How an ant manages to display individual and colonial signals by using the same channel

Three cuticular amides in the tripartite symbiosis of leafcutter ants

Cuticular hydrocarbons (CHCs) play various roles in insects' chemical ecology. As leafcutter ants live in a specific symbiosis with fungi, they harvest and with different bacteria, some of these CHCs might be associated with a mutualistic function within this symbiosis. To obtain a more precise picture in that respect we compared the CHC profiles of the leafcutter ants, Atta sexdens, Atta cephalotes, and Acromyrmex octospinosus inhabited by mutualistic bacteria with the profiles of Polyrhachis dives and Messor aciculatus by GC-EI-MS analysis and 28 other ant species by data from the literature. We were able to identify three alkyl amides (hexadecanamide, hexadecenamide, and tetradecanamide), occurring only in the CHC profiles of leafcutter ants inhabited by symbiotic bacteria. Our results lead to the conclusion that those alkyl amides could have a function in the tripartite symbiosis of leafcutter ants.

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.

Cuticular Hydrocarbon Profiles Differentiate Tropical Fire Ant Populations (Solenopsis geminata, Hymenoptera: Formicidae)

The cuticular hydrocarbons (CHCs) from hexane rinses of workers from two Florida populations (dark and red forms) of the tropical fire ant, Solenopsis geminata, were separated by silica gel chromatography and identified by GC/MS analysis. Both the dark form and the red form produce similar CHCs with carbon chain lengths ranging from 17 to 35. However, the relative percentages of these CHCs were consistently different between the two color forms. The largest CHC component in the dark form is tricosane, and (Z)-9-tricosene for the red form. There were several significant differences in percent composition. For example, the dark form was characterized by a low tricosene:tricosane ratio (ca. 0.25), whereas this ratio was > 2.5 for the red form. The ratio of tricosene:tricosane can be used as a diagnostic biomarker to delimit the dark and red forms. Cluster analysis showed that the CHCs patterns of dark form colonies are completely separated from the CHC pattern of red form colonies. Differences in social behaviors like nestmate recognition and polygyny between workers from this dark form and the red form await further investigation.

Co-adaptation of electric organ discharges and chirps in South American ghost knifefishes (Apteronotidae)

Animal communication signals that simultaneously share the same sensory channel are likely to co-evolve to maximize the transmission of each signal component. Weakly electric fish continuously produce a weak electric field that functions in communication. Fish modulate the electric organ discharge (EOD) on short timescales to produce context-specific signals called chirps. EODs and chirps are simultaneously detected by electroreceptors and processed in the electrosensory system. We analyzed these signals, first to explore whether EOD waveform is encoded in the signal received by electroreceptors and then to examine how EODs and chirps interact to influence conspicuousness. Our findings show that gross discrimination of sinusoidal from complex EOD waveforms is feasible for all species, but fine discrimination of waveform may be possible only for species with waveforms of intermediate complexity. The degree of chirp frequency modulation and chirp relative decay strongly influenced chirp conspicuousness, but other chirp parameters were less influential. The frequency difference between the interacting EODs also strongly impacted chirp conspicuousness. Finally, we developed a method for creating hybrid chirp/EOD combinations to independently analyze the impact of chirp species, EOD species, and EOD difference frequency on chirp conspicuousness. All three components and their interactions strongly influenced chirp conspicuousness, which suggests that evolutionary changes in parameters of either chirps or EODs are likely to influence chirp detection. Examining other environmental factors such as noise created by fish movement and species-typical patterns of sociality may enrich our understanding of how interacting EODs affect the detection and discrimination of chirps across species.

