Similarities in Recognition Cues Lead to the Infiltration of Non-Nestmates in an Ant Species

Chemical cues are among the most important information-sharing mechanisms in insect societies, in which cuticular hydrocarbons play a central role, e.g., from nestmate recognition to queen signaling. The nestmate recognition mechanism usually prevents intruders from taking advantage of the resources stored in the nest. However, nestmate recognition is not unconditionally effective, and foreign individuals can sometimes infiltrate unrelated nests and take advantage of the colony resources. In this study, we investigated the role of overall colony odor profiles on the ability of conspecific workers to drift into unrelated colonies. We hypothesized that drifters would have higher chances of success by infiltrating colonies with the odor profiles most similar to their own nest, avoiding being detected as non-nestmates. By performing a drifting bioassay, we found that workers of the ant Formica fusca infiltrated unrelated conspecific colonies at a rate of 2.4%, significantly infiltrating colonies displaying CHC profiles most similar to their natal nests. Notably, methyl branched hydrocarbons seem to play a role as recognition cues in this species. In addition, we show that environmental rather than genetic factors are responsible for most contributions on the CHC phenotype, presenting ca. of 50% and 27.5% of explained variation respectively, and playing a major role in how worker ants detect and prevent the infiltration of non-nestmates in the colony. Hence, relying on cuticular hydrocarbons similarities could be a profitably evolutionary strategy by which workers can identify conspecific colonies, evade detection by guards, and avoid competition with genetic relatives.

Facultative slave-making ants Formica sanguinea label their slaves with own recognition cues instead of employing the strategy of chemical mimicry

Slave-making ant species use the host workforce to ensure normal colony functioning. Slaves are robbed as pupae from their natal nest and after eclosion, assume the parasite colony as their own. A possible factor promoting the successful integration of slaves into a foreign colony is congruence with the slave-makers in terms of cuticular hydrocarbons, which are known to play the role of recognition cues in social insects. Such an adaptation is observed in the obligate slave-making ant species, which are chemically adjusted to their slaves. To date, however, no reports have been available on facultative slave-making species, which represent an earlier stage of the evolution of slavery. Such an example is Formica sanguinea, which exploit F. fusca colonies as their main source of a slave workforce. Our results show that F. sanguinea ants have a distinct cuticular hydrocarbon profile, which contains compounds not present in free-living F. fusca ants from potential target nests. Moreover, enslaved F. fusca ants acquire hydrocarbons from their slave-making nestmates to such an extent that they become chemically differentiated from free-living, conspecific ants. Our study shows that F. sanguinea ants promote their own recognition cues in their slaves, rather than employing the strategy of chemical mimicry. Possible reasons why F. sanguinea is not chemically well adjusted to its main host species are discussed in this paper.

Mitochondrial and peroxisomal population in post-pharyngeal glands of leaf-cutting ants after lipid supplementation

The post-pharyngeal gland (PPG) occurs in ants and some Sphecidae wasps. Among its several roles is the storage of lipids from food. In order to investigate the effect of lipids on the cell, especially on mitochondria and peroxisomes, the present study was aimed at examining the peroxisomal and mitochondrial population in the PPG of Atta sexdens rubropilosa after lipid supplementation by confocal laser scanning microscopy and transmission electron microscopy. Soybean oil provided as lipid supplement was not toxic for A. sexdens rubropilosa workers for the first 48 h and 120 h. However, the ultrastructural cytochemical analysis revealed an accumulation of lipid droplets in the PPGs of ants after lipid supplementation at 48 h and 120 h, and smaller lipid droplets in the basal membrane of the PPG epithelium, showing lipid mobilization from the PPG to the hemolymph. The lipid supplementation reduces the life expectancy of medium workers, probably due to the high lipid metabolism. Most importantly, the PPGs of medium workers of leaf-cutting A. sexdens rubropilosa is probably a specialized gland in the lipid metabolism, due to the increased mitochondrial and peroxisomal population inside cells after lipid supplementation; participation of peroxisomal population in the β-oxidation of long chain fatty acids into smaller chains and participation of mitochondrial population in the β-oxidation of fatty acids for energy, or mobilization of lipid derivatives from the PPG to hemolymph, a process that requires energy. However, the hypothesis that the PPGs convert lipids from food in aldehydes and/or hydrocarbons must be better investigated.

