Chemical basis of nest-mate discrimination in the ant Formica exsecta.

Distinguishing nest-mates from non-nest-mates underlies key animal behaviours, such as territoriality, altruism and the evolution of sociality. Despite its importance, there is very little empirical support for such a mechanism in nature. Here we provide data that the nest-mate recognition mechanism in an ant is based on a colony-specific Z9-alkene signature, proving that surface chemicals are indeed used in ant nest-mate recognition as was suggested 100 years ago. We investigated the cuticular hydrocarbon profiles of 10 Formica exsecta colonies that are composed almost entirely of a Z9-alkene and alkane component. Then we showed that worker aggression is only elicited by the Z9-alkene part. This was confirmed using synthetic Z9-alkene and alkane blends matched to the individual colony profiles of the two most different chemical colonies. In both colonies, only glass beads with 'nest-mate' alkene profiles received reduced aggression. Finally, changing the abundance of a single Z9-alkene on live ants was shown to significantly increase the aggression they received from nest-mates in all five colonies tested. Our data suggest that nest-mate discrimination in the social insects has evolved to rely upon highly sensitive responses to relatively few compounds.     

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.     

A quantitative threshold for nest-mate recognition in a paper social wasp

Nest-mate recognition is fundamental for protecting social insect colonies from intrusion threats such as predators or social parasites. The aggression of resident females towards intruders is mediated by their cuticular semiochemicals. A positive relation between the amount of cues and responses has been widely assumed and often taken for granted, even though direct tests have not been carried out. This hypothesis has important consequences, since it is the basis for the chemical insignificance strategy, the most common explanation for the reduction in the amount of semiochemicals occurring in many social parasites. Here we used the social wasp Polistes dominulus, a model species in animal communication studies and host of three social parasites, to test this hypothesis. We discovered that different amounts of cuticular hydrocarbons (CHC) of a foreign female evoke quantitatively different behavioural reactions in the resident foundress. The relation between CHC quantity and the elicited response supports the idea that a threshold exists in the chemical recognition system of this species. The chemical insignificance hypothesis thus holds in a host–parasite system of Polistes wasps, even though other explanations should not be discarded.     

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.     

Long-term memory of individual identity in ant queens

Remembering individual identities is part of our own everyday social life. Surprisingly, this ability has recently been shown in two social insects. While paper wasps recognize each other individually through their facial markings, the ant, Pachycondyla villosa, uses chemical cues. In both species, individual recognition is adaptive since it facilitates the maintenance of stable dominance hierarchies among individuals, and thus reduces the cost of conflict within these small societies. Here, we investigated individual recognition in Pachycondyla ants by quantifying the level of aggression between pairs of familiar or unfamiliar queens over time. We show that unrelated founding queens of P. villosa and Pachycondyla inversa store information on the individual identity of other queens and can retrieve it from memory after 24h of separation. Thus, we have documented for the first time that long-term memory of individual identity is present and functional in ants. This novel finding represents an advance in our understanding of the mechanism determining the evolution of cooperation among unrelated individuals.     

Nest-mate recognition in Manuelia postica (Apidae: Xylocopinae): an eusocial trait is present in a solitary bee

In eusocial Hymenoptera, females are more tolerant towards nest-mate than towards non-nest-mate females. In solitary Hymenoptera, females are generally aggressive towards any conspecific female. Field observations of the nest biology of Manuelia postica suggested nest-mate recognition. Experiments were performed involving two live interacting females or one live female interacting with a dead female. Live females from different nests were more intolerant to each other than females from the same nest. Females were more intolerant towards non-nest-mate than towards nest-mate dead females. When dead females were washed with pentane, no differences in tolerant and intolerant behaviours were detected between non-nest-mate and nest-mate females. Females were more intolerant towards nest-mate female carcasses coated with the cuticular extract from a non-nest-mate than towards non-nest-mate female carcasses coated with the cuticular extract from a nest-mate. The compositions of the cuticular extracts was more similar between females from the same nest than between females from different nests. The results demonstrate for the first time nest-mate recognition mediated by cuticular chemicals in a largely solitary species of Apidae. The position of Manuelia at the base of the Apidae phylogeny suggests that nest-mate recognition in eusocial species apical to Manuelia represents the retention of a primitive capacity in Apidae.     

