Intraspecific slavery inPolyergus rufescens Latr. (Hymenoptera,Formicidae): field and laboratory observations

Summary Four intraspecific slave raids involving 3 colonies of the European amazon antPolyergus rufescens Latr. occurred during hot and sunny afternoons in July 1992 near Parma, Italy. Generally, no fighting between the resident ants and the invaders was recorded, and pillage of the brood lasted just a few minutes. The brood captured during 3 raids was collected and transported ot the laboratory where slavemaker callow workers eclosed and were accepted by both the raiders and hosts present in artificial mixed colonies. This is the first case of functional intraspecific slavery recorded in the field for the obligatory dulotic formicine antP. rufescens.     

Evidence for adaptive brain tissue reduction in obligate social parasites (Polyergus mexicanus) relative to their hosts (Formica fusca)

Brain investment is evolutionarily constrained by high costs of neural tissue. Several ecological factors favour the evolution of increased brain investment; we predict reduced brain region investment will accompany the evolution of organismal or social parasitism when parasites rely on host behaviour and cognition to solve ecological problems. To test this idea we investigated whether brain region investments differed between obligate slave‐making Polyergus mexicanus ant workers and their Formica fusca slave workers. Polyergus workers perform little labour for their colonies; enslaved workers of Formica host species forage, excavate nests and tend the brood. We focused on the calyces of the mushroom bodies, central processing brain regions that are larger in social insect workers that perform complex tasks. As predicted we found lower relative investment in mushroom body calyx in P. mexicanus workers than in F. fusca workers; by contrast, enslaved and free F. fusca workers did not differ in mushroom body calyx volume. We then tested whether slave‐makers and hosts differed in brain investment among sensory modalities. Polyergus slave‐makers employ several unique classes of pheromones during raids, and eye size relative to head size was smaller in P. mexicanus workers than in F. fusca workers. The size of antennal brain tissues relative to visual tissues was greater in Polyergus, both in the peripheral sensory lobes and in the mushroom body calyx, suggesting greater relative investment in antennal processing by slave‐makers. © 2014 The Linnean Society of London, Biological Journal of the Linnean Society, 2014, 113 , 415–422.     

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.     

Eco-ethological Study on Raiding Behaviour of the European Amazon Ant,Polyergus rufescens Latr. (Hymenoptera: Formicidae)

In this paper we report the results of a detailed study on the behavioural ecology of slave raiding in the European amazon ant, Polyergus rufescens Latr. The field study, supported also by a video-tape recording technique, was conducted over an unbroken period of 53 days, during which observations of the activity of the residents (both slave-makers and slaves) were made for 10 h each day. It was possible to observe 38 slave raids distributed over 32 days, among which 27 were followed by the sack of 10 different nests of Formica cunicularia Latr., whereas 11 failed because of various reasons. Simple, compound and multiple raids occurred. We recorded the timing, frequency, distance, and direction of slave raids, including the number of participants and the type of captured brood. Moreover, particular attention was paid to the atmospheric conditions present at the moment of the raid onset. Information was also collected about the behaviour of the “activators” and the scouts before and during the movement of the storming column. Both dealate and winged P. rufescens queens, having emerged from the mixed colony during 6 sexual flights, were seen following the outbound raiding column during 4 raids. Finally, some peculiar behaviour, such as digging out the soil near the target nest to facilitate the entry of the raiding swarm, and the pillage of adult ants (eudulosis) was recorded and described. Data have been compared with what is known about the other species of the genus Polyergus.     

