of the other three supercolonies in Kobe Port provides the first evidence for multiple invasions of distinct supercolonies into a base for international trade.     

Evidence that queens do not influence nestmate recognition in Argentine ants

Summary Introduced populations of many invasive ants exhibit low levels of intraspecific aggression. Argentine ants (Linepithema humile), for example, maintain expansive supercolonies in many parts of their introduced range. Recent studies demonstrate that variation in nestmate recognition in L. humile can derive from both environmental and genetic sources. In some ants, pheromones emitted by queens also influence nestmate-recognition behavior. To test if such a phenomenon occurs in Argentine ants, we examined whether levels of intraspecific aggression vary as a function of queen presence or absence in experimental lab colonies. For each of four known supercolonies from southwestern California, we set up a pair of experimental colonies and randomly assigned replicates within each pair to treatment (queen removal) and control (no queen removal) groups. Using two different behavioral assays, we then measured aggressive behavior for ten days, removed queens from colonies in the treatment group, and continued to monitor aggression in both experimental groups for an additional 65 days. Both assays yielded qualitatively similar results: intraspecific aggression remained high throughout the experiment in both experimental groups. These results suggest that L. humile queens fail to influence levels of intraspecific aggression in introduced populations.Received 2 June 2003; revised 1 September 2003; accepted 18 September 2003.     

Ontogeny of hydrocarbon profiles in the ant Aphaenogaster senilis and effects of social isolation

In ants, cuticular hydrocarbons are used for nestmate recognition; they are stored in the postpharyngeal gland and shared among the individuals. Newly emerged ants have a very small quantity of hydrocarbons. We studied the ontogeny of the hydrocarbon profile in Aphaenogaster senilis. The total quantities of both cuticular and postpharyngeal gland (PPG) hydrocarbons increased with age from 0 to 20 days after emergence and then stabilised. These quantities are correlated with the development of the ovary. Under individual social isolation, cuticular hydrocarbons increased as normal, but the total quantity of PPG hydrocarbons never increased from the initial low level. This effect of social isolation on the PPG hydrocarbon level indicates the importance of hydrocarbon transfer between nestmates through the PPG and lends support to the gestalt model of nestmate recognition. To cite this article: K. Ichinose, A. Lenoir, C. R. Biologies 332 (2009).     

Global-scale analyses of chemical ecology and population genetics in the invasive Argentine ant

Ants are some of the most abundant and ecologically successful terrestrial organisms, and invasive ants rank among the most damaging invasive species. The Argentine ant is a particularly well-studied invader, in part because of the extreme social structure of introduced populations, known as unicoloniality. Unicolonial ants form geographically vast supercolonies, within which territorial behaviour and intraspecific aggression are absent. Because the extreme social structure of introduced populations arises from the widespread acceptance of conspecifics, understanding how this colonymate recognition occurs is key to explaining their success as invaders. Here, we present analyses of Argentine ant recognition cues (cuticular hydrocarbons) and population genetic characteristics from 25 sites across four continents and the Hawaiian Islands. By examining both hydrocarbon profiles and microsatellite genotypes in the same individual ants, we show that native and introduced populations differ in several respects. Both individual workers and groups of nestmates in the introduced range possess less diverse chemical profiles than ants in the native range. As previous studies have reported, we also find that introduced populations possess much lower levels of genetic diversity than populations in the native range. Interestingly, the largest supercolonies on several continents are strikingly similar to each other, suggesting that they arose from a shared introduction pathway. This high similarity suggests that these geographically far-flung ants may still recognize and accept each other as colonymates, thus representing distant nodes of a single, widely distributed supercolony. These findings shed light on the behaviour and sociality of these unicolonial invaders, and pose new questions about the history and origins of introduced populations.     

Wax on, wax off: nest soil facilitates indirect transfer of recognition cues between ant nestmates

Social animals use recognition cues to discriminate between group members and non-members. These recognition cues may be conceptualized as a label, which is compared to a neural representation of acceptable cue combinations termed the template. In ants and other social insects, the label consists of a waxy layer of colony-specific hydrocarbons on the body surface. Genetic and environmental differences between colony members may confound recognition and social cohesion, so many species perform behaviors that homogenize the odor label, such as mouth-to-mouth feeding and allogrooming. Here, we test for another mechanism of cue exchange: indirect transfer of cuticular hydrocarbons via the nest material. Using a combination of chemical analysis and behavioral experiments with Camponotus aethiops ants, we show that nest soil indirectly transfers hydrocarbons between ants and affects recognition behavior. We also found evidence that olfactory cues on the nest soil influence nestmate recognition, but this effect was not observed in all colonies. These results demonstrate that cuticular hydrocarbons deposited on the nest soil are important in creating uniformity in the odor label and may also contribute to the template.     

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.     

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.     

