Kurzgesagt – In a Nutshell
Sources – Slaver Ants
Thanks to our experts –
Prof. Donato A. Grasso
Department of Chemistry, Life Sciences and Environmental Sustainability, University of Parma (Italy)Dr. Alice Laciny
Konrad Lorenz Institute for Evolution and Cognition Research, Klosterneuburg
The World War of the Ants is claiming millions of lives every day. But in the brutal world of ants, straight up war is not the only way.
To dive deeper into this more or less hidden world war with millions of casualties daily, see one of our earlier videos
#Kurzgesagt (2019): The World War of the Ants - The Army Ant
https://www.youtube.com/watch?v=7_e0CA_nhaE&t=361s
Around 50 ant species practice slavery, the most extreme form of division of labor.
Here only an order of magnitude can be given, because the number of ant species in general is also only an estimate. But if one assumes these two numbers, only about 0.5% of all species practice slavery.
#P. D’Ettorre, P. & Heinze, J. (2001): Sociobiology of slave-making ants. acta ethologica Vol. 3, pp. 67–82
https://link.springer.com/article/10.1007/s102110100038
Quote: “Of the approximately 10,000 species of ants, only a tiny minority of about 50 are active slave makers (Table 1).”
Polyergus has specialized in slavery so much that they have lost their ability to care for themselves. They don’t clean, build nests, care for their brood, or feed themselves. They only exist for raiding.
This is called obligatory social parasitism. In this context, "obligate" means that the parasites are, at least in some phases, absolutely dependent on a host without which they cannot reproduce. Slavery is one of the most extreme forms of this practice.
For example, although ants of the genus Polyergus are veritable fighters, without their slaves they would starve, as they lack the anatomical abilities to feed themselves.
#Buschinger, A. (2009): Social parasitism among ants: a review (Hymenoptera: Formicidae). Myrmecological News, Vol. 12, pp. 219-235
Quote: “The slave-maker workers are usually unable to forage, to feed their larvae, or even to eat by themselves. On the other hand, they are often predisposed for effective fighting. Slave-maker workers may be equipped with specialized piercing mandibles as in Polyergus (Formicinae) and Strongylognathus (Myrmicinae), or with toothless, pincers-like mandibles for cutting off the appendages of their opponents as in Harpagoxenus sublaevis (Myrmicinae). (...)
A colony of Polyergus will conduct slave raids on nests of species of Formica (subgenus Serviformica), and worker pupae of the hosts are taken. The workers hatching from these pupae are necessary for foraging and for feeding and rearing the slave-maker's brood in the mixed society, and for nest building. Polyergus workers are incapable of surviving without slaves, even when plentiful food is available.”
In the following review you will find numerous further sources on slavery among animals in general.
#Czechowski, W. & Godzinska, E. J. (2015): Enslaved ants: not as helpless as they were thought to be. Insectes Sociaux, Vol. 62, pp. 9–22
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4291515/pdf/40_2014_Article_377.pdf
Quote: “Slavery (dulosis) is the most spectacular form of social parasitism in ants and one of the most unusual interspecific relationships in the animal world. (...) Slave-makers rob brood from colonies of their host species and transport them to their own nests. As a consequence, adult workers of the host species, the so-called slaves, emerge in the enslaver’s nest and subsequently recognise it—due to an early olfactory learning process. (...) In the case of obligate slavery, presence of slaves is an essential condition for the survival of the colony, as abilities of enslaver workers to carry out everyday tasks are usually highly restricted, among others due to morphological and functional limitations. (...) Moreover, workers of obligate social parasites often occur in limited numbers, whereas their slaves are relatively numerous.”
Slaves make up 80 to 90% of the ants in their colonies, so a few hundred Polyergus and a single queen control thousands of slave ants.
#Czechowski, W. (2005): Integration raids in the Amazon ant Polyergus rufescens (Hymenoptera, Formicidae)
Quote: “The most frequent slave species of P. rufescens in Central Europe is Formica fusca, principally a mono- or slightly polygynous and monocalic species. Its workforce usually numbers from several hundred to a thousand, rarely reaching 1,500–2,000 adults in particularly big colonies. Swarms of P. rufescens may number up to about 2,000 workers (see below), and the proportion of slaves in its mixed colonies reaches 80–90%. This means that a P. rufescens nest may support between 8,000 and 18,000 enslaved workers, what exceeds several times the common size of a free-living colony of F. fusca.”