Raiding Activity of an Obligate (Polyergus breviceps) and Two Facultative (Formica puberula and F. gynocrates) Slave-Making Ants

Slave-making ants are social parasites that exploit the labor of workers from their host species by keeping them captive in the slave-maker nest. Slave-makers vary in their degree of specialization, ranging from obligate slave-makers that cannot survive without captives, to facultative slave-makers, which are often found living independently. Our study system included one obligate slave-maker, Polyergus breviceps, two facultative slave-makers, Formica puberula and F. gynocrates, and two hosts, F. occulta and F. sp. cf. argentea. We observed all raids conducted during two raiding seasons by seven P. breviceps colonies, two F. puberula colonies, and two F. gynocrates colonies. We report on raiding frequency, average raid distances, and then compare the probability of being raided multiple times in a single raiding season for the two host species. We also report on the spatial distribution of slave raids, which suggests that slave-makers avoid raiding in areas used by other slave-maker colonies. This is the first report of raiding activity for P. breviceps in this location, and the first report of raiding activity of any kind for F. puberula and F. gynocrates.     

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.     

A new taxonomic status for Iberoformica (Hymenoptera,Formicidae) based on the use of molecular markers

The subgeneric subdivision of the genus Formica is still open. In this article, we make a phylogenetic study on several species of the genus Formica and of its closely related genera, Polyergus and Proformica, using sequences of nuclear satellite DNA (stDNA) and the mitochondrial rrnL as molecular markers. Our goal was to shed light on their phylogenetic relationships and particularly on the systematic position of F. subrufa. This species was first included in the subgenus Serviformica, but afterwards a new subgenus (Iberoformica) was established to include only this species. The results show that a stDNA family previously reported in Formica species, with a repetitive unit 129 bp long, is also found in Polyergus rufescens and P. samurai but not in Proformica longiseta. This is the first case of presence of a stDNA family in two different ant genera. In F. subrufa, this stDNA is very divergent relative to those isolated in the remaining Formica species and in the genus Polyergus. The Bayesian analysis of mitochondrial rrnL sequences shows three highly supported groups: F. subrufa, the remaining Formica species studied, and the genus Polyergus, suggesting that parasites (Polyergus species) and hosts (Formica species) are closely related but not sibling species. The combined analysis of nuclear stDNA sequences and mitochondrial rrnL showed their phylogenetic congruence despite their distinct evolutionary dynamics. This analysis did not discriminate between the remaining Formica species that were not grouped according to the subgeneric classification. According to these results, it can no longer be assumed that F. subrufa belongs to the subgenus Serviformica or of the fusca species group. This differentiation was also supported by previous studies based on the morphological characters, molecular and cytogenetic data. Therefore, taking into consideration these arguments and others explained in detail in this article, we propose that the taxon Iberoformica, formerly synonymized subgenus, be raised to a genus status. This genus would be monotypic and only composed, up to the moment, by Iberoformica subrufa (= F. subrufa Roger, 1859 ).     

Eco-ethological Factors Affecting the Scouting and Raiding Behaviour of the Slave-making Ant,Polyergus rufescens Latr. (Hymenoptera,Formicidae)

A dulotic colony of the obligatory slave-making ant Polyergus rufescens was monitored daily in the field (Parma, Italy) from 1100–2000 h over the summers of 1991 and 1992. The first P. rufescens workers to emerge from the nest each day were individually marked and their activity was accurately recorded. The path of 47 such individuals (21 over a complete trip) was followed in detail and mapped. Results confirm a clear scouting activity: the route of these workers away from the nest is generally tortuous and time consuming and is followed by a return trip along a different and straighter path. Observations also showed that these scouts recruit nestmates and lead raiding columns towards target nests, confirming their important role in the organization of slave raids. The similarity between routes taken by scouts and raiders during outbound trips indicates the close connection between scouting and raiding activity, which was also recorded in detail since 40 raids were observed. Some experiments analysing the orientation behaviour of inbound columns showed that raiders use a chemical trail (deposited during the outbound run) to lead them back home. The hypothesis of a multiple strategy for the location of host colonies operated by this slave-making species is also discussed.     

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.     

