Host suitability

Once a young queen of a socially parasitic ant species finds and accepts a host colony, it must penetrate the host society, overcome the host's defences, gain acceptance and reproduce; and the parasite's brood and eclosing offspring must also be accepted and nurtured by the host colony. Much of the literature on socially parasitic social insects is devoted to detailed descriptions of the broad range of diverse mechanisms used by social parasites to penetrate host colonies. As referred to above, social insect colonies are typically highly aggressive in the defence of their nests, with well-developed nest-mate-recognition systems, which enable them to discriminate between legitimate nest mates and various kinds of intruders (Holldobler and Michener, 1980; Stuart, 1988b). To be successful, socially parasitic social insects must have mechanisms to breech these defences, and various tactics have apparently evolved in this context.

Obligatory slave makers are highly aggressive in their attacks on host nests and exhibit various fighting and recruitment adaptations for conducting slave raids, which enable them to defeat their hosts in direct frontal assaults (see Buschinger, 1986; Holldobler and Wilson, 1990; and references therein). Workers of obligatory slave-making ants are often quite literally highly evolved fighting machines, with an array of adaptations for successful slave raiding. In many species, the workers have specialized, clipper-shaped or sickle-shaped toothless mandibles, which they use for dismembering their opponents or piercing their exoskeletons during slave raids. In some cases, the workers have enlarged poison glands and the propensity to sting their opponents to death, whereas others have apparent chemical weapons, so-called 'propaganda substances', which they use to confuse and disperse their opponents or induce them to attack one another during raids (e.g. Regnier and Wilson, 1971; Allies et al., 1986). Moreover, all known slave makers actively recruit nest mates for coordinated assaults on target nests, often using chemical trails. Once a slave-maker colony launches a raid on a target nest, the outcome is seldom in doubt.

Colony founding by slave-maker queens is also typically aggressive, with newly mated queens often appearing to use the same lethal weapons and fighting tactics during their solitary assaults on host-species nests as do their workers during slave raids. However, in some cases, less aggressive tactics come into play, and chemical propaganda or appeasement substances are sometimes used (Stuart, 1984; Buschinger, 1986, 1989; Topoff, 1997).

Queens of various Formica species in Europe that are facultative temporary social parasites illustrate a range of tactics for dealing with colony defences (see Holldobler and Wilson, 1990, and references therein). Young queens of Formica rufa plunge directly into host nests and, although many are killed, enough survive to make this ant extremely common and widespread. Queens of Formica exsecta exhibit a more cautious approach and initially 'stalk' host nests and then either enter by stealth or are carried in by host workers, which display relatively little hostility to the parasite. Queens of Formica pressilabris reportedly lie down and 'play dead', assuming a pupal position, and are then picked up by host workers and carried into the nest without any apparent hostility.

For certain Lasius species, temporary parasitism is apparently obligatory (see Holldobler and Wilson, 1990, and references therein). Colony-founding Lasius umbratis queens first attack and kill a host worker and carry it around for a time, before attempting to enter the host nest. Queens of Lasius reginae eliminate host queens by rolling them over and 'throttling' them, biting the ventral neck region until they are dead. Queens of the temporary social parasites Bothriomyrmex decapitans and Bothyriomyrmex regicidus exhibit similar behaviours (see Holldobler and Wilson, 1990, and references therein). The parasites are initially attacked and dragged into the host nest by host workers but ultimately find their way on to the back of the host queen, where they slowly and meticulously cut off her head. Often the parasite queens appear extremely attractive to host workers, probably because of chemical attractants or appeasement substances, and in some cases it is the host workers that kill their own queens, once the parasite has become accepted into the colony. Thus, social parasites appear to use a variety of behavioural and chemical means to penetrate host nests, eliminate host queens and secure adoption by host workers.

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