The global expansion of a single ant supercolony

Ants are among the most damaging invasive species, and their success frequently arises from the widespread cooperation displayed by introduced populations, often across hundreds of kilometers. Previous studies of the invasive Argentine ant (Linepithema humile) have shown that introduced populations on different continents each contain a single, vast supercolony and, occasionally, smaller secondary colonies. Here, we perform inter-continental behavioral analyses among supercolonies in North America, Europe, Asia, Hawaii, New Zealand and Australia and show that these far-flung supercolonies also recognize and accept each other as if members of a single, globally distributed supercolony. Furthermore, populations also possess similar genetic and chemical profiles. However, these ants do show aggression toward ants from South Africa and the smaller secondary colonies that occur in Hawaii and California. Thus, the largest and most dominant introduced populations are likely descended from the same ancestral colony and, despite having been established more than 100 years ago, have diverged very little. This apparent evolutionary stasis is surprising because, in other species, some of the most rapid rates of evolutionary change have occurred in introduced populations. Given the spatial extent of the Argentine ant society we report here, there can be little doubt that this intercontinental supercolony represents the most populous known animal society.

Genetic distance and age affect the cuticular chemical profiles of the clonal ant Cerapachys biroi

Although cuticular hydrocarbons (CHCs) have received much attention from biologists because of their important role in insect communication, few studies have addressed the chemical ecology of clonal species of eusocial insects. In this study we investigated whether and how differences in CHCs relate to the genetics and reproductive dynamics of the parthenogenetic ant Cerapachys biroi. We collected individuals of different ages and subcastes from several colonies belonging to four clonal lineages, and analyzed their cuticular chemical signature. CHCs varied according to colonies and clonal lineages in two independent data sets, and correlations were found between genetic and chemical distances between colonies. This supports the results of previous research showing that C. biroi workers discriminate between nestmates and non-nestmates, especially when they belong to different clonal lineages. In C. biroi, the production of individuals of a morphological subcaste specialized in reproduction is inversely proportional to colony-level fertility. As chemical signatures usually correlate with fertility and reproductive activity in social Hymenoptera, we asked whether CHCs could function as fertility-signaling primer pheromones determining larval subcaste fate in C. biroi. Interestingly, and contrary to findings for several other ant species, fertility and reproductive activity showed no correlation with chemical signatures, suggesting the absence of fertility related CHCs. This implies that other cues are responsible for subcaste differentiation in this species.

A conserved fertility signal despite population variation in the cuticular chemical phenotype of the trap-jaw ant Odontomachus brunneus

Contact pheromones in the form of cuticular hydrocarbons are widespread among insects. Eusocial insects present a special challenge for understanding the evolution of the cuticular hydrocarbon profile because this blend is responsible for multiple distinct roles such as nestmate recognition and signalling fertility status. This study investigates these two signalling roles of the hydrocarbon profile in the trap-jaw ant Odontomachus brunneus Patton. We demonstrate that the cuticular hydrocarbon profile is highly variable across populations and provide evidence that these differences are used for nestmate discrimination. Through manipulative experiments we also show that (Z)-9-nonacosene (Z9:C29) is used as a fertility signal and its role is conserved across populations. Our data demonstrate that both fertility and nestmate signalling influence the cuticular hydrocarbon profile and specifically the relative abundance of Z9:C29 on the cuticle of O. brunneus. Our study suggests that natural selection works on the cuticular chemical profile through multiple regulatory pathways, diversifying nestmate signals while conserving fertility signals.

Strong differences in chemical recognition cues between two closely related species of ants from the genus Lasius (Hymenoptera: Formicidae)

Cuticular hydrocarbons (CHCs) are increasingly recognized as important to insects and are used for constructing taxonomies. However, multiple parameters affect the expression of CHCs besides a genetic component. We propose that selection may act differently on the expression of CHCs, depending on the evolutionary context. To explore the influence of selection, the CHCs of two closely related ant species, Lasius niger and Lasius platythorax, were studied in a multidisciplinary approach. We characterized (i) CHCs and (ii) niches (through baiting, activity observations and foraging analysis). The species were distinct in both measures, although to a varying degree. Although they showed moderate niche partitioning along diet and environmental preferences, chemical differences were unexpectedly pronounced. This may be explained by divergent selection on mate recognition cues or by other influences on CHCs. Such striking chemical differences among closely related species may not be the rule and suggest that taxonomies based on CHCs should be interpreted cautiously; though, they remain useful tools for differentiating among cryptic species.

