According to a study published by a team of biologists, ant slaves trapped in their oppressors' nests covertly kill off the offspring they are left to care for in acts of rebellion that are part of an evolutionary ant "arms race"...
The study, published in the journal Evolutionary Ecology, reveals that earlier recorded instances of the behaviours were not isolated acts, but a symptom of a common tendency by enslaved Temnothorax longispinosus worker ants to rebel against their Protomognathus americanus oppressors by means of sabotage.
Lead author on the paper Susanne Foitzik of Johannes Gutenberg University Mainz witnessed the acts in ant populations located in the US in West Virginia, New York and Ohio.
The sabotage resulted in an average survival rate among the captors' offspring of just 45 per cent -- in ordinary circumstances, around 85 percent of the pupae (a life stage of ants that follows the larval stage) should survive. Instances of the slaves neglecting and tearing apart the vulnerable pupae -- alone, or in gang attacks -- were recorded.
The study deduced that, since the workers cannot reproduce, the clever tactic is designed to weaken the parasitic colony, thus giving opposing colonies a fighting chance. It is a militaristic tactic, rather than a brood defence.
"Growth of social parasite nests is reduced, which leads to fewer raids and likely increases fitness of neighbouring related host colonies," write the paper's authors.
Species such as the Protomognathus americanus, a notorious slavemaker in North America that relies on its subjects to survive, have driven neighbouring populations to devise counter measures that ensure their own survival. No longer could the common worker ant idly sit by and allow the slavemaker to reduce it to a species of day care providers -- what is referred to as "brood parasitism", where slaves are forced to tend to their captors' young.
"Parasite pressure has led to the development of defensive strategies in hosts," says the study, "which, in turn, resulted in the evolution of counter-adaptations in parasites, a process which may lock both species in a coevolutionary dynamic, potentially escalating in an evolutionary arms race."
The ants are enslaved when their colony is attacked and expelled, or their brood stolen. The workers continue with their usual behaviour, despite now being in the Protomognathus americanus' master nest -- they continue to feed and clean the larvae, and the stolen brood become new slaves. When the captors' larvae start to pupate, however, something is triggered in the slaves.
"Probably at first the slaves cannot tell that the larvae belong to another species," Foitzik said in a statement. "The pupae, which already look like ants, bear chemical cues on their cuticles that can apparently be detected."
In West Virginia, New York and Ohio, 27, 49 and 58 per cent of the pupae survived, respectively -- the study explains that the variations are most likely due to varying defensive and offensive tactics developed by different colonies. For instance, the New York host colony was more aggressive. Prior studies have focused on these factors -- how colonies evolve to protect themselves from attack. However, the curious question of what a worker ant that cannot reproduce can do to target a stronger host community that holds all the cards, is what drove Foitzik's investigation.
"Based on theoretical considerations it was long thought that defense behaviours of enslaved workers are unlikely to evolve, because slaves cannot escape and reproduce, hence no behaviour could increase their direct fitness," explains the paper.
This study proves that when a Temnothorax longispinosus looks as though it has been defeated and entirely assimilated to its new life, it still has a few tricks up its sleeve. By targeting its captors' brood, the colony is weakened and thus conducts fewer raids on its neighbours, which could be relatives to the Temnothorax longispinosus workers.