Cuticular spectra and inter-individual recognition in the slave-making ant Polyergus rufescens and the slave species Formica rufibarbis

Abstract. The relationship between behavioural tests and relative proportions of cuticular components were studied in the slave-making species Polyergus rufescens and the slave and Formica rufibarbis living in either monospecific or mixed colonies. A correlation between the relative proportions of the cuticular products and interindividual recognition exists in each of the two species Polyergus and Formica: Polyergus are fiercely aggressive towards individuals which have different cuticular spectra and originate from a geographically isolated nest. This seems to be true also in the case of Formica living in monospecific colonies. A similar correlation also exists between the two species, which have different cuticular spectra: encounters arranged between them show that free-living Formica are always fiercely aggressive towards Polyergus. The reason why no such correlation seems to exist, however, between Polyergus and Formica when the latter are enslaved and the two species coexist peacefully at the same nest is discussed.     

Cuticular spectra in the slave-making ant Polyergus rufescens and the slave species Formica rufibarbis

Abstract. The relative proportions of cuticular components having the same retention times were compared between the slave-making ant Polyergus rufescens and the slave ant Formica rufibarbis living in monospecific or mixed colonies. The two species were found to present different spectra. The Formica workers, when enslaved by Polyergus , tend to lose their colony characteristics but they do not seem to adopt the characteristics of Polyergus.     

Cuticular hydrocarbons in workers of the slave-making ant Polyergus samurai and its slave, Formica japonica (Hymenoptera: Formicidae)

Comparisons of cuticular hydrocarbons between workers of the dulotic ant Polyergus samurai and its slave, Formica japonica, were carried out. Gas chromatography–mass spectrometry showed that the slave‐maker and its slave shared the major cuticular hydrocarbon compounds, but possessed several minor products unique to each species. No difference in hydrocarbon composition was detected between enslaved and free‐living F. japonica workers, suggesting that association with P. samurai has no qualitative effect on hydrocarbon composition in these ants. Principal component analyses of the cuticular hydrocarbon profiles (CHP) revealed that (i) CHP was species specific in a given mixed colony; and (ii) among mixed colonies, P. samurai workers had species‐colony specific CHP, while the same feature was not always found in enslaved and free‐living F. japonica workers. Therefore, a ‘uniform colony odor’ in terms of CHP is not achieved in naturally mixed colonies of P. samurai nor those of its slaves, F. japonica.     

Facultative and obligate slavery in formicine ants: frequency of slavery, and proportion and size of slaves

Slave-making ants raid nests of other ant species, capture the developing offspring and rear them to slave workers. Here we compare slave-making of three formicine slave-making ants: the facultative Formica subnuda, the obligate Polyergus breviceps, and F. subintegra which previously has been considered facultative but appears to be an obligate slave-making ant. If F. subintegra is an obligate slavemaker, slave-making of F. subintegra should differ from that of F. subnuda but closely resemble slave-making of P. breviceps in the following aspects: (1) Obligate slavemakers are rarer than facultative slavemakers. (2) Slaveless colonies of facultative slavemakers are found, but obligate slavemakers always have slaves. (3) Because obligate slavemakers depend on their slaves, they should have a higher proportion of slaves than facultative slavemakers. (4) Owing to special adaptations obligate slavemakers are able to raid bigger colonies, and hence have bigger slaves than facultative slavemakers. (5) Dufour's gland of F. subintegra should be larger than that of F. subnuda. Per 100 free F. podzolica colonies, the number of P. breviceps and F. subintegra colonies with F. podzolica slaves were 1.3% and 3.9%, respectively, and the number of F. subnuda colonies with F. podzolica 3.7%, and without F. podzolica 7.5%. The proportion of slaves, when present, varied between 1–30% in the colonies of F. subnuda, and between 70–90% in the colonies of the other species. The slaves of F. subnuda were significantly smaller than those of F. subintegra and P. breviceps. The length of F. subnuda's Dufour's gland was one third of the length of F. subintegra's gland. The results show that slave-making of F. subintegra parallels that of P. breviceps, and contrary to the earlier notion, F. subintegra is an obligate slave-making ant. We suggest that F. subnuda and F. subintegra represent extreme modes of slave-making behaviour in the Formica sanguinea group.     