Nestmate recognition and the role of cuticular hydrocarbons in the African termite raiding ant Pachycondyla analis

Cuticular hydrocarbons (CHCs) are used for chemical communication among nestmates in many ant species, and they may play a role in the discrimination of nestmates and non-nestmates. Using the mandible opening response (MOR) bioassay, we tested the response of the African termite raiding ant, Pachycondyla analis, to CHC extracts of nestmates and non-nestmates. The ants were able to distinguish control chemical cues, from nestmate CHCs, and from non-nestmate CHCs, and, based on a CHC recognition threshold, aggression was demonstrated toward non-nestmates. Gas chromatography (GC) and GC-mass spectrometric analyses showed that CHC components of different ant colonies had chain lengths ranging from C(8) to C(31), comprising mainly n-alkanes, alkenes, and methyl branched alkanes, with the n-alkanes occurring in the same proportions among all colonies. The ants were grouped successfully according to their colonies of origin by using discriminant analysis of CHCs. We demonstrate that nestmate recognition occurs in P. analis, and that some of the cues involved are evidently alkenes and methyl-branched alkanes.

A review of ant cuticular hydrocarbons

We compared the published cuticular hydrocarbon (CHC) profiles of 78 ant species across 5 subfamilies. Almost 1,000 CHCs have been described for these species, composing 187 distinct homologous series and ten hydrocarbon groups. In descending order of occurrence were: n-alkanes > monomethylalkanes > dimethylalkanes > alkenes > dienes>> trimethylalkanes>> methylalkenes > methylalkadienes > trienes > tetramethylalkanes. Odd chain lengths and positions of methyl or double bonds at odd carbon numbers were far more numerous than even chain-length compounds or bond positions. Although each species possess its own unique pattern of CHCs, we found no association between CHC profile and phylogeny. The production of the biosynthetically complex compounds (e.g., methyl branched dienes) by the most primitive living ant suggests that the basic genetic architecture required to produce the rich diversity of CHCs was already present prior to their adaptive radiation. Unlike the ubiquitous n-alkanes and monomethylalkanes, there is a huge diversity of species-specific dimethylalkanes that makes them likely candidates for species and nest-mate discrimination signals.

Decision rules for egg recognition are related to functional roles and chemical cues in the queenless ant Dinoponera quadriceps

The capacity to distinguish colony members from strangers is a key component in social life. In social insects, this extends to the brood and involves discrimination of queen eggs. Chemical substances communicate colony affiliation for both adults and brood; thus, in theory, all colony members should be able to recognize fellow nestmates. In this study, we investigate the ability of Dinoponera quadriceps workers to discriminate nestmate and non-nestmate eggs based on cuticular hydrocarbon composition. We analyzed whether cuticular hydrocarbons present on the eggs provide cues of discrimination. The results show that egg recognition in D. quadriceps is related to both age and the functional role of workers. Brood care workers were able to distinguish nestmate from non-nestmate eggs, while callow and forager workers were unable to do so.

How reliable is the analysis of complex cuticular hydrocarbon profiles by multivariate statistical methods?

Numerous recent studies have correlated cuticular hydrocarbon profiles with a wide range of behaviors, particularly in social insects. These findings are wholly or partly based on multivariate statistical methods such as discriminate analysis (DA) or principal component analysis (PCA). However, these methods often provide limited insight into the biological processes that generate the small differences usually detected. This may be a consequence of variability in the system due to inadequate sample sizes and the assumption that all compounds are independent. A fundamental problem is that these methods combine rather than separate the effects of signal components. By using cuticular hydrocarbon data from previous social insect studies, we showed that: (1) in 13 species of Formica ants and seven species of Vespa hornets, at least one group of hydrocarbons in each species was highly (r(2) > 0.8) correlated, indicating that all compounds are not independent; (2) DA was better at group separation that PCA; (3) the relationships between colonies (chemical distance) were unstable and sensitive to variability in the system; and (4) minor compounds had a disproportionately large effect on the analysis. All these factors, along with sample size, need to be considered in the future analysis of complex chemical profiles.

