less, adoptions into alien conspecific nests are apparently possible and
are quite common in some species (Alloway, 1980, 1997; Stuartet al.,
1993; Bourke and Franks, 1995). The nests of closely related species might
provide cues that are very similar to conspecific cues, and the response
of young queens to these cues and their attempts to gain acceptance
probably form an important and perhaps general basis for the evolution of
interspecific socially parasitic relationships. Indeed, the similarity of both
nest cues and the cues emanating from the young queens themselves
among closely related sympatric species is likely to facilitate the adoption
of young queens into alien nests and probably explains, at least in part,
why socially parasitic relationships tend to evolve among closely related
species.
As mentioned above, the slave-raiding behaviour of slave-maker
workers also requires effective host finding. The evolutionary basis for
slave raiding in various groups of ants appears to be the aggressive
territorial behaviour of free-living species (Wilson, 1975b; Alloway, 1980;
Stuart and Alloway, 1982, 1983; Hölldobler and Wilson, 1990; see below),
and an effective aggressive response to appropriate competitors was
probably characteristic of the free-living ancestral species that gave rise to
these parasites. Again, the precise cues and mechanisms involved are
unknown, but the high level of host specificity achieved indicates that
such cues and mechanisms undoubtedly exist, and evolving parasites
would be under considerable selection pressure to further develop and
refine their responses to these host-specific cues.Host acceptance
The ability of socially parasitic social insects to recognize and accept their
hosts and to respond to them appropriately seems strongly analogous to
their ability to recognize, accept and respond appropriately to their nest
mates, an ability that social insects possess in abundance (Hölldobler
and Michener, 1980; Stuart, 1987, 1988a,b, 1992; Hölldobler and
Wilson, 1990). Typically, social insects are highly aggressive in defending
their nests, territories, trails and food sources against intruding members
of their own or other species. Nest-mate recognition in these contexts
generally involves colony-specific chemical cues (or odours), located
on the surface of the body and learned by colony members. Generally,
this learning first occurs in the early adult stage, shortly after the adult
emerges from the pupa, and, therefore, might constitute imprinting.
However, the learning of new nest-mate recognition cues can apparently
continue throughout adult life under appropriate circumstances and, in
the absence of a critical period, would not be considered imprinting
(Stuart, 1988a,b, 1992). Imprinting has been implicated in the learning of
species-specific brood-recognition cues in some species but not in others
(Jaisson and Fresneau, 1978; Le Moli and Mori, 1982; Alloway, 1997). For
social parasites, learning the species-specific chemical cues of the hosts326 R.J. Stuart