Raiding and Foraging Behavior of the Blood-Red Ant,Formica sanguinea Latr. (Hymenoptera,Formicidae)

In this paper we report the results of a detailed study on the behavioral ecology of slave raiding and foraging activity in the European blood-red ant, Formica sanguinea Latr. The field study was conducted over an unbroken period of 78 days, during which the activity of two dulotic colonies of this facultative slave-maker was observed for 10 h each day. It was possible to observe 26 raids distributed over 23 days, among which 18 were followed by the sacking of nests belonging to the species F. cunicularia, F. fusca, and Lasius emarginatus, whereas 8 failed. Simple, continuous, and simultaneous raids occurred. We recorded the timing, frequency, distance, and direction of slave raids, including the number of participants and the type of booty. Particular attention was devoted to the scouting behavior and raiding organization. Moreover, every day, we observed foraging and predatory behavior, during which adult insects (mainly ants), seeds, and berries were retrieved to the dulotic colonies. On the basis of our observations F. sanguinea seems to be a very efficient slave-maker and predatory species of the Raptiformica subgenus. Moreover, its dulotic behavior may be regarded as a continuation and an expansion of its foraging and predatory behavior, as predicted by Darwin's hypothesis for the origin and evolution of slavery in ants.     

Untersuchungen zur Orientierung der Raubzüge der AmazonenameisePolyergus Rufescens Latr.

Zusammenfassung An benachbarten Nestern vonPolyergus rufescens wurden gleichzeitig erfolgende Raubzüge genauestens hinsichtlich Richtung, Auszugslänge und Erfolg kontrolliert. Dabei wurde jeweils bei einem Nest die Auszugsrichtung durch Auslegen vonServiformica-Puppen entgegengesetzt zur erkennbaren Auszugs-Tendenz gelenkt. Die Erfolge beider Raubzüge sind nicht voneinander verschieden. Es kann daher angenommen werden, dass im Normalfall weder die einzelnen Kundschafterinnen» (nach Forel) noch die «Aktivistinnen» (nachDobrzanska et al.). wesentlich zumErfolg des Raubzuges beitragen.

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Summary During a longer period the simultaneously occurring raids, from two adjucent nests ofPolyergus rufescens, were exactly controlled with regard to direction, processional length and rate of collected pupae. The direction of the raids of one of the nests was steerd by placingServiformica pupaee opposite to the recognized directional tendency of the raid. The success of both raids was equal in both cases. It can therefore be assumed that in the normal case neither the individual «Kundschafter» (Scoucts (after Forel) nor the «activators» (afterDobrzanska et al.) contribute essentially to the suscess of the raid.

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Mitteilung     

Host nest usurpation and colony foundation in the European amazon ant,Polyergus rufescens Latr. (Hymenoptera: Formicidae)

Summary The socially parasitic mode of founding new colonies by queens of the European amazon antPolyergus rufescens was analysed in the laboratory. Newly-mated females of this obligatory slave-maker were individually introduced into queenright and queenless artificially established colonies of bothFormica cunicularia (the slave present in the natal dulotic nest) andF. rufibarbis (another potentialServiformica host). Particular attention was devoted to the behavioural patterns displayed by these young queens during the usurpation phases. Our observations, supported also by video-taping, show that the slave-making female, before laying her eggs, must penetrate the host colony, kill the resident queen, become accepted by the adult workers and appropriate the host brood. The parasite was almost always adopted in the colonies ofF. cunicularia, whereas in the presence ofF. rufibarbis it was generally killed in a short time. The failure in the attempt of usurping the colonies ofF. rufibarbis is discussed in relation to the host specificity typical of this slave-maker. Finally, egg-laying byPolyergus successful usurpers, the subsequent eclosion of the brood, and its complete social integration in the newly-established mixed colonies were also recorded.     

Characterization of microsatellite loci in red wood ants Formica (s. str.) spp. and the related genus Polyergus

Ants are interesting subjects for studies of evolution of altruism. We developed 13 microsatellite loci in a red wood ant Formica (s. str.) yessensis from random amplified polymorphic DNA fragments to study genetic structure within populations and colonies. Five loci bore two to five alleles in both F. (s. str.) yessensis and F. (s. str.) truncorum and two were also polymorphic in a related species, Polyergus samurai. Results suggest that the loci will be useful in evolutionary studies on Formica and Polyergus species.     

