Slave-making ant

File:Polyergus lucidus with host Formica archboldi.jpg

Queen and brood of the slave-maker Polyergus lucidus with Formica archboldi workers

Slave-making ants are brood parasites that capture brood of other ant species to increase the worker force of their colony. After emerging in the slave-maker nest, slave workers work as if they were in their own colony, while parasite workers only concentrate on replenishing the labor force from neighboring host nests, a process called slave raiding.

The slave-making ants are specialized to parasite a single species or a group of related species, and they are often close relatives to their hosts, which is typical for social parasites. The slave-makers may either be permanent social parasites (thus depending on enslaved ants throughout their whole lives) or facultative slave-makers. The behavior is unusual among ants but has evolved several times independently.

Terminology

Theft of brood for the purpose of employing the stolen individual's efforts in support of the thief is called dulosis (from Greek doulos, "slave"), but the term "slave-making" is used in older literature and is still common. Herbers (2007) considered the term offensive and suggested that "slave-making ants" should be replaced with "pirate ants", noting that pirates take captives and rely on forced labor. Further, "slave" could be replaced with "captive" and "dulosis" with "leistic behavior" (Greek for pirated spoils, leistos).

A related type of social parasitism is called inquilinism, in which a reproductive enters a host colony, lays eggs, and relies on the host colony to rear its offspring. Unlike brood parasitism, the inquiline remains within the nest and typically its brood does not outnumber the host's brood.

Obligate and facultative slave-makers

Slave-making ants may either be permanent social parasites, thus depending on enslaved hosts ants throughout their whole lives or facultative slave-makers. Facultative slave-making ants, like those in the Formica sanguinea complex, represent an intermediate parasitic group, between freeliving species on the one hand, and obligatory slave-making species on the other. In laboratory tests, slaves were removed from colonies of Formica sanguinea and Polyergus rufescens. The behavior of F. sanguinea changed dramatically within 30 days of slave removal, with workers becoming self-sufficient at feeding and brood care. Workers of Polyergus, by contrast, were unable to care for their brood, and experienced high mortality.

Raids

File:Polyergus lucidus returning from raid on Formica incerta.jpg

Polyergus lucidus returning from raid on Formica incerta. Two of the latter already incorporated into the mixed colony are visible to the right of the nest entrance.

Parasitized nests need to replenish the host workers periodically and this is achieved by raiding other nests in a process called slave raiding. The parasite workers are specialized for conducting raids in a two-step process. First, scouts individually search for potential host nests. When successful, the scout returns to its nest and recruits nest-mates to initiate the raid, during which slave-maker ants seize brood and bring it back home. A colony may capture 14,000 pupae in a single season. Most slave-raiders capture only brood, but Strongylognathus sp. also enslave adult workers.

In most parasite species, workers mark the way to its nest with pheromones and afterwards fellow slave-makers are attracted in a few seconds. Then they go quickly to the targeted host nest, attack it, and carry as many larvae and pupae as possible and return to their nest following the same trail marked by the pheromone. Rossomyrmex is the only reported slave-maker that exclusively uses adult transport and single recruitment chain instead of pheromones during raids, a behavior probably constrained by the arid habitat: raids take place in early summer when soil surface temperature can reach up to 30 °C (86 °F), a temperature in which pheromones would quickly evaporate.

Workers of the attacked nest can fight or flee. In the host species Proformica, the most common behaviour is flight, probably because hosts almost always lose fights. Most studies on the raiding behavior of species in the F. sanguinea complex confirm that slave raiders usually rout their opponents, who typically flee in a state of panic alarm, and that aggressive encounters, when they occur, are brief and do not result in the death of adult individuals from either species. However, when large colonies of slave species offer resistance during raids, prolonged fighting is possible, and many workers of both species can be killed.

Later, host workers emerging in the parasite nest will be imprinted on and integrated into the mixed colony where they rear the parasite brood, feed and groom the parasite workers, defend the nest against aliens, and even participate in raids, including raids against their original colony. Altruistic acts of slaves are thus directed toward unrelated individuals. One hypothesis suggests that slave deception is possible because slaves are captured as pupae and learn the slave-maker colony odour after emergence.

However, in some cases, the slave ants rebel against their slave-maker ants, killing a large number of the slave-maker ant offspring. The reason behind this is because "slaves can gain indirect fitness benefits by reducing parasite pressure on nearby host colonies, because these are often closely related to the slaves". Thus, the slave ants protect their native colonies from further raids by slave-maker ants.

Parasite–host pairs

• Rossomyrmex–Proformica
• Polyergus–Formica
• Formica–Formica
• Chalepoxenus–Leptothorax
• Epimyrma–Leptothorax
• Harpagoxenus–Leptothorax
• Leptothorax–Leptothorax
• Protomognathus–Leptothorax
• Myrmoxenus–Temnothorax
• Strongylognathus–Tetramorium

Reproduction

The reproductive behavior of slave-making ants usually consists in synchronous emergence of sexuals followed by a nuptial flight and the invasion of a host nest, but also in some cases females display a mating call around the natal nest to attract males and immediately after mating search for a host nest to usurp.

Only one slave species is usually found in a single Polyergus nest. This is in contrast to related facultative slave-makers of the genus Formica belonging to the F. sanguinea species group, found in the same habitat, whose nests commonly contain two or more species serving as slaves. Choice of a host species can occur both through the colony-founding behavior of queens and through the choice of target nests for slave raids. The parasitic Polyergus queens found colonies either by adoption, where a queen invades the nest of a slave species, killing the resident queen and appropriating workers and brood present, or by "budding", in which a queen invades or is accepted into a host species nest accompanied by workers from her nest of origin.

Evolution

The first hypothesis concerning the origins of slave-making was Darwin's (1859) suggestion in On the Origin of Species that slavery developed as a by-product of brood predation among related species. Other hypotheses focuses on territorial interactions with opportunistic brood predation or brood transport among polydomous colonies (consist of multiple nests) as the main pathway to slave-making. Slave-making behavior is unusual among ants but has evolved several times independently in the ant subfamilies Myrmicinae and Formicinae, and more than ten times in total in ants. Slave-makers and their hosts are often close phylogenetic relatives, which is typical for social parasites and their respective hosts (formalized as Emery's rule). This has major evolutionary implications since it may argue for sympatric speciation.

Raids can jeopardize host colony survival, therefore exerting a strong selection pressure upon the hosts. Reciprocally, there is some evidence that hosts also exert a selection pressure on their parasites in return, since resistance by host colonies might prevent enslavement. Coevolutionary processes between slave-making ant species and their hosts then can escalate to an evolutionary arms race.

See also

• Trophobiosis
• Host–parasite coevolution
• Kleptoparasitism

References

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