Witness this colony of over ten thousand Formica ants, genetically cousins of Polyergus, who build a thriving nest in the underground.
Formica and Polyergus are genus, which means there are other different species. There are typical slave-slaveholder relationships between specific species, such as: Polyergus breviceps and Formica gnava or Polyergus rufescens and Formica fusca.
#Greenberg, L. et al. (2007): Queen Sex Pheromone of the Slave-making Ant, Polyergus breviceps. Journal of Chemical Ecology 33 (5), pp. 935–945
#Czechowski, W. (2005): Integration raids in the Amazon ant Polyergus rufescens (Hymenoptera, Formicidae). Insectes Sociaux 52 (1), pp. 103-104
The raid in our video is largely based on the following description of a typical one by slave-maker ants:
#Trager, J. C. (2013): Global revision of the dulotic ant genus Polyergus (Hymenoptera: Formicidae, Formicinae, Formicini). Zootaxa 3722 (4), pp. 501–548
They are a widespread genus, some species are good fighters, some great builders and some cattle farmers – often welcome by humans because they hunt vermin that hurt forests.
#Goropashnaya, A. V. et al (2012): Phylogenetic Relationships of Palaearctic Formica Species (Hymenoptera, Formicidae) Based on Mitochondrial Cytochrome b Sequences. PLoS ONE, Vol. 7(7)
https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0041697
Quote: “Formica ants represent a large group of soil insects that occur mainly in the Holarctic. The genus has currently 176 recognized species, a bigger part of which are distributed in the Nearctic and a smaller part (63 species) in the Palaearctic ([5], World Catalogue of Ants, www.antweb.org, 27 September 2011). Even though the genus is well studied, these numbers are still changing (e.g. [6,7]). Many species are widespread and abundant, and they play an important role in ecosystems being active predators, tending aphids and improving soil composition. Formica species also demonstrate a great diversity of complex behavior and social organization.”
There are multi-layered relationships between Formica ants and aphids. The ants protect the aphids while they harvest the honeydew, a sugar-rich secretion. The following sources and quotes provide further sources as an introduction to this relationship, which is called "mutualism," a reciprocal relationship between two species from which both benefit.
#Kilpeläinen, J. et al. (2009): Does the mutualism between wood ants (Formica rufa group) and Cinara aphids affect Norway spruce growth? Forest Ecology and Management, Vol. 257, pp. 238–243
https://www.sciencedirect.com/science/article/abs/pii/S0378112708006683
Quote: “Many ant species have mutualistic relationships with aphids feeding on the phloem sap of a wide variety of plants (e.g. Way, 1963; Buckley, 1987; Stadler and Dixon, 2005; Styrsky and Eubanks, 2007). Wood ants (Formica rufa group) protect aphids from predators, and in return, are supplied with honeydew excreted by aphids. Ants may collect more than 80% of the excreted honeydew (Douglas and Sudd, 1978), which constitutes 62–94% of their diet (Wellenstein, 1952; Rosengren and Sundström, 1991). Honeydew contains 15–20% sugars and a small amino acid component (Zoebelein, 1956a; Stradling, 1987; Rosengren and Sundström, 1991). A wood ant colony can bring 240–1000 kg of honeydew (fresh weight) annually to its mound (Zoebelein, 1956b; Wellenstein, 1980; Stradling, 1987; Rosengren and Sundström, 1991). Simultaneously, wood ants prey on a large number of other arthropods (Way and Khoo, 1992; Martikainen et al., 2000; Punttila et al., 2004). Ants may predate on other arthropods more intensively in mutualistic relationships with aphids than otherwise (Way, 1963), which would also benefit the tree as defoliating insects are removed.”