Incomplete homogenization of chemical recognition labels between Formica sanguinea and Formica rufa ants (Hymenoptera: Formicidae) living in a mixed colony

Formica sanguinea Latreille (Hymenoptera: Formicidae) is a slave-making species, i.e., it raids colonies of host species and pillages pupae, which are taken to develop into adult workers in a parasite colony. However, it has been unclear if the coexistence of F. sanguinea with slave workers requires uniformity of cuticular hydrocarbons (CHCs), among which those other than n-alkanes are believed to be the principal nestmate recognition cues utilized by ants. In this study, a mixed colony (MC) of F. sanguinea and Formica rufa L. as a slave species was used to test the hypothesis that CHCs are exchanged between the species. Chemical analysis of hexane extracts from ants' body surfaces provided evidence for interspecific exchange of alkenes and methyl-branched alkanes. This result was confirmed by behavioral tests during which ants exhibited hostility toward conspecific individuals from the MC but not toward ones from homospecific colonies of their own species. However, it seems that species-specific differences in chemical recognition labels were not eliminated completely because ants from the MC were treated differently depending on whether they were con- or allospecific to the individuals whose behavioral reactions were tested. These findings are discussed in the context of mechanisms of colony's odor formation and effective integration of slaves into parasite colony.

Eusocial evolution and the recognition systems in social insects

Eusocial species, animals which live in colonies with a reproductive division of labor, typically have closed societies, in which colony members are allowed entry and nonmembers, including animals of the same species, are excluded. This implies an ability to discriminate colony members ("self") from nonmembers ("nonself"). We draw analogies between this type of discrimination and MHC-mediated cellular recognition in vertebrates. Recognition of membership in eusocial colonies is typically mediated by differences in the surface chemistry between members and nonmembers and we review studies which support this hypothesis. In rare instances, visual signals mediate recognition. We highlight the need for better understanding of which surface compounds actually mediate recognition and for further work on how differences between colony members and nonmembers are perceived.

Nestmate recognition in social insects: overcoming physiological constraints with collective decision making

Social insects rank among the most abundant and influential terrestrial organisms. The key to their success is their ability to form tightly knit social groups that perform work cooperatively, and effectively exclude non-members from the colony. An extensive body of research, both empirical and theoretical, has explored how optimal acceptance thresholds could evolve in individuals, driven by the twin costs of inappropriately rejecting true nestmates and erroneously accepting individuals from foreign colonies. Here, in contrast, we use agent-based modeling to show that strong nestmate recognition by individuals is often unnecessary. Instead, highly effective nestmate recognition can arise as a colony-level property from a collective of individually poor recognizers. Essentially, although an intruder can get by one defender when their odor cues are similar, it is nearly impossible to get past many defenders if there is the slightest difference in cues. The results of our models match observed rejection rates in studies of ants, wasps, and bees. We also show that previous research in support of the optimal threshold theory approach to the problem of nestmate recognition can be alternatively viewed as evidence in favor of the collective formation of a selectively permeable barrier that allows in nestmates (at a significant cost) while rejecting non-nestmates. Finally, this work shows that nestmate recognition has a stronger task allocation component than previously thought, as colonies can nearly always achieve perfect nestmate recognition if it is cost effective for them to do so at the colony level.