Blending in with the crowd: social parasites integrate into their host colonies using a flexible chemical signature

Social parasites are able to exploit their host's communication code and achieve social integration. For colony foundation, a newly mated slave-making ant queen must usurp a host colony. The parasite's brood is cared for by the hosts and newly eclosed slave-making workers integrate to form a mixed ant colony. To elucidate the social integration strategy of the slave-making workers, Polyergus rufescens, behavioural and chemical analyses were carried out. Cocoons of P. rufescens were introduced into subcolonies of four potential host species: Formica subgenus Serviformica (Formica cunicularia and F. rufibarbis, usual host species; F. gagates, rare host; F. selysi, non-natural host). Slave-making broods were cared for and newly emerged workers showed several social interactions with adult Formica. We recorded the occurrence of abdominal trophallaxis, in which P. rufescens, the parasite, was the donor. Social integration of P. rufescens workers into host colonies appears to rely on the ability of the parasite to modify its cuticular hydrocarbon profile to match that of the rearing species. To study the specific P. rufescens chemical profile, newly emerged callows were reared in isolation from the mother colony (without any contact with adult ants). The isolated P. rufescens workers exhibited a chemical profile closely matching that of the primary host species, indicating the occurrence of local host adaptation in the slave-maker population. However, the high flexibility in the ontogeny of the parasite's chemical signature could allow for host switching.     

Chemical integration of Thorictus myrmecophilous beetles into Cataglyphis ant nests

Thorictus beetles of the Dermestidae are obligate myrmecophiles. To understand how these beetles are integrated into and tolerated by their host colonies, the cuticular hydrocarbon profiles of different species of the Thorictus castaneus group that are generally associated with Cataglyphis were examined. The beetles are characterized by small amounts of cuticular hydrocarbons, which render them partly chemically “insignificant”. They also have the same cuticular hydrocarbon profiles as their hosts and thus likely use chemical mimicry to evade worker hostility but, like slaves in slave-maker species, they maintain some partial chemical identity. Thorictus martinezi from Burkina Faso were immediately adopted by conspecific colonies of their host, Cataglyphis sp. aff. bicolor, but were never adopted by colonies of other species (i.e. Cataglyphis viatica and Formica selysi). Thorictus buigasi from Morocco also mimicked the chemical profile of its host, C. viatica, but, in contrast to T. martinezi, individuals were adopted by colonies of Cataglyphis velox from Spain. This result can be explained by the similarity between the hydrocarbon profiles of C. viatica and C. velox, which may facilitate adoptions. T. buigasi beetles remained in Formica selysi colonies for some time but were ultimately rejected, probably due to their very different hydrocarbon profiles. In contrast, they were sometimes adopted by Camponotus herculeanus colonies and eventually chemically matched their new hosts, probably by passive camouflage. These data suggest that Thorictus of castaneus group myrmecophily is the result of coevolution with Cataglyphis hosts and that the mimicry is plastic, such that beetles can live with different hosts if the hosts show very limited CHC differences.     

Chemical deterrent enables a socially parasitic ant to invade multiple hosts

Social parasites are involved in a coevolutionary arms race, which drives increasing specialization resulting in a very narrow host range. The Formicoxenus ants are a small group of social parasites with a xenobiotic lifestyle. Formicoxenus quebecensis and Formicoxenus provancheri are highly specialized ants using chemical mimicry to blend into their respective Myrmica ant host colonies. However, Formicoxenus nitidulus is unique in being able to survive in over 11 different ant host species. We observed that when live or dead F. nitidulus adults are seized by their host they are immediately dropped undamaged, despite possessing a cuticular hydrocarbon profile that differs markedly from its host. Hexane extracts of the F. nitidulus cuticle made previously acceptable prey items unattractive to their Formica host, indicating a chemical deterrent effect. This is the first time that a social parasite has been shown to exploit the generalized deterrence strategy to avoid host aggression over long periods of time. This supports the idea that coevolved and generalist diseases or parasites require fundamentally different defence mechanisms. We suggest that F. nitidulus uses its cuticular chemistry, possible alkadienes, as a novel deterrent mechanism to allow it to switch hosts easily and so become a widespread and abundant social parasite.     

