Creature Feature: Slave-making ants

“Take no prisoners!” said no slave-making ant ever. In fact, kidnapping and, for lack of a better term, enslavement is essentially what these ant species do best. Only about 50 out of over 10,000 ant species practice this form of social parasitism. It involves the co-existence of two ant species within a single colony through the kidnapping and rearing of the host colony’s brood by the parasitic colony’s workers, otherwise known as dulosis. Most parasitic ant colonies are found within two subfamilies of ant species: Formicinae and Myrmicinae. Some slave-making ant species in the Formicinae subfamily include Polyergus mexicanus, Raptiformica sanguinea, and Rossomyrmex minuchae. In the Myrmicinae subfamily, species such as Leptothorax duloticus, Harpagoxenus americanus, and Temnothorax americanus, which used to be known as Protomognathus americanus, engage in slave-making activity. Amongst these various genera, parasitic species differ from each other in colony size, with or without the kidnapped host workers. The number of host workers within a colony can range from a few dozen to several thousand [1]. That’s a whole lot of dulosis!

Slave-maker Polyergus mexicanus ants and their hosts Formica argentea together in one colony. [Photo taken by Alex Wild, Source]

The parasitic process of dulosis can be characterized by two types of invasions into the host colony: invasion by slave workers and invasion by the parasitic queen. The occurrence of either invasion is dependent on the size of the queen’s colony. If the queen already has adult workers of her own species, the workers will invade the host nest. Otherwise, a solo queen will invade a host colony alone. This is no small feat. The queen, who has already mated with a male, must invade the colony, usurp the host queen either by killing her or driving her away, secure the host’s brood, and have the host workers accept her as their own queen [2]. All of this in a day’s work? No thank you! The reason this queen flies solo in her attack is because she requires workers to help care for her brood. Now that she has the support of the host workers, they can rear her own brood [2]. How does she get this support, you may ask? Workers can usually identify whether or not other worker ants are part of their colony; however, slave-making queens can disrupt this recognition process. The parasitic queen manipulates the host ants to engage in cooperative behavior towards her that normally would have been reserved for fellow colony members. One way that Polyergus queens manipulate the host workers is by acquiring the scent of their original host queen. This can be done by grooming herself near the injured or dead host queen or by rolling around and covering herself in the nest debris [1]. With the help of these manipulated host workers, the queen can grow her colony with workers of her own species.

Polyergus mexicanus parasitic queen (in red) with her Formica argentea host worker (in black). [Photo taken by Alex Wild, Source]

So far, we’ve only discussed the queen’s invasion into a host colony. What about the workers? Once the queen has given rise to a suitably-sized colony with workers of her own species, the raids begin. An individual slave-making worker or a small group of slave-making workers will head out of the colony as scouts to find a new host colony. Once an unfortunate and unsuspecting host colony is found, the scouts return to their own nest to form a raiding party. An assembled party will then head to the host colony where it will kidnap the brood to bring back to their own nest [2]. Some raiders stand guard at the raided nest’s entrance to prevent any brood from escaping with a host worker. These raids might seem aggressive, however some species, such as Leptothorax duloticus’s, are especially brutal as they kill most of the adult workers upon invasion and sometimes take over the nest as their own. But not all species are quite as deadly: Harpagoxenus americanus will kill very few host adults and will simply take the brood back to their nest. [3]. Once a raid is complete and the host brood have been acquired, the  newly-acquired ants become the housekeepers of the colony when they develop into adults. They provide a variety of services, from foraging for food and feeding nestmates to removing waste from the nest and expanding the nest through excavation [2]. What a life for the parasitic ants. They don’t even have to lift a leg!

Temnothorax curvispinosus, otherwise known as Acorn ants, care for brood whilst their parasitic nestmate Temnothorax americanus lounges next to them. [Photo taken by Alex Wild, Source]

Now that you understand the social parasitic behavior of these ant colonies, I would like to note that there is some controversy over the name of these ant colonies. Slave-making has been the official and colloquial term for this group of parasitic ants. However, the term “slave” or “enslavement” carries a lot of weight due human history’s relationship with enslavement. Using the term slave, even in a scientific setting, can cause discomfort and negativity to such a degree that some may not want to study these ant behaviors. Therefore, a search is afoot for a new term! It has been difficult to find a term that both accurately portrays the behavior and does not carry with it negative connotations. Some have suggested pirate ants, but this would be an inaccurate description of what these colonies actually do. Pirates loot items but do not really kidnap individuals. This is why some have started calling them kidnapper ants instead. Seeing as there hasn’t been a definitive consensus for a new name till now, I have chosen to use the original, albeit problematic, name in this feature. Will this group officially become kidnapper ants in the near future? I wouldn’t be surprised. 

So, what have we learned?

1) Ants can be hostile. Based on the sheer fact that they steal others’ babies, they can be quite ruthless.

2) These queens don’t take no for an answer. They do whatever it takes to get their “crown” when they usurp the host queen and convince the colony that they are the new queen.

3) Ants can be lazy. Once stolen, the host ants do all the work and the slave-makers just raid.

Formica pergandei (red-colored parasitic worker) and Formica glacialis (black-colored host worker) battle over the brood of the Formica glacialis. [Photo taken by Alex Wild, Source]

Nicole W. Korzeniecki is a graduate student in the Animal Behavior Graduate Group at UC Davis. She’s interested in the dynamics of social insect colonies, specifically in collective decision-making and the self-organization of complex behaviors.

Main featured image: Polyergus mexicanus slave-making ant carries away a brood member from its Formica host colony. [Photo taken by Alex Wild, Source]


References:

1. D’Ettorre, P., & Heinze, J. (2001). Sociobiology of slave-making ants. Acta ethologica, 3(2), 67-82.

2. Topoff, H. (1990). Slave-making ants. American Scientist, 78(6), 520-528.

3. Alloway, T. M. (1979). Raiding behaviour of two species of slave-making ants, Harpagoxenus americanus (Emery) and Leptothorax duloticus Wesson (Hymenoptera: Formicidae). Animal Behaviour, 27, 202-210.

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