Harpagoxenus sublaevis is interesting in that, despite its relatively small size, it is a 'slave-maker'. In this instance it was apparently using Leptothorax acervorum as slaves, though it also enslaves L. muscorum and L. gredleri.
Describing ants as slave-makers is generally a misuse of the term, as it's only possible to enslave members of the same species. Instead, myrmecologists have given this form of social parasitism its own name: dulosis.
H. sublaevis conducts raids on Leptothorax colonies, carrying larvae and pupae away to rear as slaves. This can be quite drawn out process, as H. sublaevis recruits through tandem running, so it can take quite a while until there are enough H. sublaevis workers to conduct the raid. During the raid, H. sublaevis produces a propaganda pheromone, which causes the Leptothorax workers to attack one another, rather than the intruders. Nevertheless, H. sublaevis is heavily built, with antennal scrobes into which antennae can be drawn and straight edged mandibles that can be used for shearing through the appendages of attacking Leptothorax.
The evolutionary origins of dulosis in ants still has a lot of questions that need answering. There are three main hypotheses for the origin of slave-making:
- Predation: This was the only possible origin for slave-making suggested by Darwin in the Origin of Species. In essence, he suggested that slave-making emerged when one colony raids another colony and steals brood to be used as food. If any of the brood were allowed to survive then these would be adopted as slaves within the host colony. There is currently no evidence for this as the prime mover leading to slavery.
- Territoriality: This hypothesis suggests that territorial aggression between neighbouring colonies, and the stealing of rival brood for food, led to some captured brood to be allowed to survive to adulthood and be accepted as nestmates. There is some evidence in support of this hypothesis.
- Transport: This was originally suggested by Buschinger. He suggested that slave-making may originate from transporting brood between nests in polydomous (multi-nest) colonies. If this was extended to other colonies then it could lead to an early version of slave-raiding. Again, there is little evidence in support of this theory.