Kin recognition in the ant Rhytidoponera confusa II. Gestalt odour

Two models have been proposed for the transfer of genetically determined colony odour pheromones in social insects. The ‘Gestalt’ model suggests a complete transfer of pheromones amongst all nestmates whereas the ‘Individualistic’ model suggests no significant odour transfer with nestmates bearing individually distinct odours. In experiments with the ant Rhytidoponera confusa, colonies showed significantly more aggression (12% aggression) towards nestmates that had been kept in cages with non-nestmates than they did to nestmate controls (0% aggression). Colonies also showed significantly less aggression against non-nestmates that had been kept with nestmates in a separate cage (84% aggression) than they did to non-nestmate controls (100% aggression). A second experiment indicated that workers can absorb colony-specific odours from the nest materials of other colonies. However, whilst both experiments indicated a partial Gestalt component to the colony odour (ca. 28%), the Individualistic component seemed to be more important (ca. 72%).     

Absence of aggression but not nestmate recognition in an Australian population of the Argentine ant Linepithema humile

Intraspecific aggression is rare within introduced populations of the Argentine ant Linepithema humile, and colonies exhibit a structure known as unicoloniality, in which aggression among nests is atypical. We document a similar form of colony structure in an introduced population of Argentine ants in Victoria, Australia, in which aggression is extremely rare among nests ranging over hundreds of kilometres. However, using a highly sensitive behavioural bioassay we found that workers display subtle differences in their behaviour towards non-nestmates and nestmates. In particular, non-nestmates consistently engage in antennating behaviour with greater frequency than nestmates, perhaps providing a mechanism for homogenization of nest odour. Further, we found that non-nestmates at seaport sites (where populations may derive from multiple introductions) antennate each other with greater frequency than their counterparts from non-seaport sites. These data suggest that the Victorian population of L. humile may comprise multiple independent introductions. Received 4 July 2007; revised 15 January and 4 March 2008; accepted 4 March 2008.     

Not one odour but two: A new model for nestmate recognition

Recognition systems play a key role in a range of biological processes, including mate choice, immune defence and altruistic behaviour. Social insects provide an excellent model for studying recognition systems because workers need to discriminate between nestmates and non-nestmates, enabling them to direct altruistic behaviour towards closer kin and to repel potential invaders. However, the level of aggression directed towards conspecific intruders can vary enormously, even among workers within the same colony. This is usually attributed to differences in the aggression thresholds of individuals or to workers having different roles within the colony. Recent evidence from the weaver ant Oecophylla smaragdina suggests that this does not tell the whole story. Here I propose a new model for nestmate recognition based on a vector template derived from both the individual’s innate odour and the shared colony odour. This model accounts for the recent findings concerning weaver ants, and also provides an alternative explanation for why the level of aggression expressed by a colony decreases as the diversity within the colony increases, even when odour is well-mixed. The model makes additional predictions that are easily tested, and represents a significant advance in our conceptualisation of recognition systems.     

Social wasp parasites affect the nestmate recognition abilities of their hosts (<Emphasis Type="Italic">Polistes atrimandibularis</Emphasis> and <Emphasis Type="Italic">P. biglumis</Emphasis>, Hymenoptera,Vespidae)

Summary. Queens of the parasitic social wasp, Polistes atrimandibularis, temporarily mimic the odor of their host species, Polistes biglumis, but their offspring have parasite-specific odors. As a consequence, in parasitized colonies individuals with different odors co-inhabit the colony and host workers, who are responsible for colony defense, accept wasps with different odors. In order to verify whether this particular condition causes a change in recognition abilities of hosts, we tested nestmate/non-nestmate discrimination in field colonies invaded by social parasites (and in non-parasitized colonies as controls). Results show that parasitized colonies distinguish between nestmates and non-nestmates, distinguish their parasite queen from those that usurped alien colonies, and accept their parasite's non-mimetic offspring but make more recognition errors than non-parasitized colonies. The optimal acceptance threshold model predicts that when the frequency of encountering non-kin increases, residents become less permissive towards intruders. However, my data show that parasitized colonies are more permissive towards non-nestmates with respect to non-parasitized colonies but they are also more aggressive towards nestmates, suggesting that host workers' learning abilities are impaired.     

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.     

