By Ella Davies
Earth News reporter
Slavemaker ants prefer to target the strong over the weak when seeking new servants, researchers have found.
Ants were observed actively choosing to attack larger, better defended colonies over smaller, weaker ones.
Scientists suggest that the intelligent ants identify strong defences as a sign of a strong population.
By conducting fewer raids on strongly defended targets, the slave-making ants actually limit the risks and come away with the most pupae to enslave.
Slavemaker ants such as Protomognathus americanus are known to demonstrate unusual colony behaviour.
The queen produces offspring but crucially, they do not perform the everyday worker tasks of foraging or caring for broods.
Instead, nominated scout workers identify nearby "host" ant colonies suitable for attack.
During the attack the slavemaker ants steal host pupae and take them back to their own colony.
The pupae are imprinted on the odour of the slavemaker colony and grow up to perform all of the ordinary worker tasks.
This exploitation of another species' workforce is called social parasitism.
In their study published in Animal Behaviour, researchers from the Ludwig Maximilian University of Munich, Germany theorised that slavemaker ants chose "easy targets" over more strongly defended colonies as they offered the lowest risk.
The ants in Sebastian Pohl's study acted in the opposite manner: raiding parties were more likely to attack stronger colonies.
"At first, we were quite surprised, as we expected that attacking slavemaker colonies prefer host colonies that provide a better benefit to risk ratio," Mr Pohl explained.
"We hence had to look at the slavemakers' decision in more detail and had to consider more aspects of the complete raiding behaviour."
Mr Pohl and his colleagues identified that the "slave raids" presented considerable risk to P. americanus.
In small slavemaker colonies consisting of one queen, two to five workers and 30 to 60 slaves, scout ants were very valuable.
It was essential that scouts made the right decision about suitable raid targets or "host colonies" without being discovered and attacked.
"Losing a single worker might very likely be synonymous with losing half of the colony members," Mr Pohl told the BBC.
Therefore, a smaller number of scouting events and subsequent raids presented the lowest risk to the slavemaker colony.
However, the colony still needed new slaves to be able to survive to the next season.
From their behaviour, researchers suggested that the scout ants associated strong colonies with high numbers of pupae and a high benefit.
The tactic of fewer raids on stronger targets consequently offered the best risk to benefit ratio.
P. americanus are 2-3mm in size and live in the deciduous forests of the northeastern USA and adjacent Canadian regions.
Due to their small size a whole colony can inhabit one hollow acorn.