#Styrsky, J. D. & Eubanks, M. D. (2007): Ecological Consequences of Interactions between Ants and Honeydew-Producing Insects. Proceedings: Biological Sciences, Vol. 274 (1607), pp. 151-164
https://royalsocietypublishing.org/doi/full/10.1098/rspb.2006.3701
Quote: “Mutualism, defined as a reciprocally beneficial interaction between individuals of two species, is increasingly recognized as a common and important ecological interaction (Boucher et al. 1982; Bronstein 1994a; Stachowicz 2001). Perhaps one of the most familiar examples is the food-for-protection mutualism between ants (Hymenoptera: Formicidae) and honeydew-producing insects in the hemipteran suborders Sternorrhyncha (specifically aphids, whiteflies, scales and mealy bugs) and Auchenorrhyncha (specifically treehoppers and leafhop pers; formerly grouped together as the single order/ suborder 'Homoptera')- Honeydew is a sugary excretion of carbohydrates, amino acids and water derived from plant phloem upon which many hemipterans feed (Way 1963). Ants are attracted to honeydew as a predictable, renewable food resource and, consequently, 'tend' honeydew-producing hemipterans, protecting them from predators and parasitoids (Way 1963; Buckley 1987).”
In one study, coniferous seedlings were planted around ant mounds of red wood ants (Formica rufa group) and sugar baits were added as an attractant. Other trees were left without attractant. Afterwards the scientists examined which trees had how many feeding scars of the pest pine weevil Hylobius abietis (L.). The result: on the seedlings that had the bait, i.e. that attracted ants, there were 35% fewer feeding scars or 20% fewer in a second round compared to trees without bait.
#Maňák,V. et al. (2013): Ants protect conifer seedlings from feeding damage by the pine weevil Hylobius abietis. Agricultural and Forest Entomology (2013), Vol. 15, pp. 98–105
Their importance goes far beyond that as pest controllers. Wood ants are "keystone species" as they have a major impact on carbon cycling. They are a kind of omnivore and through their activities they transport various substances around and are therefore of great importance for entire ecosystems.
#Finér L. et al. (2012). The Role of Wood Ants (Formica rufa group) in Carbon and Nutrient Dynamics of a Boreal Norway Spruce Forest Ecosystem. Ecosystems Vol. 16, pp. 196–208
Quote: “Wood ants (Formica rufa group) are keystone species in boreal and mountain forests of Europe and Asia, as they are considered to be ecosystem engineers that affect carbon (C) and nutrient pool sizes and fluxes. They transfer C and nutrients from the forest floor to their nests and from the nests back to the forest floor and from the tree canopy to nests. The area of forest floor and the number of trees affected by ants as well as the magnitude of the element transfer depend on the nest density (number and size of nest ha-1), colony size (number and size of workers), the extent of the foraging area and the length of the growing season available for the foraging within the forest ecosystem. Wood ants collect plant litter, preferably conifer needles, twigs and resin from the forest floor to build large, long-lived nests on the soil surface. The diet of wood ants consists of elements collected from different trophic levels: honeydew excreted by aphids (Hemiptera, Aphidina) living in the tree canopy, and invertebrate prey from both the canopy and forest floor.”
Some wood ants don’t simply gather pine needles. Their large nests are made of different materials, controlled temperature and moisture conditions and they contain different microclimates in which various other insects live (RWA = red wood ants).
#Parmentier, T. et al. (2014): A highly diverse microcosm in a hostile world: a review on the associates of red wood ants (Formica rufa group). Insectes Sociaux Vol. 61, pp. 229–237
https://link.springer.com/article/10.1007/s00040-014-0357-3
Quote: “The striking diversity of RWA myrmecophiles can mainly be explained by the nest structure of RWAs. Their huge mounds provide stable and long-lasting habitats with controlled temperature and moisture. The mounds are also heterogenous in temperature, moisture and material (organic thatch material, inorganic soil, central stem) which create a large variety of microhabitats. Furthermore, there is a constant supply of food and organic material which can sustain different trophic groups such as parasites, predators, scavengers, detritivores and mycophages”
An example of Formicas’s fighting ability are fighting tactics within a group. There is evidence that the ants change their tactics depending on the size and composition of the fighting troop, e.g. they fight harder when the group is small.