Thievery,home ranges,and nestmate recognition inEctatomma ruidum

Summary Thievery of food items among colonies of a ponerine ant,Ectatomma ruidum was common; nonnestmates in colonies or near the colony entrances receive incoming food items and carry them to their own colony. In our study area 7 of 10 colonies were victimized by thief ants. Colonies have discrete home ranges and home range size is correlated with the number of workers in the colony. Worker ants discriminate nestmates from non-nestmates when non-nestmates are presented at colony entrances, but individuals from different colonies were not observed to engage in agonistic interactions away from nest entrances. Non-nestmates gain entrance to colonies when the entrance is unguarded. Many instances of non-nestmates being removed from colonies by residents were observed. The costs and benefits of theft under these circumstances are considered.     

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.     

Aliens among us: nestmate recognition in the social huntsman spider, Delena cancerides

Unlike all other social spiders, the social huntsman spider, Delena cancerides, has been reported to rapidly respond to non-nestmates with lethal aggression, similar to the behavior of some eusocial insects. We tested for the presence of nestmate recognition in D. cancerides under laboratory conditions by introducing 105 unrelated alien conspecifics into foreign colonies and comparing their behavior to 60 control spiders removed and returned to their natal colony. Spiders demonstrated nestmate recognition by investigating alien spiders far more than nestmates and by resting closer to nestmates than to aliens. Serious attacks or deaths occurred in 23% of all trials; however, aggression was not directed significantly more toward aliens than to nestmates. Most notably, aggression was largely mediated by the adult females (resident or alien), who were most likely to attack or kill other subadult or mature individuals. Young individuals (resident or alien) were largely immune from serious aggression. Spiders recently collected from the field tended to be more aggressive than spiders born and raised in the laboratory, possibly due to blurring of recognition cues related to laboratory husbandry. Our findings support the prediction that nestmate recognition should evolve when there is a benefit to discriminating against non-kin, as in this social spider system where foraging individuals may enter a foreign colony and the colony retreat is a limited resource.     

Brood adoption in the leaf-cutting ant <Emphasis Type="Italic">Acromyrmex echinatior</Emphasis>: adaptation or recognition noise?

The ability to discriminate between nestmates and non-nestmates is an important prerequisite for the evolution of eusociality. Indeed, social insect workers are typically able to discriminate between nestmate and non-nestmate workers. Adult non-nestmate workers are readily detected and rejected from the colony. Whether social insects can discriminate between nestmate and non-nestmate brood, however, is less clear. Here, we show that workers of the leaf-cutting ant Acromyrmex echinatior discriminate between nestmate and non-nestmate brood, and among brood of different stages. Initially, non-nestmate brood is attacked, but it is adopted after a delay. Adoption could occur due to inefficiency of the recognition system, or it could be adaptive because it is an inexpensive way to increase the workforce. Our results suggest that brood adoption may occur accidentally. We also report how workers replace fungal hyphae on the brood’s surface before transporting the brood into their fungus garden.     

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.     

Odour convergence and tolerance between nestmates through trophallaxis and grooming in the ant <Emphasis Type="Italic">Camponotus fellah</Emphasis> (Dalla Torre)

Summary Social isolation provides a useful tool to study nestmate recognition in ants. In Camponotus fellah, reintroduction of 10-day isolated (IS) workers to their colony resulted in intensive trophallaxis and grooming, while longer isolation periods generally provoked rejection of the IS ants. In the first experiment the behaviour of queenless (QL) and queenright (QR) workers towards 10-day IS workers was tested. Trophallaxis of QL or QR with IS workers was of similar magnitude, but was significantly higher than that among the QL or QR, or that between QL and QR workers. Allogrooming was mostly initiated by the resident non-isolated ants (QL or QR) possibly because they detected a slight mismatch between the IS ants odour and their own template, which represents the group odour. It appears that the presence/absence of the queen did not affect nestmate recognition cues of workers.The second experiment demonstrated that 20-day IS workers were strongly aggressed by colony guards, irrespective of whether they were QL or QR. However, if they were permitted to exchange trophallaxis and grooming with 5 young nestmates (companion ants) for 5 days before reintroduction to their colony, aggression was greatly reduced, irrespective of the origin of the companion ants (QR or QL). Chemical analysis showed a significant divergence between the hydrocarbon profiles of IS and both QL and QR groups, but a prior contact of the IS workers with companion ants resulted in re-convergence of their profile with that of the colony. These results demonstrate that nestmate recognition cues are exchanged between workers via trophallaxis and grooming and that they are not dominated by queen cues, two conditions that fulfil Gestalt nestmate recognition signals requirements.Received 26 February 2003; revised 24 July 2003; accepted 1 August 2003.     