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.     

Comparative dynamics of gestalt odour formation in two ant species Camponotus fellah and Aphaenogaster senilis (Hymenoptera: Formicidae)

Abstract. Ant colonies experience continuous shifts in worker populations, which may affect odour composition in the nest. A major question regarding the dynamics of gestalt formation is that of the speed at which the scent of a new individual will be incorporated into the gestalt. It is predicted from the gestalt model of colony odour that workers have to exchange recognition cues continuously to maintain themselves within the gestalt and become well integrated within their colony. Using radioactive tracers the rates of transfer were measured between a labelled donor ant and one or 10 recipient ants, as a close approximation to the within-nest situation. The labelled hydrocarbons were first transferred to a small number of individuals and progressively to all the individuals of the group so that the distribution of hydrocarbon transfer rate approached a normal distribution. Furthermore, in Camponotus fellah Dalla Torre, which performs trophallaxis, homogeneity was reached more rapidly than in Aphaenogaster senilis Mayr, which does not show this behaviour. In the latter species, the gestalt seems to be maintained mainly by allogrooming. These experiments were accompanied by behavioural observations to ascertain the respective importance of trophallaxis and allogrooming in the behavioural time-budget of the ants. In A. senilis , allogrooming was more frequent than in ants that trophallax, which corroborates the role of allogrooming in the establishment of the gestalt in this species.     

Frequent colony relocations do not result in effective dispersal in the gypsy ant Aphaenogaster senilis

Dispersal is an important step in animal's life cycle, one consequence of which is reducing local mate and resource competition. Dispersal is often achieved during one unique special movement, from the birthplace to a new appropriate area where to settle and reproduce. However, in species in which this special movement is limited by life history tradeoffs, we may expect dispersal to be promoted also by routine movements occurring throughout the animal’s life and stimulated by other activities like foraging or the search of nesting conditions. Here we employ a multidisciplinary approach consisting of computer simulations, mark–recapture and genetic data to better understand the role of colony relocations as dispersal strategy in the gypsy ant Aphaenogaster senilis. Contrary to expectations, our results show that colony relocations do not result in effective dispersal as evidenced by mark–recapture and genetic data. Furthermore, simulations showed that successive colony relocations did not follow a constant direction, but occurred either in a randomly changing direction or followed a circular trajectory, indicating limited effective dispersal. We also found a general lack of inbreeding and significant population viscosity between neighbouring colonies suggesting that relocations may act as a balancing strategy between these two processes. We discuss the results in terms of their evolutionary and ecological significance, and highlight future directions of research towards the understanding of dispersal strategies in colonial species.     

Kin-informative recognition cues in ants

Although social groups are characterized by cooperation, they are also often the scene of conflict. In non-clonal systems, the reproductive interests of group members will differ and individuals may benefit by exploiting the cooperative efforts of other group members. However, such selfish behaviour is thought to be rare in one of the classic examples of cooperation--social insect colonies--because the colony-level costs of individual selfishness select against cues that would allow workers to recognize their closest relatives. In accord with this, previous studies of wasps and ants have found little or no kin information in recognition cues. Here, we test the hypothesis that social insects do not have kin-informative recognition cues by investigating the recognition cues and relatedness of workers from four colonies of the ant Acromyrmex octospinosus. Contrary to the theoretical prediction, we show that the cuticular hydrocarbons of ant workers in all four colonies are informative enough to allow full-sisters to be distinguished from half-sisters with a high accuracy. These results contradict the hypothesis of non-heritable recognition cues and suggest that there is more potential for within-colony conflicts in genetically diverse societies than previously thought.     

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.     

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.     

Genetics, behaviour and chemical recognition of the invading ant Pheidole megacephala

Introduced species often become ecologically dominant, displacing native species and posing a serious threat to ecosystem function and global biodiversity. Ants are among the most widespread and damaging alien species; introductions are often accompanied by population-level behavioural and genetic changes contributing to their success. We investigated the genetic structure, chemical profile and nestmate recognition in introduced populations of the invasive big-headed ant, Pheidole megacephala, in Australia. Behavioural analyses show that workers are not aggressive towards conspecifics from different nests, even at large geographical scales (up to 3000 km) and between populations encompassing a wide range of environmental conditions. By contrast, interactions with workers of other species invariably result in agonistic behaviours. Genetic analyses reveal that populations have low genetic diversity. No genetic differentiation occurs among nests of the same population; differentiation between populations, though significant, remains weak. Chemical analyses indicate that cuticular lipids are similar between colonies of a population, and that differentiation between populations is low. Altogether, these results indicate that the big-headed ant P. megacephala forms a large unicolonial population across northern/eastern Australia.     