Is the postpharyngeal gland of a solitary digger wasp homologous to ants? Evidence from chemistry and physiology

The postpharyngeal gland (PPG) was thought to be restricted to ants where it serves a crucial function in the generation of the colony odour. Recently, head glands that closely resemble the PPG of ants were discovered in females of a solitary digger wasp, the European beewolf. The function of this gland necessarily differs from ants: beewolf females apply the secretion of their PPG onto the bodies of paralysed honeybees that serve as larval provisions in order to delay fungus growth. Since ants and digger wasps are not closely related, the occurrence of this gland in these two taxa might either be due to convergent evolution or it is a homologous organ inherited from a common ancestor. Here we test the hypothesis that the PPGs of both taxa are homologous by comparing characteristics of chemical composition and physiology of the PPG of beewolves and ants. Based on reported characteristics of the PPG content of ants, we tested three predictions that were all met. First, the PPG of beewolves contained mainly long-chain hydrocarbons and very few compounds with functional groups. Second, the composition of hydrocarbons in the beewolf PPG was similar to that of the hemolymph. Taking the structure of the gland epithelium and the huge requirements of beewolf females for gland secretion into account this result suggests that the content of the PPG is also sequestered from the hemolymph in beewolves. Third, the chemical composition of the PPG and the cuticle was similar in beewolves since cuticular hydrocarbons derive either from the hemolymph or the PPG. Taking the considerable morphological similarities into account, our results support the hypothesis of a homologous origin of the PPG in beewolves and ants.

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.

Colony fusion in Argentine ants is guided by worker and queen cuticular hydrocarbon profile similarity

Introduced populations of the Argentine ant, Linepithema humile, have experienced moderate to severe losses of genetic diversity, which may have affected nestmate recognition to various degrees. We hypothesized that cuticular hydrocarbons (CHC) serve as nestmate recognition cues, and facilitate colony fusion of unrelated L. humile colonies that share similar CHC profiles. In this study, we paired six southeastern U.S. L. humile colonies in a 6-month laboratory fusion assay, and determined if worker and queen CHC profile similarity between colonies was associated with colony fusion and intercolony genetic similarity. We also compared worker and queen CHC profiles between fused colony pairs and unpaired controls to determine if worker and queen chemical profiles changed after fusion. We found that colony fusion correlated with the CHC similarity of workers and queens, with the frequency of fusion increasing with greater CHC profile similarity between colonies. Worker and queen CHC profile similarity between colonies also was associated with genetic similarity between colonies. Queen CHC profiles in fused colonies appeared to be a mix of the two colony phenotypes. In contrast, when only one of the paired colonies survived, the CHC profile of the surviving queens did not diverge from that of the colony of origin. Similarly, workers in non-fused colonies maintained their colony-specific CHC, whereas in fused colonies the worker CHC profiles were intermediate between those of the two colonies. These results suggest a role for CHC in regulating interactions among mutually aggressive L. humile colonies, and demonstrate that colony fusion correlates with both genetic and CHC similarities. Further, changes in worker and queen chemical profiles in fused colonies suggest that CHC plasticity may sustain the cohesion of unrelated L. humile colonies that had fused.

The chemistry of the postpharyngeal gland of female European beewolves

Females of the European beewolf, Philanthus triangulum, possess a large glove-shaped gland in the head, the postpharyngeal gland (PPG). They apply the content of the PPG to their prey, paralyzed honeybees, where it delays fungal infestation. Here, we describe the chemical composition of the gland by using combined GC-MS, GC-FTIR, and derivatization. The PPG of beewolves contains mainly long-chain unsaturated hydrocarbons (C23-C33), lower amounts of saturated hydrocarbons (C14-C33), and minor amounts of methyl-branched hydrocarbons (C17-C31). Additionally, the hexane-soluble gland content is comprised of small amounts of an unsaturated C25 alcohol, an unknown sesquiterpene, an octadecenylmethylester, and several long-chain saturated (C25, C27) and unsaturated (C23-C27) ketones, some of which have not yet been reported as natural products. Surprisingly, we found a dimorphism with regard to the major component of the PPG with some females having (Z)-9-pentacosene, whereas others have (Z)-9-heptacosene as their predominant component. The biological relevance of the compounds for the prevention of fungal growth on the prey and the significance of the chemical dimorphism are discussed.