The Effect of Social Parasitism by Polyergus breviceps on the Nestmate Recognition System of Its Host,Formica altipetens

Highly social ants, bees and wasps employ sophisticated recognition systems to identify colony members and deny foreign individuals access to their nest. For ants, cuticular hydrocarbons serve as the labels used to ascertain nest membership. Social parasites, however, are capable of breaking the recognition code so that they can thrive unopposed within the colonies of their hosts. Here we examine the influence of the socially parasitic slave-making ant, Polyergus breviceps on the nestmate recognition system of its slaves, Formica altipetens. We compared the chemical, genetic, and behavioral characteristics of colonies of enslaved and free-living F. altipetens. We found that enslaved Formica colonies were more genetically and chemically diverse than their free-living counterparts. These differences are likely caused by the hallmark of slave-making ant ecology: seasonal raids in which pupa are stolen from several adjacent host colonies. The different social environments of enslaved and free-living Formica appear to affect their recognition behaviors: enslaved Formica workers were less aggressive towards non-nestmates than were free-living Formica. Our findings indicate that parasitism by P. breviceps dramatically alters both the chemical and genetic context in which their kidnapped hosts develop, leading to changes in how they recognize nestmates.     

Integration raids in the Amazon ant Polyergus rufescens (Hymenoptera, Formicidae)

Summary. Groups of enslaved Formica fusca workers from mixed colonies of Polyergus rufescens with numerous slave workforce tend to split off and found small and almost homospecific nests around the main nest, with at least some of them connected with the latter with underground passages. Their inhabitants are able, at least temporarily, to adopt young F. fusca gynes. P. rufescens invades these satellite nests in a manner similar to the normal slave raids, and carries the slaves back to the main nest. The supposed evolutionary cause of this behaviour is to keep integrity of mixed colonies and prevent possible emancipation of slaves.Received 18 August 2004; revised 27 September 2004; accepted 11 October 2004.     

Threat,Signal or Waste? Meaning of Corpses in two Dulotic Ant Species

Ant corpses, besides representing threat of infection by pathogens and parasites, can also be used during interspecific conflicts to inhibit the activity of the attacked colony, or they can be consumed as food. In the view of the former, the signal properties of corpses can be manifold. Besides discriminating nestmates and foes, the corpses of different ant species may act as cues for foragers, signaling the presence of other rival species, and triggering appropriate responses (e.g., alarm, retreat or foraging). In our study, we examined the responses of the facultative slave-maker Formica sanguinea and those of the obligate Polyergus rufescens towards corpses of nestmates, non-nestmate conspecifics, heterospecific slave-makers and their slaves, and corpses of non-enslaved host species under laboratory conditions. Both dulotic species responded differently to corpses of different origin. In F. sanguinea, the most intensive response was elicited by the corpses of P. rufescens and its slave, but also the corpses of non-nestmate conspecifics and their slaves elicited many adverse responses. In P. rufescens, the corpses of non-nestmate conspecifics and their slaves elicited the most adverse response. Both dulotic species distinguished corpses of their slaves from corpses of non-enslaved hosts. Based on our results, ant corpses are not meaningless objects scattered in the field, but cues carrying information that trigger different behavioral responses, and in F. sanguinea they can even represent an important food source.     

Chemical basis for inter-colonial aggression in the stingless bee Scaptotrigona bipunctata (Hymenoptera: Apidae)

Inter-colonial aggression was tested using three colonies of Scaptotrigona bipunctata in a natural setting when their nests were moved and by artificial contact between individuals. Examination of the cuticular lipids of individuals from two colonies kept under identical conditions showed clear differences in their cuticular hydrocarbon profiles. The cuticular lipids were a mixture of hydrocarbons (saturated and unsaturated alkanes and alkenes) within the range of C23-C29. The use of multivariate analysis (PCA and discriminant analysis) showed that seven of the identified surface compounds are enough to separate workers from colonies A and B from each other.     