Responses of Two Potential Host Species (Formica gnava and Formica occulta) to Pupae of the Obligatory Slave-Making Ant, Polyergus breviceps (Hymenoptera: Formicidae)

Queens of the slave-making ant, Polyergus breviceps, take over nests of adult Formica workers when establishing new colonies. Although naïve to slave-maker brood, the usurped Formica rear Polyergus offspring and nests containing both host and parasite species forms. Host worker acceptance of parasite brood has been attributed to the similarity of brood tending signals between these closely related taxa and/or the presence of an attractive pheromone in the slave-maker brood. By presenting single-species groups of Formica occulta and Formica gnava (two host species of P. breviceps) with a choice of Formica pupae of both species or with a choice of P. breviceps pupae from both types of mixed-species nests, it seems that neither close phylogenetic relatedness nor an attractive brood odor alone can account for the propensity of host workers to adopt slave-maker pupae. Significantly greater numbers of P. breviceps pupae were adopted by enslaved workers than by free-living workers, and within the enslaved groups and the free-living F. gnava group, greater numbers of P. breviceps pupae were adopted if they were from nests where the host species was conspecific to workers used in tests. When presented with F. gnava and F. occulta pupae, Formica workers adopted conspecific pupae almost exclusively and ignored or consumed pupae of the other host species. Taken together, these results imply that P. breviceps pupae have both a species-specific odor and a general brood-tending pheromone, upon which a host odor may be imposed. The disparate requirements of immatures at different stages of development for cue specificity or generality in maintaining nest exclusivity and maximizing inclusive fitness are discussed.     

Raiding behavior of the dulotic antChalepoxenus muellerianus (Finzi) in the field (Hymenoptera: Formicidae,Myrmicinae)

Summary Little information is available regarding the raiding behavior in nature of dulotic ants belonging to the tribeLeptothoracini. Between July 17 and 24, 1991, several raids ofChalepoxenus muellerianus (Finzi) were observed in nature near Tignale/Lago di Garda, Italy. Apparently, this species raids frequently during its summer raiding season (5 raids were observed during 7 days spent observing 8 colonies). A singleChalepoxenus colony sometimes raids more than one host colony more or less simultaneously. Observations during which oneChalepoxenus colony raided another and captured slavemaker brood indicate that intraspecific raids can occur either accidentally, or as a result of competition or territoriality when there is a sufficient dense slavemaker population.     

Selective acceptance of the brood of two formicine slave-making ants by host and non-host related species

Summary In a laboratory choice-test, free-living ant workers ofFormica cunicularia andF. rufibarbis (subgenusServiformica), both potentially slave species of the obligatory slave-makerPolyergus rufescens, cared for cocoons of this parasite and for homocolonial cocoons at comparable rates. Both potential hosts did not differ in their capacity to rear the parasite brood. This fact is discussed in relation to host selection and specificity inP. rufescens. No such attraction and/or tolerance was found towards cocoons of the facultative slave-makerFormica sanguinea, which also enslaves both host species. Workers ofF. lugubris, a species which is never enslaved, destroyed cocoons from both slave-making species. The attractiveness of the brood ofP. rufescens for both potentially slave species could be due to an interspecific brood pheromone in addition to brood mimicry. An alternative hypothesis is a close phylogenetic distance between this slave-maker andServiformica species. The capacity to gain acceptance by adult slave workers might be one of the crucial evolutionary steps separating obligatory from facultative slave-making ants.     

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.     