#Batchelor, T. P. & Briffa, M. (2011): Fight tactics in wood ants: individuals in smaller groups fight harder but die faster. Proceedings of The Royal Society, Vol. 278, pp. 3243–3250
Quote: “When social animals engage in inter-group contests, the outcome is determined by group sizes and individual masses, which together determine group resource-holding potential (‘group RHP’). Individuals that perceive themselves as being in a group with high RHP may receive a motivational increase and increase their aggression levels. Alternatively, individuals in lower RHP groups may increase their aggression levels in an attempt to overcome the RHP deficit. We investigate how ‘group RHP’ influences agonistic tactics in red wood ants Formica rufa. Larger groups had higher total agonistic indices, but per capita agonistic indices were highest in the smallest groups, indicating that individuals in smaller groups fought harder. Agonistic indices were influenced by relative mean mass, focal group size, opponent group size and opponent group agonistic index. Focal group attrition rates decreased as focal group relative agonistic indices increased and there was a strong negative influence of relative mean mass. The highest focal attrition rates were received when opponent groups were numerically large and composed of large individuals. Thus, fight tactics in F. rufa seem to vary with both aspects of group RHP, group size and the individual attributes of group members, indicating that information on these are available to fighting ants.”
Nobody noticed the lone Polyergus scout that briefly showed up this morning, before she bolted away again. The attack begins in the early afternoon. The scout returned from her mission to find the Formica nests.
Although the study has some limitations, there is evidence that the scouts also check for other factors (e.g., how many offspring are in the target colony). Scouts were seen spending extended periods of time at the nest and even attempting to enter. It seems that nests with a stronger defense are preferred. The reasoning is probably to use the same strong defense for their own nest in the future.
#Visicchio, R. et al. (2003): How raiders of the slave-making ant Polyergus rufescens (Hymenoptera Formicidae) evaluate a target host nest, Ethology, Ecology & Evolution, Vol. 15 (4), pp. 369-378
https://www.tandfonline.com/doi/abs/10.1080/08927014.2003.9522663
Quote: “In the binary choice experiments, the raiders seemed to prefer the host colony where the residents defended their nest more and, therefore, where the scouts were attacked to a greater extent. Slave-making ant species completely depend on the tasks performed by their slaves, and colony defence is one of these. Thus, by sacking host colonies whose resident workers act more aggressively, P. rufescens would obtain future slaves that would be particularly efficient at defend the parasitic colony.”
The attack begins in the early afternoon. The scout returned from her mission to find the Formica nests. She dances erratically and spreads pheromones that excite and mass recruit more and more ants, until a large raiding party of a thousand warrioresses has formed.
Chemical communication is the main language within social insects (i.e. insects that form colonies, like ants). Each colony has its specific chemical language. If an insect does not speak this language, it cannot participate in social life or, in the extreme case, is seen as an enemy. Chemical communication takes on central functions: Group membership, belonging to a caste, coordination of activities.
A good overview and start into the topic is provided by the following article from which the quote is taken:
#Richard, F.J. & Hunt, J. H. (2013): Intracolony chemical communication in social insects. Insectes Sociaux (60), pp. 275–291
Quote: “Widespread intracolony distribution of chemical messengers gives each colony a specific odor whereby colony members are integrated into the social life of the colony and non-members of the colony are excluded. (...)
Communication among members of a social group is necessary to delineate group membership, coordinate activities, and identify castes or individuals and their roles in the society. Indeed, without communication there could be no society.”
Polyergus also seems to be somewhat resistant to the acid sprays of the defenders – and so even if a defense is forming, the attackers use their mandibles to pierce and kill the defenders.
In one study, Polyergus and Formica were placed in two containers that were connected but one passage was blocked by a mesh. Thus, only substances such as pheromones could exchange between the species.
It turns out that Formica died due to their own secretions whereas Polyergus were practically immune.