Epidermal glands in the abdomen of a basal ant Dinoponera lucida (Formicidae: Ponerinae)

The basal ant Dinoponera (Hymenoptera: Ponerinae) has lost the morphologic queen caste so that all females may be potential reproductive individuals, and the nestmate recognition results from cuticular hydrocarbons cues. However, data about the origin of that substance in Ponerinae ants are scarce. This study reports the occurrence of epidermal glands in the abdomen of the ant Dinoponera lucida. In this ant, the epidermis of the abdominal sternites has tall cells with well-developed nucleus contrasting with flattened and collapsed epidermis in the tergites, suggesting a glandular function in the epidermis of the sternites. The possible role of the glandular epidermis in the synthesis of cuticular hydrocarbons for the nestmate recognition is discussed.

Colony-specific hydrocarbons identify nest mates in two species of Formica ant

The possession of a colony identity is a fundamental property that underlies much animal behavior. In insect societies, it is widely accepted that nest-mate recognition cues are encoded within the cuticular hydrocarbons. Despite numerous studies over the past 30years, the identification of these nest-mate specific signatures is only just starting to occur. In this paper, we show two different methods by which nest-mate-specific signatures can be encoded within the hydrocarbon profile of two species of Formica ants. In F. exsecta, nest-mate-specific signatures rely on the distribution of chain lengths of a single type of hydrocarbon, various (Z)-9-alkenes, which are present in colony-specific proportions. In F. fusca, variation in nine different positional isomers of C(25)-dimethylalkanes is sufficient to produce unique colony profiles. By using this information alone, we correctly assigned 97 F. exsecta workers into their respective 20 colonies and 111 F. fusca workers into their respective 30 colonies. These two systems or variations of them may be expected to occur in many insect societies that have a strong colony identity.

Chemical profiles of mated and virgin queens, egg-laying intermorphs and workers of the ant Crematogaster smithi

Many colonies of the North American ant Crematogaster smithi contain a "third female caste" in addition to queens and workers. These "intermorphs" are morphological intermediate of queens and workers and have well-developed ovaries but lack a spermatheca for the storage of sperm. They are specialised for laying large numbers of unfertilised, viable eggs, most of which serve as food for larvae and adults, though a few may eventually develop into males. Based on the assumption that cuticular hydrocarbons (CHCs) in social insects honestly signal the reproductive status of an individual we investigated the CHC of mated mature queens, virgin queens, intermorphs and workers. We expected intermorphs to show chemical profiles intermediate between those of mated queens and non-reproductive workers. A discriminant analysis of the chemical profiles reliably separated queens, virgin queens, and workers, but failed to distinguish between queens and intermorphs even though workers were apparently capable of doing so.

Cuticular hydrocarbons are heritable in the cricket Teleogryllus oceanicus

The ability of individuals to respond differentially to conspecifics depending on their genetic relatedness is a widespread phenomenon across the animal kingdom. Despite this, little is known about the selection processes that act on the phenotypic variation of traits used during recognition. Here we use a quantitative genetic approach to examine the patterns of genetic variation in cuticular hydrocarbon (CHC) profiles, a pheromonal system used extensively in insect communication. Using gas chromatography, we found family specificity in the CHC profiles of male crickets, Teleogryllus oceanicus. Across CHC peaks, our mean coefficient of additive genetic variation was 10.8%. Multivariate principal component analysis showed that most axes of variation were weighted by CHC peaks with significant additive genetic variation. Our results provide evidence that variation in CHC profiles can reflect genetic relatedness, supporting the widely held belief that this phenotypic trait is used as a mechanism for chemosensory kin recognition.