Intraspecific variation of cuticular hydrocarbon composition in Formica japonica Motschoulsky (Hymenoptera: Formicidae)

Cuticular hydrocarbons and morphological features were compared among 80 Formica japonica colonies collected in Japan. Although a few morphological differences were found in workers among the colonies, four different types of cuticular hydrocarbon composition were observed. This was supported by a principal component analysis. We further compared the cuticular hydrocarbons among a total of approximately 400 F. japonica colonies, and categorized the hydrocarbon components into four types based on the result of discriminant analyses for the first 80 colonies. Type 1 was observed in colonies mainly collected in southern Honshu, Shikoku, and Kyushu. Types 2, 3, and 4 were from colonies with primary collections in Southern Honshu, central and Pacific coast northern Honshu, and the Sea of Japan coasts of northern Honshu and Hokkaido, respectively. The occurrence of four distinct types of CHC composition suggests that the colonies that produce them are separate species.     

The role of an attractive brood pheromone in the obligatory,slavemaking ant,Polyergus breviceps (hymenoptera: Formicidae)

Freeliving workers of Formica occulta, an ant species enslaved by the obligatory slavemaking ant Polyergus breviceps, retrieve and nurse Polyergus pupaejust as well as conspecific pupae in a choice test. No such attraction was found toward pupae of the facultative slavemaker; Formica wheeleri,which also enslaves F. occulta. Formica neogagates,a sympatric species which is not parasitized by either slavemaker, preferentially retrieves and tends conspecific brood over that of Polyergusand F. wheeleri.It is proposed that brood of obligatory slavemaking species must possess an attractive pheromone for slavemaker colony foundation to be successful, since slavemaker brood must be nursed by adult slave workers with no prior exposure to slavemaker brood. An attractive pheromone is not necessary in the brood of facultative slavemakers, since this brood is cared for by newly eclosed slave workers who imprint on the slavemaker brood.     

Is parasite pressure a driver of chemical cue diversity in ants?

Parasites and pathogens are possibly key evolutionary forces driving recognition systems. However, empirical evidence remains sparse. The ubiquitous pioneering ant Formica fusca is exploited by numerous socially parasitic ant species. We compared the chemical cue diversity, egg and nest mate recognition abilities in two Finnish and two UK populations where parasite pressure is high or absent, respectively. Finnish populations had excellent egg and nest mate discrimination abilities, which were lost in the UK populations. The loss of discrimination behaviour correlates with a loss in key recognition compounds (C₂₅-dimethylalkanes). This was not owing to genetic drift or different ecotypes since neutral gene diversity was the same in both countries. Furthermore, it is known that the cuticular hydrocarbon profiles of non-host ant species remain stable between Finland and the UK. The most parsimonious explanation for the striking difference in the cue diversity (number of C₂₅-dimethylalkanes isomers) between the UK and Finland populations is the large differences in parasite pressure experienced by F. fusca in the two countries. These results have strong parallels with bird (cuckoo) studies and support the hypothesis that parasites are driving recognition cue diversity.     

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

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

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

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

A developmental study of the cuticular hydrocarbons of Sarcophaga bullata.

The accumulation of cuticular hydrocarbon was measured throughout the life of Sarcophaga bullata. Less than 5 μg hydrocarbon per insect are present until the third larval instar when synthesis increases the quantity present to 10 to 20 μg by pupariation. The rate of synthesis increases at this time to 5 to 8 μg/day and continues until 40 to 45 μg are present per insect. This amount remains constant until several days before the pupal-adult ecdysis when synthesis again occurs. The rate of synthesis by these pharate adults is >20 μg/day. When the adult emerges it contains between 90 and 100 μg which increases slightly during the adult stadium. Two periods of rapid accumulation of cuticular hydrocarbon are observed: (1) during pupariation and the 3 day period following pupariation, and (2) during the 4 day period preceding the pupal-adult ecdysis. When pupariation is inhibited by contact with water, the rate of hydrocarbon biosynthesis also fails to increase.     