#Topoff, H. et al. (1989): Behavioral Adaptations for Raiding in the SlaveMaking Ant, Polyergus breviceps. Journal of lnsect Behavior, Vol. 2 (4), pp. 545-556
https://link.springer.com/article/10.1007/BF01053353
Quote: “This experimental test series demonstrated that workers of Formica gnava are indeed susceptible to the lethal effects of toxic pheromones (Fig. 4). The mean number of dead Formica at the end of 10 rain was 18.0, and our close observations (with a dissecting microscope) revealed that the ants were not biting each other. Almost-identical results were obtained in the condition when only Formica adults were confined together (mean number of dead ants = 20).
In contrast, the Polyergus were virtually immune to the secretions [F(3,16) = 115.55, P < 0.01]. Thus, when confined with Formica, the mean number of dead Polyergus was t.2, and no Polyergus workers died when confined with nestmates. These results support the hypothesis that the death of Formica workers in both the experimental and the control series was indeed caused by their own secretions and that workers of Polyergus are essentially immune.”
There are a few different ideas why Polyergus attacks are so efficient and one of the most fascinating ones is chemical warfare. Instead of relying on brute force, they release a propaganda pheromone that makes the defender panic, unable to mount an organized defense.
#Regnier, F. E. & Wilson, E. O. (1971): Chemical communication and "propaganda" in slave-maker ants. Science Vol. 172 (3980), pp. 267-269
https://pubmed.ncbi.nlm.nih.gov/5548706/
Quote: “The results, illustrated in Fig. 3, show conclusively that the defenders are sprayed by relatively large quantities of the acetates. The amount of acetates individual F. subsericea workers received, on the average, was equal to or greater than the entire volatile contents of their own gland reservoirs, and more than enough to cause an alarm reaction. We feel certain that this accounts not only for the disorientation observed in many defenders during the raids but also. to somile extent, for the panic and rapict retreat displayed by the slave-species colonies, and the relative ease with which their nests are breached by the slave-mnakers. (...)
It thus appears that the acetates of the Dufour's gland of F. pergandei and F. subintegra perform no less than three distinct fiunctions in the life of the slave-maker colonies: as defensivc and offensive chemical weapons, as alarm pheromones for communication within the colony, and as offensive “propaganda substanices” directed at alien colonies during slave raids.”
Hundreds are abducted and brought back to the Polyergus colony in this raid alone. Well, most of them. A few unlucky ones are eaten as a sort of snack.
#Talbot, M. (1967): Slave-raids of the ant polyergus lucidus mayr. Psyche: A Journal of Entomology 74(4), pp. 299-313
https://www.researchgate.net/publication/40632073_Slave-Raids_of_the_Ant_Polyergus_Lucidus_Mayr
Quote: “If an estimated 4o successful raids took place during the season, the Polyergus captured approximately 4,800 young ants (3,920 pupae and 880 larvae). Since there were probably not more than one-and-a-half times that number of adult Formica in the mixed colony and since many workers live for a number of years,
it would seem that a fair proportion of the brood brought in is used for food.”
A decent sized colony can forfeit thousands of pupae in a single raiding season and still survive.
In the study below, researchers observed two colonies for about three weeks, the slave maker ants launched almost daily and with an average of about 2600 attackers per day (one day there were almost 6000) and captured an average of about 800 pupae per raid. In 10 days alone they captured more than 8000 pupae.
#Topoff, H. et al. (1985): Behavioral Ecology of the Slave-Making Ant, Polyergus breviceps, in a Desert Habitat. The Southwestern Naturalist, Vol. 30 (2), pp. 289-295
#Czechowski, W. (1975): Mixed polycalic colony of Formica ( Servifortnica) cinerea MAYR
and Polyergus rufescens Latr ( Hymenoptera, Formicidae). Annales Zoologici 33, pp. 67-76
https://rcin.org.pl/Content/58410/PDF/WA058_2426_P255-T33_Annal-Zool-nr-5.pdf
Quote: “The total number of brood (larvae and pupae) carried away from the neighbouring nests during the season should have been between 150,000 to 200,000.”
Slave-making ants Polyergus are much less social than other ant species. Some species are missing a lot of the genes other ants have to make communication possible. In a sense, they are bad at talking to each other. So it may be that as Polyergus ancestors started to abduct other ants they lost the ability to collaborate and work together productively.