Antenna contact and agonism in the male lobster cockroach, Nauphoeta cinerea

On any given day, about 35% of 80- to 85-day-old socially naïve male (SNM) lobster cockroaches (Nauphoeta cinerea) spontaneously adopted an aggressive posture (AP) without encountering another male [spontaneous AP (SAP)]. Although SAP SNMs showed significantly higher release of the pheromone 3-hydroxy-2-butanone (3H-2B) than non-SAP SNMs, there was no significant difference in hemolymph juvenile hormone (JH) III titer. When different body parts were tested for induction of the attack behavior, the antenna was found to be the most effective. After 1 min of contact with an antenna from another SAP SNM, attack behavior was induced in 100% of SAP and 76.2% of non-SAP SNMs, and the JH III titer was significantly increased in all responders. Among the non-SAP SNMs, the JH III titer before antenna contact was significantly lower in the non-responders than in the responders, and, although the JH III increase induced by 1 min antenna contact was similar between responders and non-responders, the final JH III titer of the non-responders was significantly lower. A similar attack response, JH III titer change, and 3H-2B release were seen when the individual's own antenna was used. After 5 min of contact with an antenna from another SAP SNM, attack behavior was induced in 100% of SAP and 82% of non-SAP SNMs; in the former, 3H-2B release was similar before and after antenna contact, but the JH III titer was significantly increased after antenna contact, while, in the latter, both 3H-2B release and JH III titer were significantly increased after antenna contact. Among the non-SAP SNMs, JH III titer in the non-responders was not elevated after 5 min antenna contact, and was significantly lower than that in the responders. A pentane-washed antenna did not induce attack behavior or increase the hemolymph JH III titer, and a pentane-washed antenna coated with 3H-2B also failed to induce attack behavior. These results indicate that N. cinerea male-male agonistic interactions, to which the vertebrate challenge hypothesis can be applied, are due to contact pheromone on the antenna, resulting in the concomitant expression of attack behavior and an increase in 3H-2B release and JH III titer.

A 'social' gland in a solitary wasp? The postpharyngeal gland of female European beewolves (Hymenoptera, Crabronidae)

Exocrine glands play an important role in maintaining the integrity of colonies of social Hymenoptera. The postpharyngeal gland (PPG) of ants is crucial for the generation of a nest odour that enables nestmate recognition. The evolutionary history of this gland is unknown and it was thought to be restricted to ants. Here we describe an exocrine head gland in females of a solitary crabronid wasp, the European beewolf, Philanthus triangulum, that resembles the PPG of ants in many respects. The newly described gland has the same location and the same glove like shape as in ants, and it also has a monolayered epithelium with similar ultrastructure. Unlike in ants, the epithelium bears hairs that reach into the lumen of the gland. Although the PPG of beewolves serves a completely different function it is also associated to an allogrooming behaviour as in ants. Based on these morphological and behavioural similarities as well as similarities in the chemical composition of the content of the PPG of both taxa, we hypothesise that the PPGs of ants and beewolves have a common evolutionary origin. Thus, our results suggest that the PPG in ants might not have evolved in response to social requirements but might have already existed in solitary predecessors.

Males of a solitary wasp possess a postpharyngeal gland

The postpharyngeal gland has long been thought to occur only in ants. Here we characterize, by use of light and electron microscopy as well as 3D reconstruction based on nuclear magnetic resonance (NMR) imaging data, a large cephalic gland reservoir of males of a solitary digger wasp, the European beewolf, Philanthus triangulum. Several lines of evidence suggest that this reservoir is a postpharyngeal gland. The gland reservoir originates from the posterior part of the pharynx and consists of two pairs of unbranched tubular structures that occupy a large portion of the head capsule. Its wall is composed of a unicellular epithelium that is lined by a cuticle. The gland contains a blend of hydrocarbons and compounds with functional groups, and we show that the hydrocarbon fraction of the pheromone is congruent with the hydrocarbons on the cuticle. We discuss the implications of our findings for the evolution of the postpharyngeal gland in ants.