Friend or foe? Differential aggression towards neighbors and strangers in the ant Liometopum microcephalum (Hymenoptera: Formicidae)

Intra- and interspecific aggression is quite common in ants, from occasional conflicts to large-scale territorial disputes. The “nasty neighbor” phenomenon describes the differential aggressive treatment of neighbors versus foreign intruders. Due to the fact that workers of a given colony meet rival neighbors more often at food resources, they treat them as threats to their colony. The reverse can also happen: the “dear enemy” effect arises when an already known rival is treated less aggressively due to accommodation, than an unknown distant one. In the current study, we analyzed the effect of distance on the aggressiveness of Liometopum microcephalum, a territorial arboricolous ant, towards non-nestmates in two large populations. Our results show that aggression did not increase with distance, but it was higher among neighbors than among workers coming from distant nests. The results of the study are consistent with the nasty neighbor scenario, and do not support the hypothesis that the studied populations would be polydomous systems of interconnected nests.     

The Effect of Queen Number on Nestmate Discrimination in the Facultatively Polygynous Ant Pseudomyrmex pallidus (Hymenoptera: Formicidae)

We present evidence indicating that the level of nestmate discrimination in the facultatively polygynous ant Pseudomyrmex pallidus varies predictably with the number of queens in the colony. P. pallidus workers were introduced into observation arenas in either nestmate or non-nestmate pairs. During the 5-min period immediately following the first contact between test ants, all interactions and relative distances were recorded. Aggression between non-nestmates was negatively correlated with the number of queens in the colony and distance between nestmates was positively correlated. These results are consistent with predictions of Reeve's (1989) optimal acceptance threshold model.     

Nestmate discrimination in the harvester termite <Emphasis Type="Italic">Hodotermes mossambicus</Emphasis>

Summary Nestmate discrimination was studied in the African harvester termite Hodotermes mossambicus in trophallactic feeding experiments. The results show that Hodotermes mossambicus is able to discriminate nestmates from non-nestmates, feeding nestmates significantly more than non-nestmates. Experiments in which termites were treated with the antibiotic tetracycline for 48 hours and then used either as donors or as recipients in the trophallactic assay indicate that the degree of similarity in the intestinal flora strongly affects nestmate discrimination. Antibiotic-treated termites were fed more frequently by treated non-nestmates than by untreated nestmates. Pairs of non-nestmates, which were both treated with the antibiotic, exchange food as frequently as untreated nestmates. Pairs of nestmates, on the other hand, out of which only one animal was treated, exhibit nearly as little trophallactic contacts as the non-nestmate controls. The results parallel similar recent results in Reticulitermes speratus and indicate that termites can make use of recognition cues which are quite different from those of social hymenopterans, cues that are produced by intestinal symbionts.Received 5 November 2002; revised 17 and 27 February 2003; accepted 9 March 2003.     

The Effect of a Wasp's Age on Its Cuticular Hydrocarbon Profile and Its Tolerance by Nestmate and Non-Nestmate Conspecifics (Polistes fuscatus, Hymenoptera: Vespidae)

The effect of a wasp's age on its cuticular hydrocarbon profile and its tolerance by nestmate and non-nestmate conspecifics was investigated in a laboratory study of newly eclosed wasps ( Polistes fuscatus ) that were isolated from their comb at eclosion. In blind observations, mature females did not discriminate between young nestmates and non-nestmates that were ≤ 48 h-old. However, mature females did discriminate between young nestmates and non-nestmates that were 72 h old. Specifically, mature females were significantly more tolerant of (and significantly more likely to accept) 72 h-old nestmates than 72 h old non-nestmates. The abundance, relative abundance, and colony specificity of cuticular hydrocarbons changed significantly between 24 h-old and 72 h-old wasps. Our behavioral and chemical evidence indicates that wasps begin to develop a colony signature between 2 and 3 d of age.     

Does lack of intraspecific aggression or absence of nymphs determine acceptance of foreign reproductives in <Emphasis Type="Italic">Macrotermes</Emphasis>?