One study looked into the genomes of 3 slavemaking-host ant pairs and found out that up to 30% of odorant receptor genes (Or) and up to 56% of gustatory receptor genes (Gr) are lost. Also slave-making ants had smaller olfactory receptor repertoires than all host and nonhost control species. Slave-making ant genomes contained on average 311 olfactory receptors, whereas host genomes had more than 400.
#Jongepier, E. et al. (2021): Convergent Loss of Chemoreceptors across Independent
Origins of Slave-Making in Ants. Molecular Biology and Evolution, Vol. 39 (1)
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC8760941/pdf/msab305.pdf
Quote: “Slave-making ants exhibited smaller Or repertoires than all host and nonhost species. Slave-making ant genomes contained on average 311 Ors (range 308–315), which did not vary significantly among slave-making ant species. In contrast, each host and nonhost genome harbored more than 400 Ors (range 403–421, supplementary table S14, Supplementary Material online), again with Or numbers similar across all host species.
(...)
Slave-making ants experienced more Or losses than their respective hosts across all three origins of parasitism . The percentage of Ors that were lost in each slave-making lineage was very similar, ranging from 25.08% in H. sublaevis to 27.69% in T. ravouxi and 29.97% in T. americanus.
(...)
Gr loss was much more prevalent in slave-making ants than in their respective hosts. The proportion of Grs that were lost in slave-making ants was comparable across the three origins of parasitism, ranging from 55.56% in H. sublaevis to 47.22% in T. ravouxi and 44.44% in T. americanus. This indicates that the transition to parasitism is associated with extensive losses of both Ors and Grs in the parasites’ genome.”
Soon after the stolen Formica baby ants have been brought to the slaver colony, they are progressively covered in Polyergus pheromones. They are chemically imprinted, similar to a duckling, who imprints on its mother after birth.
An overview and further literature on the complex topic of deception by odor (via “cuticular hydrocarbons”, chemicals that cover the body of all insects), can be found in the following source.
#Bagnères, A.-G. & Lorenzi, M. C. (2010): Chemical deception/mimicry using cuticular hydrocarbons. Insect Hydrocarbons Biology, Biochemistry, and Chemical Ecology. Pp. 282-324
Quote: “In contrast, slave-making workers (as well as their immature stages) are cared for by enslaved host workers. Typically, in ants, workers learn colony odor at eclosion and accept or reject individuals depending upon whether or not their recognition cues match the learned template (e.g., Isingrini et al ., 1985 ; Crozier and Pamilo, 1996 ; although template and acceptance thresholds adjust with time, see D’Ettorre et al ., 2004 a; Brandt et al ., 2005 ; Errard et al ., 2006 a). Enslaved host workers eclose in parasite colonies, and thus their template resembles the parasite odor, a condition which may let them accept parasites.”
This sort of brainwashing goes so far that if they encounter free Formica ants in the wild, they will treat them as enemies.
An exemplary study had ants of the species F. cunicularia (all from the same colony) hatch under different conditions: the first group (cFc) hatched in the presence of workers of their own species, the second group (tFc) hatched in the presence of F. sanguinea workers (a slave-making ant species like Polyergus), and the third group (iFc) hatched in the absence of any adult ants.
Subsequently, each group was confronted with conspecifics and the slave-making ants.
The result: the hatched workers that grew up among the slave-making ants (tFc) were highly aggressive towards their former colony mates. Towards the slave-making ants F. sanguinea they showed no aggression at all.
#Le Moli, F. & A. Mori (1985): The influence of the early experience of worker ants on enslavement. Animal Behaviour, Vol. 33 (4),pp. 1384–1387
https://psycnet.apa.org/record/1986-26443-001
Quote: “The data obtained indicate that early experience strongly influences the intra- and interspecific relationships of F. cunicularia: workers born and reared in the presence of F. sanguinea adults do not accept their sisters as nestmates, but fight against them in overt combat. In contrast, there is reciprocal acceptance between these F. cunicularia workers and F. sanguinea, as between ants of the same colony. This behaviour is rather different from that in the control situation, where F. cunicularia individuals housed from hatching with adults from the same colony discriminate between conspecifics and aliens (F. sanguinea), as is the rule among ants. Isolation also disrupts the ability to recognize conspecifics, which are frequently attacked by F. cunicularia workers reared from hatching without adults. It seems evident, therefore, that early experience can account for the enslavement of this Serviformica species in nests of F. sanguinea.”
