8800 described ant species (Hölldobler and Wilson, 1990), and they
encompass almost every conceivable mode of commensalism and para-
sitism. Commensalism is a relationship in which one species benefits
and the other is unaffected, whereas, in parasitism, one species (the
parasite) benefits and the other (the host) suffers negative consequences.
Mutualism, in which two species cooperate for the benefit of both, has
yet to be clearly documented among ant species, with all known cases
of symbiosis that have been closely examined being at least somewhat
unilateral and, in most cases, decidedly parasitic. Hölldobler and Wilson
(1990) provide an extensive list of the ant species involved in socially
parasitic relationships.
Why are social insects so prone to the formation of socially parasitic
relationships? Social-insect colonies, with their nests, territories and
trails and the various food sources that they defend, control or maintain,
constitute valuable resources that can be exploited by any organism that
can successfully penetrate and either elude or be accepted by colony
members. Indeed, to paraphrase Wilson (1971), the lines of communi-
cationamongthemembersofsocial-insectcoloniesaredecidedlytenuous
and, through the course of evolution, have been repeatedly opened,
tapped, pried apart or rerouted through a variety of mechanisms to
encompass alien individuals from a broad range of taxonomic groups. In
some cases, these relationships between species involve little more than
banditry,asstealthy,aggressive,well-armouredorfast-movinginterlopers
appropriate colony resources and evade colony defences. However, on
occasion, they have evolved into long-lasting and intimate associations
and the interlopers have become highly specialized obligatory social
parasites, with various behavioural, physiological and morphological
adaptations that facilitate their penetration and integration into the
societies of their hosts. Although social parasitism among social insects
has been recognized and studied for almost 200 years, we are only begin-
ning to understand the various causes, constraints and consequences of
parasitic evolution in these societies, and there is much that remains
unexplained.
The study of social parasitism among social insects has a long and
illustrious history. The dramatic slave raids of the legendary amazon-ants
(see below) were first described by Huber (1810) and were commented
upon by Darwin inTheOriginofSpecies(1859). Indeed, Darwin (1859)
was the first to suggest a plausible scenario for the evolution of slavery
in ants (see above quotation). Subsequently, a vast bestiary of socially
parasitic social insects, with a variety of life-history strategies, has been
documented (see Wilson, 1971; Hölldobler and Wilson, 1990). Many
of these species are highly evolved, obligatory parasites and exhibit
an astounding array of adaptations that rival the raids of the amazons.
For students of behavioural ecology and evolution, the socially parasitic
social insects provide a rich menagerie for detailed study and experimen-
tation. Indeed, since socially parasitic social insects are often exceedingly316 R.J. Stuart