A selfish function of a "social" gland? A postpharyngeal gland functions as a sex pheromone reservoir in males of the solitary wasp Philanthus triangulum

The postpharyngeal gland (PPG) has long been assumed to be restricted to ants, where it mainly functions in the maintenance of social integrity. Recently, a PPG has been described in both sexes of a solitary digger wasp, the European beewolf, Philanthus triangulum (Hymenoptera, Crabronidae). Female beewolves use the contents of their PPG to embalm their honeybee prey to delay microbial growth. Here we show that in male beewolves, the PPG serves as a reservoir of the pheromone used to scent-mark their territories. Gas chromatography-mass spectrometry (GC-MS) analysis of PPG contents identified 55 substances including long-chain aliphatic hydrocarbons, and 13 substances with functional groups. The composition was consistent with the composition of the marking pheromone of male European beewolves described earlier. Comparisons of the PPG contents, and total-head extracts showed a strong congruency, suggesting that total-head extracts can be used for the analysis of marking secretion in beewolves. Furthermore, we found a dimorphism in the composition of the PPG contents, based on significant differences in the proportions of seven compounds between the two morphs.

Behavioural and chemical studies of discrimination processes in the leaf-cutting ant Acromyrmex laticeps nigrosetosus (Forel, 1908)

Leaf-cutting ants live in symbiosis with a basidiomycete fungus that is exploited as a source of nutrients for ant larvae. Tests of brood transport revealed that Acromyrmex laticeps nigrosetosus workers did not discriminate a concolonial brood from an alien brood. The same result was observed with tests of fungus transport. Adult workers showed no aggressive behaviour to workers from other alien colonies (non-nestmates). There was no qualitative variation in the chemical profiles of larvae, pupae and adult workers from the different colonies. However, quantitative differences were observed between the different colonies. Hypotheses about the lack of intraspecific aggression in this subspecies of ants are discussed.

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

Cuticular hydrocarbons are used by some ants to discriminate nestmates from nonnestmates. Every member of the colony bears the same pattern because they are continuously exchanged among nestmates. The postpharyngeal gland (PPG) stores the blend of hydrocarbons and is involved in the distribution of this common mixture. However, some individuals might display individual information on the cuticle (such as a chemical signal of fertility) that must not be mixed within the common pool. We investigated how this paradox is solved in the ant Pachycondyla goeldii by analyzing the nature and localization of colonial and fertility signals. Workers in a queenless condition showed a dominance hierarchy that was correlated with ovarian development. Hydrocarbons from the cuticle and the PPG analyzed by gas chromatography (GC) and identified by GC-mass spectrometry showed a clear discrimination among colonies, supporting the involvement of the PPG in the colonial identity signal. We identified and selected 11 cuticular hydrocarbons that permitted us to discriminate ovarian development classes and that might function as a fertility signal. They allowed clear colony discrimination as well, which suggests that the two signals (the individual signal of fertility and the common signal of colony identity) can be conveyed by the same compounds. However, the hydrocarbons in the PPG did not discriminate among ovarian developmental classes, suggesting that the portion of variation in the cuticular hydrocarbons constituting the fertility signal is superimposed on the signal of colony identity.

Individual Recognition in Ant Queens

Personal relationships are the cornerstone of vertebrate societies, but insect societies are either too large for individual recognition, or their members were assumed to lack the necessary cognitive abilities . This paradigm has been challenged by the recent discovery that paper wasps recognize each other's unique facial color patterns . Individual recognition is advantageous when dominance hierarchies control the partitioning of work and reproduction . Here, we show that unrelated founding queens of the ant Pachycondyla villosa use chemical cues to recognize each other individually. Aggression was significantly lower in pairs of queens that had previously interacted than in pairs with similar social history but no experience with one another. Moreover, subordinates discriminated familiar and unfamiliar dominants in choice experiments in which physical contact, but not odor perception, was prevented and in tests with anaesthetized queens. The cuticular chemical profiles of queens were neither associated with dominance nor fertility and, therefore, do not represent status badges , and nestmate queens did not share a common odor. Personal recognition facilitates the maintenance of stable dominance hierarchies in these small societies. This suggests that the ability to discriminate between individual traits is selected for when it incurs net benefits for the resolution of conflict.

