There are three main types of social parasites that form mixed species ant nests: temporary social parasites, permanent inquilines and slave-makers. Temporary social parasites depend on a host species only during the establishment of new colonies. Usually the parasitism is initiated by young queens following their insemination in a mating flight (Figure 2). The queens try to penetrate host colonies, replace the original queens, and gain acceptance by the workers. The parasitic queens then lay eggs that develop, with the care of the host colony workers, into a worker force of their own offspring. Eventually the host workers die leaving only the parasitic queens and their offspring. As a result, a mature colony contains only members of the parasitic species.



Figure 2: This temporary social parasite (Lasius claviger) initiates a colony following her nuptial flight by penetrating a host colony and killing the host queen. Courtesy of Alex Wild (www.alexanderwild.com).

Whereas temporary social parasites typically kill the host queens, queens of permanent inquilines are usually tolerant of the host queens. With a few exceptions, inquilines do not produce worker offspring, but instead invest most of their energy into producing eggs that eventually develop into sexual forms. In the extreme case of the Swiss ant (Teleutomyrmex schneideri), the inquilines have evolved special modifications like concave abdomens (i.e., gasters) and long tarsal claws which enable them to grip onto the host queens and ride their backs as ectoparasites (Hölldobler & Wilson 1990). Despite the burden, the host queens continue to produce worker offspring and so the mixed species colony is permanent. The host workers simultaneously rear the brood of both the parasitic and non-parasitic queens.



The antics of the slave-makers have made them a favorite among myrmecologists. Besides initiating their nests much like the temporary social parasites, the slave-makers also raid other ant colonies to steal the brood (Figure 3). The pilfered larvae and pupae that are not consumed eventually eclose into worker slaves that are chemically imprinted and completely integrated into the society of their enslavers. The slaves tend brood, gather food, feed their enslavers, care for the queen, and defend the nest against threats. If the colony moves to a new location, the slaves carry their enslavers to their new nest. Sometimes, the slaves even participate with the slave-making workers in slave raids against other ant colonies of their own or closely related species.



Figure 3: Workers of the slave-maker Formica subintegra returning to their nest with a pupa pilfered from a slave-raid. Courtesy of Alex Wild (www.alexanderwild.com).

Not all slave-makers are equally adept at enslaving other ants. Facultative slave-makers like Formica subnuda are commonly found without slaves in their colonies. The slaves, if present, are small and comprise on average only about 10% of workers, because the slave-maker only successfully raids small incipient colonies of its host species. Further, slave raids by F. subnuda are met with great resistance by the host species, and thus the raids last for hours and mortality is high for both slave-makers and the workers of the raided colonies (Savolainen & Deslippe 2001).



In sharp contrast, obligate slave-makers like P. breviceps always have slaves that comprise even over 90% of the work force. They can successfully raid large colonies, sometimes several in sequence on a raiding day, and thus they have more slaves and larger slaves than facultative slave-makers. Their efficiency during raids is associated with special adaptations like enlarged glands and sharp sickle-shaped mandibles (Figure 4). Glandular secretions released during raids serve to pacify or turn workers from target colonies against each other, reducing aggression towards the slave-makers. Workers that do offer resistance to the slave-makers are readily immobilized with a quick piercing of the head or body with the mandibles. Although the modified mandibles of obligate slave-makers are effective weapons, they also make the slave-makers inept nurses and foragers, and incapable of managing their colony tasks without slaves. Indeed, P. breviceps cannot survive on its own even if plenty of food is available. This ant must have slaves to survive, and mature colonies must obtain a minimum of about 6000 slaves per season per colony to maintain a high proportion of slaves (Savolainen & Deslippe 1996).