When supercolonies collide: territorial aggression in an invasive and unicolonial social insect

Some species of ants possess an unusual form of social organization in which aggression among nests is absent. This type of social organization, called unicoloniality, has been studied in only a handful of species and its evolutionary origins remain unclear. To date, no study has examined behavioural and genetic patterns at points of contact between the massive supercolonies that characterize unicoloniality. Since interactions at territory boundaries influence the costs of aggression and the likelihood of gene flow, such data may illuminate how supercolonies are formed and maintained. Here we provide field data on intraspecific territoriality for a widespread and invasive unicolonial social insect, the Argentine ant (Linepithema humile). We observed abrupt and well-defined behavioural boundaries at 16 contact zones between three different pairs of supercolonies. We visited nine of these zones weekly during a six-month period and observed consistent and intense intercolony aggression that resulted in variable, but often large, levels of worker mortality. Microsatellite variation along six transects across territory borders showed that F(ST) values were lower within supercolonies (0.08 +/- 0.01 (mean +/- SE)) than between supercolonies (0.29 +/- 0.01) and that this disparity was especially strong right at territory borders, despite direct and prolonged contact between the supercolonies. Matrix correspondence tests confirmed that levels of aggression and genetic differentiation were significantly correlated, but no relationship existed between geographic distance and either intraspecific aggression or genetic differentiation. Patterns of F(ST) variation indicated high levels of gene flow within supercolonies, but little to no gene flow between them. Overall, these findings are inconsistent with a model of relaxed ecological constraints leading to colony fusion and suggest that environmentally derived cues are not the prime determined of nestmate recognition in field populations of Argentine ants.     

Pavement Ant Workers (<Emphasis Type="Italic">Tetramorium caespitum</Emphasis>) Assess Cues Coded in Cuticular Hydrocarbons to Recognize Conspecific and Heterospecific Non-Nestmate Ants

Most ants live in closed societies from which non-members are excluded through fighting or ritualized displays to protect colony resources. Nestmate recognition is the process by which ants discriminate nestmate from non-nestmate ants. Ants use cues coded in mixtures of long-chain hydrocarbon compounds on the cuticle as nestmate recognition cues. Pavement ants (Tetramorium caespitum) form conspicuous wars between neighboring colonies that are organized after workers meet and make the decision to fight after assessing nestmate recognition cues. These wars involve thousands of individuals. Fighting is ritualized and few ants die in the process. We identified 24 cuticular hydrocarbon compounds, above 1% in relative abundance, in the profile of pavement ants with chain lengths ranging from 15 to 31 carbon atoms. Cuticular lipids contained, in order of abundance: mono-methyl alkanes (45–56%), n-alkanes (range: 16–40% relative abundance), and alkenes (10–20%), with small or trace amounts of di-methyl, tri-methyl alkanes and fatty acids. Results from behavioral tests show that pavement ants assess information in cuticular hydrocarbon profiles to recognize both conspecific and heterospecfic (Pogonomyrmex occidentalis and Camponotus modoc) non-nestmate ants and that the relative abundance of methyl-branched alkanes and alkenes codes for nestmate status, at least for conspecific interactions. Our data add to a growing body of knowledge about how ants use cuticular hydrocarbon based nestmate recognition cues to prevent the intrusion of non-nestmates in to colony space.     

Nest Hydrocarbons as Cues for Philopatry in a Paper Wasp

Philopatric behavior has been demonstrated in a wide taxonomic spread of animals. In temperate environments, overwintered Polistes wasp foundresses often return to their natal nest prior to initiating colony construction. Previous research has shown that these spring foundresses can identify the natal nest in the absence of landmark and gross morphological cues. Hydrocarbons are essential recognition cues for Polistes nest and nestmate discrimination, but cuticular hydrocarbon profiles can become homogenized when foundresses overwinter in mixed colony groups. We examined the hydrocarbon profiles of Polistes dominulus foundresses and nests before and after an overwintering period, and found that the hydrocarbon profiles of nests remain unique over time and that this uniqueness is influenced by the original foundresses. Our data raise the possibility that in returning to the natal nest, foundresses reacquire their colony‐specific signature, which may play a role in the formation of cooperative associations.     

Ants recognize foes and not friends

Discriminating among individuals and rejecting non-group members is essential for the evolution and stability of animal societies. Ants are good models for studying recognition mechanisms, because they are typically very efficient in discriminating ‘friends’ (nest-mates) from ‘foes’ (non-nest-mates). Recognition in ants involves multicomponent cues encoded in cuticular hydrocarbon profiles. Here, we tested whether workers of the carpenter ant Camponotus herculeanus use the presence and/or absence of cuticular hydrocarbons to discriminate between nest-mates and non-nest-mates. We supplemented the cuticular profile with synthetic hydrocarbons mixed to liquid food and then assessed behavioural responses using two different bioassays. Our results show that (i) the presence, but not the absence, of an additional hydrocarbon elicited aggression and that (ii) among the three classes of hydrocarbons tested (unbranched, mono-methylated and dimethylated alkanes; for mono-methylated alkanes, we present a new synthetic pathway), only the dimethylated alkane was effective in eliciting aggression. Our results suggest that carpenter ants use a fundamentally different mechanism for nest-mate recognition than previously thought. They do not specifically recognize nest-mates, but rather recognize and reject non-nest-mates bearing odour cues that are novel to their own colony cuticular hydrocarbon profile. This begs for a reappraisal of the mechanisms underlying recognition systems in social insects.