The rejection or acceptance of a foreign reproductive by an alien colony may not always be as straightforward as cue recognition between worker termites. This paper aims to determine whether adoption of foreign reproductives is caused simply by lack of intraspecific aggression or is contingent on the reproductive status of the host colony. In the fungus-culturing termites, Macrotermes gilvus (Hagen) and Macrotermes carbonarius (Hagen), major workers showed low intraspecific aggression towards non-nestmates irrespective of geographic distance between source colonies. Our results indicated that workers were hardly aggressive towards non-nestmates. In royal cell-swapping experiments, both species responded in a similar way: (1) in host colonies with nymphs present, the foreign reproductives were rejected; while (2) in host colonies without nymphs the foreign reproductives were either accepted and breeding resumed or the host colonies died eventually. Workers from the host colonies preferentially maintained offspring nymphs from which adultoid replacement reproductives develop rather than accepting foreign reproductives. There is no fitness gain for the queenless workers in accepting foreign reproductives; however, there is overall benefit to the newly born population.     

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

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

Specific recognition of reproductive parasite workers by nest-entrance guards in the bumble bee <Emphasis Type="Italic">Bombus terrestris</Emphasis>

Background

The impact of social parasites on their hosts’ fitness is a strong selective pressure that can lead to the evolution of adapted defence strategies. Guarding the nest to prevent the intrusion of parasites is a widespread response of host species. If absolute rejection of strangers provides the best protection against parasites, more fine-tuned strategies can prove more adaptive. Guarding is indeed costly and not all strangers constitute a real threat. That is particularly true for worker reproductive parasitism in social insects since only a fraction of non-nestmate visitors, the fertile ones, can readily engage in parasitic reproduction. Guards should thus be more restrictive towards fertile than sterile non-nestmate workers. We here tested this hypothesis by examining the reaction of nest-entrance guards towards nestmate and non-nestmate workers with varying fertility levels in the bumble bee Bombus terrestris. Because social recognition in social insects mainly relies on cuticular lipids (CLs), chemical analysis was also conducted to examine whether workers’ CLs could convey the relevant information upon which guards could base their decision. We thus aimed to determine whether an adapted defensive strategy to worker reproductive parasitism has evolved in B. terrestris colonies.

Results

Chemical analysis revealed that the cuticular chemical profiles of workers encode information about both their colony membership and their current fertility, therefore providing potential recognition cues for a suitable adjustment of the guards’ defensive decisions. We found that guards were similarly tolerant towards sterile non-nestmate workers than towards nestmate workers. However, as predicted, guards responded more aggressively towards fertile non-nestmates.

Conclusion

Our results show that B. terrestris guards discriminate non-nestmates that differ in their reproductive potential and respond more strongly to the individuals that are a greatest threat for the colony. Cuticular hydrocarbons are the probable cues underlying the specific recognition of reproductive parasites, with the specific profile of highly fertile bees eliciting the agonistic response when combined with non-colony membership information. Our study therefore provides a first piece of empirical evidence supporting the hypothesis that an adapted defensive strategy against worker reproductive parasitism exists in B. terrestris colonies.

     

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.     

Nest volatiles as modulators of nestmate recognition in the ant Camponotus fellah

When ants from alien colonies encounter each other the stereotypic reaction is usually one of aggressive behavior. It has been shown that factors such as queen-derived cues or nest-odors modulate this reaction. Here we examined whether nest volatiles affect nestmate recognition by observing the reaction of nestmates towards individual workers under one of four regimes: completely isolated; isolated but receiving a constant airflow from the mother colony; as previous but with the passage of nest volatiles towards the isolated ants blocked by adsorption on a SuperQ column; or reversed airflow direction-from the isolated ants to the nest interior. Ants that had been completely isolated for three weeks were subjected to aggressive behavior, but not those that had continued to receive airflow from the mother colony. Adsorbing the nest volatiles from the airflow by SuperQ abolished this difference, with these ants now also being subjected to aggression, indicating that nest volatiles can modulate nestmate recognition. Reverse airflow also reduced the level of aggression but to a lesser extent than airflow directed from the mother colony. In queenless colonies the overall aggression was reduced under all regimes, and there was no effect of flow, suggesting that the volatiles involved are queen-borne. The SuperQ adsorbed volatiles originated from Dufour's gland secretions of both workers and queen, implicating them in the process. Cuticular hydrocarbon profiles were not affected by exposure to nest volatiles, suggesting that the latter either constitute part of the recognition cues or affect worker behavior via a different, as yet elusive mechanism.