Molecular phylogenetic analysis of the ant genus <Emphasis Type="Italic">Formica</Emphasis> L. (Hymenoptera: Formicidae) from Palearctic region

Sixty-five sequences of the mitochondrial DNA cytochrome b gene fragment (759 bp) and 23 sequences of the NADH dehydrogenase subunit 6 gene fragment (224 bp) were compared in ants of the genus Formica L. from different regions of the Palearctic and in Polyergus rufescens Latr. as outgroup. In total, 28 species of the genus Formica were examined. As a result, dated trees with a molecular clock were constructed showing the phylogenetic relationships of Formica ants. The topology of the obtained tree based on the Cyt-b sequences was found to be not consistent with the generally accepted opinion on the Formica rufa and F. rufibarbis groups. New data on the formation history of the present-day fauna of Formica ants of the Palearctic were obtained. It was demonstrated that a considerable fraction of the examined species (about a third) were formed in the Quaternary Period.     

Chemical communication in mating behaviour of the slave-making ant <Emphasis Type="Italic">Polyergus rufescens</Emphasis> (Hymenoptera,Formicidae): 3-ethyl-4-methylpentanol as a critical component of the queen sex pheromone

The aim of the research reported here was to determine whether 3-ethyl-4-methylpentanol, a minor but crucial component of the sex pheromone of the North American slave-making ant species Polyergus breviceps, was also a component of the sex pheromone of the European congener Polyergus rufescens. Thus, the contents of mandibular glands of P. rufescens virgin queen were extracted and analysed. The main component of the extracts was methyl 6-methylsalicylate and 3-ethyl-4-methylpentanol was identified as one of several minor components. Further analyses showed that the insects produce mainly the (R)-enantiomer of the alcohol. Males’ responses to various blends of methyl 6-methylsalicylate with the racemate or the pure enantiomers of 3-ethyl-4- methylpentanol were tested in field behavioural bioassays. The data showed that blends of methyl 6- methylsalicylate and 3-ethyl-4-methylpentanol were strongly synergistic, with the most active ratios being biased toward the first component. The addition of other minor components to the binary blend neither increased nor decreased responses by males. Only the (R)-enantiomer of the alcohol was biologically active; its antipode did not inhibit attraction. The results are discussed in terms of the evolution of signals, and are compared with the results previously obtained for the allopatric species Polyergus breviceps. Received 12 November 2007; revised 10 January 2008; accepted 11 January 2008.     

Nest raiding by the invasive Argentine ant on colonies of the harvester ant, Pogonomyrmex subnitidus

Invasive ants often displace native ants, and published studies that focus on these interactions usually emphasize interspecific competition for food resources as a key mechanism responsible for the demise of native ants. Although less well documented, nest raiding by invasive ants may also contribute to the extirpation of native ants. In coastal southern California, for example, invasive Argentine ants (Linepithema humile) commonly raid colonies of the harvester ant, Pogonomyrmex subnitidus. On a seasonal basis the frequency and intensity of raids vary, but raids occur only when abiotic conditions are suitable for both species. In the short term these organized attacks cause harvester ants to cease foraging and to plug their nest entrances. In unstaged, one-on-one interactions between P. subnitidus and L. humile workers, Argentine ants behaved aggressively in over two thirds of all pair-wise interactions, despite the much larger size of P. subnitidus. The short-term introduction of experimental Argentine ant colonies outside of P. subnitidus nest entrances stimulated behaviors similar to those observed in raids: P. subnitidus decreased its foraging activity and increased the number of nest entrance workers (many of which labored to plug their nest entrances). Raids are not likely to be the result of competition for food. As expected, P. subnitidus foraged primarily on plant material (85% of food items obtained from returning foragers), but also collected some dead insects (7% of food items). In buffet-style choice tests in which we offered Argentine ants food items obtained from P. subnitidus, L. humile only showed interest in dead insects. In other feeding trials L. humile consistently moved harvester ant brood into their nests (where they were presumably consumed) but showed little interest in freshly dead workers. The raiding behavior described here obscures the distinction between interspecific competition and predation, and may well play an important role in the displacement of native ants, especially those that are ecologically dissimilar to L. humile with respect to diet. Received 15 July 2005; revised 19 October 2005; accepted 26 October 2005.