Nest odor dynamics in the social wasp Vespula vulgaris

We investigated nest odor dynamics in the common yellow jacket, Vespula vulgaris. In six isolated colonies, we tested the aggression rates toward dead nestmates that had been stored for 10 min, 10 and 19 days outside their colonies at -76 degrees C. The aggression rate increased from about 12% toward recently killed nestmates up to 30% toward nestmates killed 19 days before the experiment. Obviously, the conserved nest odor profile of the nestmates frozen for several days did not match with that of their colony anymore. This indicates a change of the nest odor within the colony. In a second experiment, we kept two colonies each in one nest box with a complete separation of both neighbor nests by a solid wall inside the box for 28 days. In confrontation experiments, the colony members treated dead foragers from the neighbor nest as aggressively as dead foreign, non-neighbor workers (about 39% each) whereas only about 14% reacted aggressively toward dead nestmates. Seventeen days after the replacement of the solid wall by a metallic grid, which allowed no physical contact but air exchange between the two neighbor colonies, the aggression rates toward foreign workers and nestmates remained relatively unaffected whereas it decreased significantly toward dead neighbors to about 11%. These results suggest a nest odor dynamic caused by volatiles transferred between two adjacent colonies, resulting in an equalization of the former colony specific nest odors. A change of nest odor dynamics influenced by volatiles was so far described only for one ant species at all.

Hydrocarbon circulation and colonial signature in Pachycondyla villosa

In ants, both cuticular and postpharyngeal gland (PPG) hydrocarbons (HCs) have been involved in nestmate recognition. However, no detailed comparison is available. A comparative study including also high density lipophorin (HDLp), an internal HC carrier, was therefore undertaken on Pachycondyla villosa. Purified HDLp is an 820 kDa lipoprotein with a density of 1.114 g/ml and two 245 and 80 kDa apo-proteins. Its hydrocarbon profile is very similar with the cuticular one, in agreement with its hydrocarbon carrier function. Conversely, n-alkanes and externally branched monomethylalkanes are markedly decreased in the PPG. According to their physical properties, this suggests that they are involved in waterproofing on the cuticle. The PPG actually contains only internally branched mono-, dimethylalkanes or monomethylalkenes; their greater fluidity is more adequate for chemical communication. The percentages of some of them are statistically not different between the cuticle and PPG. Their mixtures vary with colonies and they may thus be involved in colonial signature. A scheme for hydrocarbon circulation is discussed, involving lipophorin, cuticle, PPG and self-grooming in one individual, a pathway complementary or alternative to the selective delivery by lipophorin in some other insects. HCs are then distributed between nestmates' cuticles through allo-grooming and physical contacts.

Parasitic exploitation as an engine of diversity

Parasitic exploitation occurs within and between a wide variety of taxa in a plethora of diverse contexts. Theoretical and empirical analyses indicate that parasitic exploitation can generate substantial genetic and phenotypic polymorphism within species. Under some circumstances, parasitic exploitation may also be an important factor causing reproductive isolation and promoting speciation. Here we review research relevant to the relationship between parasitic exploitation, within species-polymorphism, and speciation in some of the major arenas in which such exploitation has been studied. This includes research on the vertebrate major histocompatibility loci, plant-pathogen interactions, the evolution of sexual reproduction, intragenomic conflict, sexual conflict, kin mimicry and social parasitism, tropical forest diversity and the evolution of language. We conclude by discussing some of the issues raised by comparing the effect of parasitic exploitation on polymorphism and speciation in different contexts.

