Though ants are famous for their industriousness and work ethic, one particular species, called Temnothorax americanus, prefers to kick back and let slaves do their bidding for them. In retaliation, enslaved ants have been observed slaughtering the young of their captors, despite the fact that this does not improve their chances of being freed. Since it is unlikely that ants are capable of acting out of spite, researchers are currently seeking to understand the benefit of this type of sabotage, and how such behavior evolved.
Temnothorax americanus obtains its slaves by raiding colonies of other ant species, such as Temnothorax longispinosus. Once inside the colony of their oppressors, these ants react to the chemical profiles of the pupae of slavemaker ants, killing them in large numbers. However, genetic analysis has revealed that enslaved ants never go on to reproduce, suggesting that this behavior brings them no benefit.
It has therefore been proposed that this instinct evolved as part of a species-wide defence mechanism, whereby enslaved ants eliminate future generations of slavemaker ants, thereby reducing their capacity to conduct further raids and take more slaves. Such behaviors are known as “altruistic defence traits.” However, whether or not this type of genetic tendency could feasibly spread through a population of ants depends on the costs associated with it.
For instance, the initial genetic mutation leading to a propensity for slaughtering pupae carrying certain chemical profiles could potentially backfire, causing some ants to kill pupae of their own species. Alternatively, the development of the alleles – or gene variants – necessary for such behavior could disrupt the ants’ ability to conduct other tasks.
Temnothorax longispinosus is often enslaved, but it doesn't go down without a fight. April Nobile via Wikimedia Commons
To determine whether or not genes leading to altruistic defence mechanisms could feasibly spread through species of ants, an international team of researchers used computer simulation models to observe how the development of these genes might affect populations.
When simulations were run under the assumption that such genes carried zero cost – meaning none of the wrong pupae would be accidentally killed – the spread of so-called “rebel alleles” through individual colonies of Temnothorax longispinosus was found to be plausible. According to these models, the presence of these alleles results in a decreased capacity of slavemaker ants to enslave future generations, which in turn allows the rebel alleles to proliferate. Consequently, more slave ants are driven to kill the pupae of their captors, which further reduces the slave-raiding ability of Temnothorax americanus, creating a cycle.
However, if the costs associated with these rebel alleles are set at 1 percent or above, then the adverse effects they produce are unsustainable, leading to repeated extinctions and recolonizations. For this to occur, however, the presence of other nearby colonies of Temnothorax longispinosus is vital, as migrant ants from these populations are required to ensure that numbers bounce back after extinctions.
Publishing their findings in the Journal of Evolutionary Biology, the study authors conclude that the spread of rebel alleles is indeed possible, thereby verifying the altruistic explanation for the ants’ behavior. However, depending on the costs associated with these genetic tendencies, the spread of rebel alleles may not be possible within individual colonies, instead requiring migration of ants between nearby populations.
In a statement, lead author Dirk Metzler explained: “We conjectured that slave rebellion can evolve because it is beneficial for the slaves' relatives living close to the slavemaker nest. Indeed, we found that kin selection could be at work, but on a much larger geographical and temporal scale than we expected.”