Patterns of biosynthesis and accumulation of hydrocarbons and contact sex pheromone in the female german cockroach,Blattella germanica

De novo synthesis of contact female sex pheromone and hydrocarbons in Blattella germanica was examined using short in vivo incubations. Accumulation of pheromone on the epicuticular surface and the internal pheromone titer were related to age-specific changes in hydrocarbon synthesis and accumulation in normal and allatectomized females. The incorporation of radiolabel from [1-¹⁴C]propionate into the cuticular methyl ketone pheromone fraction was positively related to corpora allata activity during two gonotrophic cycles. During peak pheromone production the total internal lipid fraction contained greater titers of pheromone than the cuticular surface, and it too exhibited a cycle internally, preceding the rise in external pheromone. This suggests that synthesis and accumulation of pheromone internally are followed by transport of pheromone to the epicuticular surface where it accumulates. Radiolabel was incorporated efficiently into both cuticular and internal hydrocarbons after the imaginal molt and until the peak of pheromone synthesis, but it declined to lower levels before ovulation and throughout pregnancy. The internal hydrocarbon titer decreased 58% after oviposition, suggesting deposition in the egg case. It remained relatively unchanged during pregnancy and increased again during the second gonotrophic cycle. In allatectomized females, hydrocarbon synthesis was reduced relative to control females until oviposition in the latter. However, subsequent rates of hydrocarbon synthesis in allatectomized females (without oothecae) exceeded the rates in sham-operated females (with oothecae). In the absence of ovarian uptake of hydrocarbons, the internal titer increased without the decline found in control females at oviposition. As internal hydrocarbons increased, so did cuticular hydrocarbons and both internal and cuticular methyl ketone pheromones. These patterns corresponded well with feeding patterns in sham-operated and allatectomized females, suggesting that pheromone production is normally regulated by stage-specific feeding-induced hydrocarbon synthesis (precursor accumulation internally) and juvenile hormoneinduced conversion of hydrocarbon to pheromone. They also suggest that both the cuticle and the ovaries might be target sites for hydrocarbon and possibly methyl ketone deposition. © 1994 Wiley-Liss, Inc.     

Comparative study of the cuticular hydrocarbon composition of Melipona bicolor Lepeletier, 1836 (Hymenoptera, Meliponini) workers and queens

In social insects, cuticular hydrocarbons are involved in species, kin, caste and nestmate recognition. Gas chromatography and mass spectrometry were used to compare the cuticular hydrocarbon composition of workers, males and queens of Melipona bicolor. The cuticular hydrocarbon composition of this species was found to consist mainly of C23, C25:1, C25, C27:1, C27, C29:1 and C29, which are already present in imagoes that have not yet abandoned the brood cell. This composition varied quantitatively and qualitatively between and within the castes and sexes. The newly emerged workers and young queens (virgins) had similar cuticular hydrocarbon profiles, which were different from those of the males. When the females start executing their tasks in the colony, the cuticular hydrocarbon profile differences appear. The workers have less variety, while the queens conserve or increase the number of cuticular hydrocarbon compounds. The queens have more abdominal tegumentary glands than the workers, which apparently are the source of the new cuticular compounds.     

Dynamics of cuticular chemical profiles of Polistes dominulus workers in orphaned nests (Hymenoptera, Vespidae)

We analysed changes in cuticular hydrocarbon signatures of workers in orphaned colonies of the paper wasp Polistes dominulus. In natural conditions, workers and foundresses possess characteristic cuticular signatures, and foundresses are further distinguishable, both behaviourally and chemically, on the basis of their rank in a reproductive dominance hierarchy. In our study, several workers were found to develop their ovaries and produce cuticular signatures resembling those of dominant foundresses, while remaining workers possessed undeveloped ovaries and had cuticular blends characteristic of subordinate foundresses. Workers that did not develop their ovaries had changed epicuticular signatures, demonstrating that the mixture of hydrocarbons of worker individuals is strongly dependent on social role and environment. Our results suggest that the composition of epicuticular lipids is not determined at the pre-imaginal stage, and that physiological pathways leading to cuticular chemical changes are similar in foundresses and workers of P. dominulus.