Tuesday, October 7, 2008

Science Vocab: Dulosis - Slave-making in Ants

Ants, like other social Hymenoptera, are some of the most fascinating animals there are. They live in societies – fully functioning, every-individual-has-a-role-and-must-contribute-society. Though I talk about the ups and downs of human societies, animal societies are just as interesting and complicated. Even the unattractive aspects of human social behavior exists among our animal cousins. Dulosis is the slave making behavior of some species of ants.
Here is how it works:

The Parasite species exploits the parent care (brood care) of another species. They steal
pupae from another species (the host species). An all-out fight ensues between the species and it is not pretty. Ants die…lots and lots of ants die. If the raiding party is successful they will kill the workers of the host species leaving too few to worry about. The raiding party carry the kidnapped pupae to their nest where they complete their development. The pupae stage is the last metamorphosis. Pupae require no special care for food. It is very much like the cocoon stage of a butterfly.

Days later pupae emerge as fully adult worker ants and accept the Parasite Queen and parasite workers as their own kin and commence with the jobs they were kidnapped to do – raising the larvae of the Queen.
Sounds eerily familiar to slave dynamics in the Americas. Slaves were kidnapped from Africa, brought to the New World and forced to work the plantations and females often nursed and cared for the young of the owners.

But like all tenuous relationships, there are is co-evolutionary war at hand. This has and will always occur when two individuals or groups have a relationship but conflicting goals. This, too is eerily familiar.

The host species have some defenses against this colony attack and pupae kidnapping, for a long time, everyone thought that these defenses must all be pre-take-over defenses. And that makes sense. These Pre-take over defenses include enemy recognition and physical defenses (fighting) against the parasites. At the same time, no one really thought that post-taker defenses were possible. For one, once the ants emerge they smell the odor of the colony and imprint to it, and everyone thought this locked everything in place. Plus, no one imagined a scenario of rebellion. What good would it do the kidnapped ants to rebel? They can’t “go back home”. They wouldn’t know where it is, besides, everyone was killed in the battle to take them as pupae.

But according to some research by Susanne Foitzik, rebellion can happen and she has documented it. She studies ant species are found in the northeast United States, Atlantic Region: slavemaking ants Protomognathus americanus and Harpagoxenus sublaevis and their hosts Temnothorax spp. and Leptothorax spp.

Photo of a Harpagoxenus sublaevis– credit: antclub.ru

Photo of a Temnothorax – credit: bugguide.net

Even after the slave ants have accepted their “new home” there have been some recorded incidences that slave workers will actually kill the larvae of the parasite brood they were supposed to raise...but only the female larvae. In some cases slave workers actually attack, mutilate and rip the larvae apart. In other cases, the workers will simply neglect the larvae by not feeding them and discarding them in “dump piles” in the nest. Some host species are more vicious than others with some species more likely to rebelling than others.

Why would this happen? Well the running reason has to do with the fact that perhaps the slave workers are keeping the Brood colony in check. By doing so, they prevent or decrease the number of raids on nearby host colonies that are likely to be comprised of their relatives…a sort of indirect group selection.
But how do they know which pupae belong to which species? It seems the chemical and olfactory cues – hydrocarbon profiles are different for each species; and the slave worker ants can distinguish between the female pupae of their own species and that of the parasite species. However, they cannot distinguish between male pupae of their own or parasite species.

And this is why some think the workers don’t kill male pupae. The proposed answer is believed to be related to odor…male larvae “smell” different than female larvae and some experiments suggest that the male larvae of parasite species smell a lot like the male larva of the host species. Moreover, male ants aren’t considered a real threat. Males don’t participate in colony affairs like work, defense, or raiding other ant species nests. Simply put, males mate, then die.

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