The more dangerous one might unfold during a raid like the one we witnessed before. A young Polyergus queen silently followed the raiding party. Using the chaos of the invasion, she was able to find her way to the Formica queen and kill her, taking over the shaken colony.
It has been reported that the queens not only use the raids to take over another nest, but also mate immediately before or during the fighting. They can then lay their eggs in the conquered nest and these eggs are then taken care of by the enslaved ants.
#Greenberg, L. & Sherman, P. (1988): Colony Founding by Queens of the Obligatory Slave-making Ant, Polyergus breviceps: The Role of the Dufour's Gland. Ethology, Vol. 78(3), pp. 209 - 218
Quote: “In an early field study of P. breviceps in California, Wheeler (1916) reported that both winged and dealate females may accompany workers on raids, an observation that we recently confirmed for the Arizona study site (Topoff et al. 1985). In the present study, we found that mating takes place primarily during slave raids. During such raids, winged queens ceased running in the swarm, and positioned themselves on rocks or other raised objects. Within minutes, numerous males descended and mated with these stationary females. After mating, the queens immediately shed their Colony Founding by Slave-making Ants 217 wings and resumed running until they reached the raided Formica colony. The result of this process is that newly-mated Polyergus queens arrive at disorganized Formica colonies whose workers and queen are scattered across the substrate, a situation which could facilitate the takeover of even large Formica colonies in the field.”
Although such a victory may be very short lived. Poyergus do not tolerate other slaver colonies within their hunting ground. They raid each other fiercely too and can destroy the competing colonies nearby. So while this tactic works sometimes, it is pretty dangerous.
#Topoff, H., et al. (1984): Social and orientation behavior of Polyergus breviceps during slave-making raids. Behavioral Ecology and Sociobiology, Vol. 15, pp. 273-279
https://link.springer.com/article/10.1007/BF00292989
Quote: “Of particular interest, however, was a raid on June 29, which Polyergus colony 2 launched towards a Formica colony 45 m to the north. While the raiders were mid-way to the target, their own colony was attacked by a raid swarm from Polyergus colony 4, located 50 m to the southeast. Fighting was intense as resident Polyergus (i.e., those not participating in the slave raid) and Formica both attempted to repel the intruders.
(...)
Polyergus colony 2 was again attacked on July 30, this time by Polyergus colony 5, located 79 m to the southwest. On this day the resident colony conducted no slave raid, so most of the Polyergus were available for nest defense. Although the conflict extended 1 m in all directions from the residents' nest, and lasted more than 1 h, the nest was not penetrated and no brood was captured. Polyergus worker mortality was again extremely high, although we could not determine to which colony the dead ants belonged. Colony 2 was subsequently attacked by colony 5 on August 3, 5, and 19. On each of these raids, many adults were killed but no brood was taken. The final attack on colony 2 was made by Polyergus colony 4 on August 23, during which the worker population of colony 2 was destroyed.”
Another young queen is going for a different strategy: she is looking for a Formica colony that is further from her birthplace, attacking a whole colony on her own.
#Topoff, H. (1999): Slave-Making Queens. Scientific American, Vol. 281 (5)
https://www.jstor.org/stable/26058487
Quote: “The alternative tactic for an up-and-coming Polyergus queen is to bolt from the raiding column and on her own locate a more distant colony of Formica. Although there are no guarantees, this behavior at least increases the likelihood of finding an appropriate host nest outside the raiding territory of a resident Polyergus population.”
She bolts through an entrance, pushing aside confused ants that try to stop her, releasing a powerful pheromone that drives defenders away. She has only a short time window to find the Formica queen deep in the hostile nest.
There are several studies showing that certain substances from the so-called Dufour's gland of the queen play an important role in the usurpation. What effect exactly these substances have, however, is a matter of debate. On the one hand there might be an appeasement effect, on the other hand a repellent effect.
