Social Parasites: The Case of the Pirate Ant

    You’re probably thinking, what exactly is a social parasite? Consider a person who samples ice cream without actually paying for the product afterwards. We can think of a social parasite as someone who reaps the benefits of society without any cost to themselves. In the animal kingdom, a social parasite is any organism that takes advantage of the social organization of eusocial animals including insects, crustaceans, and mammals. One prominent example of a social parasite is the pirate (sometimes called “slave-maker”) ant.

    So what exactly is a pirate ant? As with its name, a colony of pirate ants steals the brood (eggs and immature stages) of another ant species and raises them to use as workers for their own colony. The captive ants do tasks that would normally be performed by the workers of the pirate ant colony, like foraging for food and taking care of the young ants. Some species of pirate ants can't survive without captives of another species -- because their workers have lost the ability to perform these critical tasks! The species that lack the ability to perform such tasks are called “obligate” pirate ants. For example, ants in the genus Polyergus are thought to be obligate pirate ants because they suffer a high mortality rate and colony death when deprived of a worker force.

Polyergus mexicanus (red) and its host, the silver field ant Formica argente (black). Photo by Alex Wild [1]

    On the other hand, in so-called “facultative” pirate ant species, the workers retain the ability to carry out nurturing tasks. Some examples of facultative pirate ants include the blood red ant or Formica sanguinea. Even if deprived of a host workforce, blood red ant colonies can continue to function unlike the genus Polyergus. 

A parasitic Formica (possibly, F. subintegra) queen. Photo by Alex Wild [2]

    To support themselves, pirate ants often engage in raids. Typically, small parties of pirate ants will scout for potential host colonies and will return to their nest for further recruitment of their nestmates. Gathering a larger force, pirate ants will conduct raids on the host colony, killing adults and defenders alike while stealing eggs, pupae, and larvae in the process. Upon return to their nests, pirate ants will coat their newly captured eggs with the colony pheromone, marking them as members of the colony and the new workforce in the making. Potential host colonies may offer resistance but may face even more casualties in defense of their brood rather than retreating. Some host species have evolved to counteract pirate ants, favoring a colony level retreat over resistance, including the genus Proformica.

    I stated in the beginning that pirate ants are also known by their more widely used name, slave-maker ants. I think it's very important to understand that there’s a lot of exclusionary metaphors rooted in such terminology. Take the more common name of the pirate ant, the “slave-maker” ant. Given the racial-charge behind the institution of slavery, the name “slavemaker” can only be best described as something archaic. Even worse is the term “negro ant” to describe the species Formica fusca, a potential host colony for pirate ants. There can be no denying that there is a responsibility of both scientists and audience alike to pay attention to social impact and change terminology in the field if needed. And while the field continues its discussion around what should be appropriate, I would definitely prefer the name of the pirate ant. 

A raiding party of Polyergus mexicanus. Photo by Alex Wild [3] 

Further Reading:

Howard Topoff, Ellen Zimmerli, "Formica Wheeleri: Darwin's Predatory Slave-Making Ant?", Psyche: A Journal of Entomology, vol. 98, Article ID 034829, 9 pages, 1991. https://doi.org/10.1155/1991/34829

Joan M. Herbers, Watch Your Language! Racially Loaded Metaphors in Scientific Research, BioScience, Volume 57, Issue 2, February 2007, Pages 104–105, https://doi.org/10.1641/B570203

Kleeberg, I., Foitzik, S. The placid slavemaker: avoiding detection and conflict as an alternative, peaceful raiding strategy. Behav Ecol Sociobiol 70, 27–39 (2016). https://doi.org/10.1007/s00265-015-2018-6

Stoldt, M., Foitzik, S. (2021). Slave-Making in Ants (Dulosis). In: Starr, C.K. (eds) Encyclopedia of Social Insects. Springer, Cham. https://doi.org/10.1007/978-3-030-28102-1_105

Zamora-Muñoz, Carmen & Ruano, F & Errard, Christine & Lenoir, Alain & Hefetz, Abraham & Tinaut, Alberto. (2003). Coevolution in the slave-parasite system Proformica longiseta-Rossomyrmex minuchae (Hymenoptera: Formicidae). Sociobiology. 42. 299-317. 

Media Credits:

[1]: Photo by Alex Wild. 

Polyergus - Alex Wild Photography 

[2]: Photo by Alex Wild. 

Formica - Alex Wild Photography (alexanderwild.com)

[3]: Photo by Alex Wild.

Polyergus - Alex Wild Photography (alexanderwild.com)