The look of royalty: visual and odour signals of reproductive status in a paper wasp

Reproductive conflicts within animal societies occur when all females can potentially reproduce. In social insects, these conflicts are regulated largely by behaviour and chemical signalling. There is evidence that presence of signals, which provide direct information about the quality of the reproductive females would increase the fitness of all parties. In this study, we present an association between visual and chemical signals in the paper wasp Polistes satan. Our results showed that in nest-founding phase colonies, variation of visual signals is linked to relative fertility, while chemical signals are related to dominance status. In addition, experiments revealed that higher hierarchical positions were occupied by subordinates with distinct proportions of cuticular hydrocarbons and distinct visual marks. Therefore, these wasps present cues that convey reliable information of their reproductive status.     

Differential learning and memory by co-occurring ant species

Foragers of the antsMessor pergandei andPogonomyrmex rugosus experience differing levels of variability in the distribution of seeds they harvest due to species-specific differences in foraging behavior.Messor pergandei foragers experience more variable seed distributions and densities, learn to recognize a novel seed faster but forget this information faster thanP. rugosus, which experiences more constant seed distributions even in the same habitat. Rate of learning to recognize a novel seed species was negatively associated with measures of seed species diversity for both ants.Messor pergandei foragers respond to variation in seed density by varying number of seeds handled per seed harvested, whileP. rugosus foragers do not. Memory of a novel seed exceeds forager longevity, due perhaps to use of seed caches as a type of information center.     

A chemical level in the coevolutionary arms race between an ant social parasite and its hosts

Here we investigate the coevolutionary interactions between the slavemaking ant Protomognathus americanus and its Temnothorax hosts on a chemical level. We show that, although this social parasite is principally well-adapted to its hosts' cuticular hydrocarbon profile, there are pronounced differences in the fine-tuning of this adaptation. Between populations, chemical adaptation varies with host community composition, as the parasite faces a trade-off when confronted with more than one host species. In addition to adaptation of its own chemical signature, the slavemaker causes a reciprocal adjustment in its slaves' cuticular profile, the degree of which depends on the slave species. On the host side, successful parasite defence requires efficient enemy recognition, and in behavioural aggression trials, host colonies could indeed discriminate between invading slaves, which commonly accompany slavemakers on raids, and free-living conspecifics. Furthermore, hosts shifted their acceptance threshold over the seasons, presumably to reduce the costs of defence.     

Development of interspecific recognition behavior in the antsManica rubida andFormica selysi (Hymenoptera: Formicidae) reared in mixed-species groups

The purpose of this study was to determine the role of early social experience on the ontogeny of kin and nestmate recognition in ants by means of both behavioral and chemical analysis. Workers of two ant species,Manica rubida (Myrmicinae) andFormica selysi (Formicinae), were reared in homospecific groups (control) or in artificial heterospecific groups (mixed), created less than 5 h after their emergence. Recognition was evaluated between unfamiliar individuals of different species reared in control and mixed groups for periods of 5, 12, 24, 48, and 72 h and 8, 15, 30, 60, or 90 days after emergence. Heterospecifically reared individuals of both species gradually became tolerant of allospecific individuals from control groups. Moreover, homospecifically reared individuals did not aggress allospecific individuals reared in mixed groups. During the course of familiarization between the species, there were modifications of the chemical recognition signals. In mixed groups, hydrocarbon profiles of both species acquire gradually some of the components characteristic of their heterospecific nestmates. These experiments showed that allospecific recognition required the acquisition of a minimal quantity of allospecific cues. The phenomenon provided an another example of the relationship between tolerance and the chemical cues displayed by both species. The results suggested that the individual recognized the allospecific cues borne on each individual's body surface and/or that each individual learned and memorized allospecific cues during its early life. Therefore, each individual might develop a template encoding the allospecific and the conspecific cues to characterize nestmates.