Cuticular hydrocarbons mediate discrimination of reproductives and nonreproductives in the ant Myrmecia gulosa

In many species of social insects, the cuticular hydrocarbons of adults vary with both colony identity and individual physiology (oogenesis). Such variations have been shown in some ants and social wasps to function in nestmate recognition, but as yet there is no demonstration of their use by workers to recognize egg layers. We report that in the ant Myrmecia gulosa, workers can discriminate queens and fertile workers from infertile individuals based on distinctive blends of long-chained hydrocarbons present both on the cuticle and in the postpharyngeal gland. The purified hydrocarbon fraction of cuticular extracts from queens elicited high interest in workers, unlike the nonhydrocarbon fraction. However, both fractions were necessary to trigger a response of maximal intensity. In contrast, extracts of mandibular and Dufour glands from queens or infertile workers were not treated differentially by workers. We suggest that cuticular hydrocarbons function as pheromones allowing for recognition of the queen as well as egg-laying workers.

Colony insularity through queen control on worker social motivation in ants

We investigated the relative contribution of the queen and workers to colony nestmate recognition cues and on colony insularity in the Carpenter ant Camponotus fellah. Workers were either individually isolated, preventing contact with both queen and workers (colonial deprived, CD), kept in queenless groups, allowing only worker-worker interactions (queen deprived, QD) or in queenright (QR) groups. Two weeks post-separation QD and QR workers were amicable towards each other but both rejected their CD nestmates, which suggests that the queen does not measurably influence the colony recognition cues. By contrast, aggression between QD and QR workers from the same original colony was apparent only after six months of separation. This clearly demonstrates the power of the Gestalt and indicates that the queen is not a dominant contributor to the nestmate recognition cues in this species. Aggression between nestmates was correlated with a greater hydrocarbon (HC) profile divergence for CD than for QD and QR workers, supporting the importance of worker-worker interactions in maintaining the colony Gestalt odour. While the queen does not significantly influence nestmate recognition cues, she does influence colony insularity since within 3 days QD (queenless for six months) workers from different colony origins merged to form a single queenless colony. By contrast, the corresponding QR colonies maintained their territoriality and did not merge. The originally divergent cuticular and postpharyngeal gland HC profiles became congruent following the merger. Therefore, while workers supply and blend the recognition signal, the queen affects worker-worker interaction by reducing social motivation and tolerance of alien conspecifics.

Nestmate recognition signals of the leaf-cutting ant Atta laevigata

Behavioral tests with field colonies of Atta laevigata were performed in order to identify the source of the odors used in nestmate recognition. We tested the postpharyngeal (PPG) and mandibular glands (MG) as putative organs producing chemical signals for nestmate recognition. Chemical analyses of PPG were also undertaken. With a series of bioassays, we confirmed that nestmate recognition is based on cephalic odors and that these odors come mainly from the mandibular gland secretion. We show chemical evidence that odors from MG are dispersed all over the cuticle. Although odors from PPG elicited colony-specific behavioral responses, the types of behaviors they elicited differed from those of nestmate recognition of whole ants or MG extracts. PPG secretion was characterized by long-chain alkanes and methyl branched alkanes of low volatility, whereas MG contained volatile ketones and alcohols.

Social isolation of mature workers affects nestmate recognition in the ant Camponotus fellah

This study investigates the role of social stimulation on nestmate recognition in mature workers of Camponotus fellah. We isolated 4-week-old workers before examining their behaviour in dyadic reunion tests. At the age of 4 weeks, workers are normally intolerant towards both allospecific and homospecific but allocolonial individuals. However, when they were isolated for up to 20 days, allocolonial aggressions decreased while allospecific aggression remained constant. Workers isolated for 20 days also engaged in allocolonial trophallaxis. These results suggest that workers need to be reinforced by social stimulation during their adult life to keep precise nestmate recognition capacities. We discuss our data under the perspective of recent neuroethological data in social insects to propose a mechanism for the formation of the neural template used in the nestmate recognition process.

