This article is reposted from the old Wordpress incarnation of Not Exactly Rocket Science.
An ant nest is sheltered, well defended and stocked with food, but one that takes time to build and protect. That's why some species of ants don't bother to do it themselves - they just squat in the nests of others.
These ants are 'social parasites' - they don't feed off their hosts' tissues, but instead steal their food, sleep in their homes and use their resources. They're like six-legged cuckoos
An ant colony is too dangerous a target to victimise lightly and the social parasites use several tricks to stop their hosts from ripping them apart. Some escape reprisal by chemically camouflaging themselves, either by mimicking their hosts' odour, or by acquiring it through contact.
This specialised strategy ties the parasite's fates into those of its host. Both are caught in an evolutionary arms race, with the hosts becoming more discriminating and the parasites' deception becoming more accurate. But Stephen Martin from the University of Sheffield has found one ant species with a completely different and more flexible strategy - it tastes really, really bad.
Ants of the genus Formicoxenus raise their young in the colonies of other ants. Some species have earned the nickname of 'shampoo ants' for their tendency to spend almost half their time licking their hosts. As they do so, they acquire the hosts' odour and blend into the colony, escaping discovery and reprisals.
The shampoo ants are strictly one-host parasites, faithfully sticking to their species of choice. But the Formicoxenus nitidulus, the 'shining guest ant' (so called for its glossy abdomen and not its manners or gifts), is an exception.
It's a promiscuous parasite and has been found in the nests of at least nine species of Formica wood ants. Individuals can even move freely between nests, something that would get other social parasites quickly dismembered.
Unlike its camouflaged cousins, F.nitidulus doesn't lick its hosts and mostly ignores them - clearly it has no need to blend in with the colony. In fact, its strategy lies in not blending in.
Martin found that its cuticle (its hard outer 'skin') contained certain chemicals not found in any other Formicoxenus or Formica species. And these chemicals proved to be repulsive to their hosts. Every single time the parasites were seized by a host ant, they were immediately dropped. They weren't doing anything beyond tasting foul, for even dead ants warded off their attackers.
The strategy works because ants don't use weapons; they have no raking talons or stabbing horns. They attack intruders with their powerful jaws and as formidable as they are, they become quite ineffective if the intruder tastes too repulsive to hang on to.
As further proof of F.nitidulus's chemical deterrent, Martin prepared extracts from their skin and applied to fruit flies. These coated flies enjoyed newfound protection from biting ants; 80% were dropped unharmed, while only 15% of untreated flies were.
This is the first time that this strategy has been observed in ants, but it's clearly a successful one. F.nitidulus is a widespread species, and its hosts' nests are often rife with hundreds of parasitic colonies.
Its cousins have becomes specialists, abjectly depending on their lone host species for survival. As such, their continued survival is poised on a knife edge and the extinction of their host could see them toppling over.
F.nitidulus, on the other hand, has chosen the lifestyle of a generalist, a much more viable long-term strategy. It can exploit a huge variety of hosts, and if one of them dies out, it will simply move on.
Reference: Martin, Jenner & Drijftout. Chemical deterrent enables a socially parasitic ant to invade multiple hosts. Proc Roy Soc B doi:10.1098/rspb.2007.0795
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