The Occasional Hymenoptera: Slave revolts among ants

Slave-making ants are highly derived species which depend on other ant species for survival. There are different techniques for enslaving another species but the basic method is to replace the “host” queen with the enslaving one (the parasite). The “host” workers then care for the offspring of the new slave-master queen, including feeding and defending them. One technique by the invading queen is to kill the queen of the hosts and bathe herself or consume the pheromones of the old queen. Another method is for workers of the slave-making species to capture eggs or larva of the host and take them back to the parasites’ nest to raise them to work for the new colony.

Protomognathus leading two Leptothorax slaves (photo: University of Toronto)

Protomognathus leading two Leptothorax slaves (photo: University of Toronto)

Protomognathus americanus is a species of slave-making ants endemic to the northeastern United States. Professor Tom Alloway of the University of Toronto makes laboratory nests for them and uses as the hosts for P. americanus Leptothorax ambiguus, L. curvispinosus, and L. longispinosus. The hosts nest “in tiny preformed cavities found in acorns, hickory nuts, milkweed stems, and very small pieces of rotting hardwood. Their favourite nest sites are old acorns that have been partly hollowed out by beetle larvae.” Both the slave-makers and the Letothorax hosts are very small animals. The Protomognathus, which is slightly larger than the slaves species,  is less than 3 mm long (see the photo with scale bar by Dan Kjar at Discovery Life).

The Protomognathus go on raids (leading slaves with them by means of trail pheromones for the slaves to follow. The slaves feed the workers. How the slave-maker treats the enslaved hosts differs within the geographical range. (See results of 2000 study.)

Because the host species once enslaved has no opportunity to reproduce, it was originally thought that selective pressures did not operate on the hosts (once captured) in connection with the parasite-host relationship. Then Susanne Foitzik of Ludwig-Maximilians University in Munich observed in 2008 a slave revolt by Temnothorax, where the hosts killed the Protomognathus larvae (as well as some parasite guards).

A paper by Alexandra Achenbach, Volker Witte, and Susanne Foitzik just published in Behavior Ecology (vol 21, n5, Sept-Oct 2010), entitled “Brood exchange experiments and chemical analyses shed light on slave rebellion in ants” (abstract; paper behind paywall), describes the results of experiments to investigate the acceptance of host and parasite brood by host workers in parasitized and unparasitized colonies. When the slavemakers transferred brood, the hosts commonly killed them, even when the parasites ants were present. The brood of the slave-makers survived better when transferred to host colonies from the same site than to colonies from different areas. The authors propose that it is the parasite that is adapted to local hosts. They find that the selective pressures are on the parasite ant (the Protomognathus) because the chemical profiles on their cuticles are very different from the slaves’. Therefore they are under pressure to adapt its brood recognition cues to those of its hosts to circumvent slave rebellion. The instinct to destroy the unrecognized brood, evidently, must already exist in the host (which of course has the benefit of reducing parasitism overall) and is disabled only by adaptation of the parasite.

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