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rare, many more species with diverse and perhaps bizarre life-history

strategies probably still await discovery.

In this chapter, I review social parasitism among ant species and

emphasize the behavioural aspects of these relationships. I examine the

major life-history strategies of socially parasitic ants, explore some of the

behavioural mechanisms exhibited by these species and discuss some

especially salient aspects of their evolution, focusing on their behavioural

evolution. The major emphasis will be on the relatively well-studied and

most highly specialized species, particularly the obligatory slave-making,

inquiline and xenobiotic ants. However, other free-living and facul-

tatively parasitic species will also be discussed, especially when their

characteristics provide insight into the evolution of the more specialized

forms. For more extensive reviews, see Wilson (1971, 1975a) and

Hölldobler and Wilson (1990).

Life-history Strategies

Traditionally, the life histories of socially parasitic ants are considered to

be of two fundamental types: those that involve ‘compound nests’ and

those that involve ‘mixed colonies’. In compound nests, the parasite

species and the host species maintain their broods separately, each

tending their own in separate areas. In mixed colonies, the brood is kept

communally, with the parasite’s brood mixed in with that of the host

(Wheeler, 1910; Wilson, 1971; Hölldobler and Wilson, 1990).

The distinction between compound nests and mixed colonies

provides a useful framework for ordering and distinguishing various

kinds of associations among ant species and also underlines an important

variable in socially parasitic relationships, namely, the degree of intimacy

obtained between parasite and host. Compound nests occur between pairs

of species that tend to be only distantly related taxonomically. Relation-

ships involving compound nests range from somewhat casual and fairly

unspecialized or facultative associations that are not necessarily parasitic

to highly evolved and obligatory forms of social parasitism. In contrast,

mixed colonies often reflect a very close taxonomic relationship between

the species and tend to be characteristic of some form of either facultative

or obligatory social parasitism. Thus, the distinction between compound

nests and mixed colonies often entails important and fundamental differ-

ences in the origin and evolution of parasitic relationships.

Generally, the degree of integration between closely related species

in mixed colonies far exceeds that in even the most highly evolved cases

involving distantly related species in compound nests. Indeed, close

taxonomic relatedness appears to be an important prerequisite for

the formation of fully integrated mixed colonies and, therefore, for the

evolution of some of the more advanced forms of social parasitism. This

pattern of close taxonomic relationship between social parasites and their

Social Parasitism in Ants 317