The latter are recent studies and are based on a chemical study of the queen's Dufour's gland and experiments on how host ants respond to various substances.
#Errad, C. et al. (2000): Sneak in or repel your enemy: Dufour's gland repellent as a strategy for successful usurpation in the slave-maker Polyergus rufescens. Chemoecology Vol 10(3), pp. 135-142
Quote: “Behavioral observations during nest usurpation by queens of P. breviceps showed that not all the host workers attacked the alien queen in a concerted effort to drive her away and prevent her from reaching the host queen (Topoff et al. 1988). This slight indifference to the parasite invasion was mimicked by laboratory experiments, where it was demonstrated that host workers exposed to the queen Dufour’s gland became docile and did not attack the treated alien ant. This pacifying effect of the glandular secretion led to the conclusion that Dufour’s gland secretion acts as an appeasement pheromone (Topoff et al. 1988). Reduction of aggression was also observed in the case of Dufour’s gland secretion of the European P. rufescens (Mori et al. 2000 and the present study). Adopting Topoff’s terminology, Mori et al. (2000) suggested that also the secretion in P. rufescens acts as an appeasement allomone.
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Alternatively, we hypothesize that decyl butanoate is used as a repellent during usurpation. In our bioassay, the starved ants consistently avoided the offered droplet of honey when it was applied with decyl acetate. However, these starved ants willingly fed on untreated honey or even on honey treated with octadecyl butanoate (the workers’ Dufour’s major compound) or limonene, a compound that is reported for its ant repellency effects. From our experience satiated, but not starved ants, avoid contact with limonene. This strengthens our observation that decyl butanoate is an effective ant repellent. We, therefore, attribute the reduced aggression of the host ants when exposed to alien ants applied with decyl butanoate to the repellency effect of the compound. In these dyadic encounters we never observed trophallaxis attempts, as predicted from an appeasement allomone, just mutual avoidance. In our view, the queen Polyergus employs a dual strategy. By not exposing any cuticular signature it renders itself odorless with respect to nestmate recognition. This may be sufficient for eluding most, but not all workers. Indeed, some of the workers that encountered the invading queen attacked her. This host defensive aggression is sometimes successful and the Polyergus queen is driven off or killed. As a countermeasure to these attacks, the Polyergus queen may then use Dufour’s gland secretion to repel the aggressive workers, at least until it can reach the host queen and acquire its body odor in the process of killing her.”
#Mori, A. et al. (2000): Colony founding in Polyergus rufescens: the role of the Dufour’s gland. Insectes Sociaux 47, pp. 7–10
Quote: “In fact, our experiments indicate that the aggressiveness of the F. cunicularia workers towards the workers of F. rufibarbis, daubed with the Dufour’s extract, decreases drastically: at the beginning they tend to ignore the intruder, which is successively adopted. Thus, the secretion of the Dufour’s gland is likely to act as an “appeasement” allomone towards the residents of the target host colony rather than as a “propaganda” substance. This chemical strategy probably allows an easier invasion and usurpation of host colonies by newly mated females of P. rufescens. A further development of this study is the chemical analysis of the Dufour’s gland secretion in P. rufescens, in order to verify possible similarities with the chemical communication systems adopted by their host species (Visicchio et al., in prep.). This secretion, in fact, might imbue the P. rufescens queen with an odour similar to the Serviformica one (changing its cuticular hydrocarbon profile), or reduce aggression until the usurper queen acquires the odour from the environment of the invaded nest.”
Once she finds her target, both queens engage in a fight for life and death. The Polyergus queen is well equipped with her sharp mandibles. She bites and rips into her victim for about half an hour before she finally calms down. Between her bites, she licks the chemical surface of the dead Formica queen, covering herself in her pheromones. When she is done with this macabre ritual, Formica workers approach her. Subdued by her intense smells, they start grooming and feeding her, pledging their allegiance to their new ruler.
#Topoff, H., & Zimmerli, E. (1993): Colony takeover by a socially parasitic ant, Polyergus breviceps: the role of chemicals obtained during host-queen killing. Animal Behaviour, 46(3), pp. 479–486.
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