Chemical ecology and social parasitism in ants

The chemical strategies by which parasites manage to break into the social fortresses of ants offer a fascinating theme in chemical ecology. Semiochemicals used for interindividual nestmate recognition are also involved in the mechanisms of tolerance and association between the species, and social parasites exploit these mechanisms. The obligate parasites are odorless ("chemical insignificance") at the time of usurpation, like all other callow ants, and this "invisibility" enables their entry into the host colony. By chemical mimicry (sensu lato), they later integrate the gestalt odor of this colony ("chemical integration"). We hypothesize that host and parasite are likely to be related chemically, thereby facilitating the necessary mimicry to permit bypassing the colony odor barrier. We also review the plethora of chemical weapons used by social parasites (propaganda, appeasement, and/or repellent substances), particularly during the usurpation period, when the young mated parasite queen synthesizes these chemicals before usurpation and ceases such biosynthesis afterwards. We discuss evolutionary trends that may have led to social parasitism, focusing on the question of whether slave-making ants and their host species are expected to engage in a coevolutionary arms race.

Hydrocarbon site of synthesis and circulation in the desert ant Cataglyphis niger

Chemical analyses revealed that in Cataglyphis niger both the hemolymph and the crop contain the same hydrocarbons that are found in the postpharyngeal gland (PPG). On the cuticle, on the other hand, alkanes, and in particular nonacosane, were more abundant than in the PPG. Studies of their biosynthesis in vivo, using intact ants, revealed the presence of newly synthesized hydrocarbons in both the PPG and the crop. In decapitated ants (in the absence of the PPG), however, the crop did not contain any newly synthesized hydrocarbons, indicating the PPG as the major source of crop hydrocarbons. The fat body, as demonstrated by in vitro studies, is the major tissue that biosynthesizes hydrocarbons. The PPG failed to do so, but showed good de novo biosynthesis of other lipid constituents. The large amount of hydrocarbons in the crop suggests that the alimentary canal may serve as an outlet for the overflow of PPG hydrocarbons, or as a route for the directed clearance of hydrocarbons from the PPG.These results confirm and enlarge the model proposed for hydrocarbon circulation in C. niger. They are synthesized by the fat body, released to the hemolymph and transported to the cuticle and the PPG. The PPG hydrocarbons are applied to the cuticle by self-grooming, but can also be cleared via the alimentary canal. Partial emptying of the PPG may facilitate the admixing of recognition cues that the ant may acquire from nestmates by trophallaxis. The reason for the dissimilarity in hydrocarbon composition between the PPG and the cuticle is not yet clear; it may be due to secretions from additional glands, or reflect deviant hydrocarbon transport mechanisms between the PPG and the cuticle.

Camponotus fellah colony integration: worker individuality necessitates frequent hydrocarbon exchanges

Our aim was to test the existence of Gestalt colony odour in Camponotus fellah. We isolated individual workers to prevent trophallaxis, allogrooming and body contact. After 20 days, the cuticular hydrocarbon profile of the isolated ants diverged from that of the parent colony. Moreover, each isolated individual had its own specific blend. This procedure showed that after about 20 days of isolation there was a turnover of the colony odour, revealing the genetically expressed hydrocarbon profile of each individual. It also showed that the cuticular hydrocarbon profile is polymorphic, and that its homogeneity within a colony is maintained by frequent exchanges of hydrocarbons between workers. Behavioural observations of resident workers, in their nest, towards nestmates reintroduced after isolation indicated that a short isolation period (3-5 days), which induced a minor change in hydrocarbon profile, provoked frequent trophallactic solicitations. These were likely to permit the isolated ants to readjust their hydrocarbon profile to that of the ants in the mother colony. Longer isolation periods (20-40 days) induced a greater change in hydrocarbon profile and made the residents intolerant towards their introduced nestmates. Therefore, our results clearly support the existence of a Gestalt colony odour in C. fellah. They also show that since individual hydrocarbon production is dynamic, workers are obliged to exchange hydrocarbons continually (mainly by trophallaxis) in order to be in the Gestalt, and properly integrate into the colony. Copyright 2000 The Association for the Study of Animal Behaviour.