Questions 21-30

Social parasitism involves one species relying on another to raise its young. Among vertebrates, the best known social parasites are such birds as cuckoos and cowbirds; the female lays an egg in a nest belonging to another species and leaves it for the host to rear.

The dulotic species of ants, however, are the supreme social parasites. Consider, for example, the unusual behavior of ants belonging to the genus Polyergus. All species of this ant have lost the ability to care for themselves. The workers do not forage for food. feed their brood or queen, or even dean their own nest. To compensate for these deficits, Polyergus has become specialized at obtaining workers from the related genus Formica to do these chores.

In a raid, several thousand Polyergus workers will travel up to 500 feet in search of a Formica nest, penetrate it, drive off the queen and tier workers, capture the pupal brood, and transport it back to their nest. The captured brood is then reared by the resident Formica workers until the developing pupae emerge to add to the Formica population, which maintains the mixed-species nest Th e Formica workers forage for food and give it to colony members of both species. They also remove wastes and excavate new chambers as the population increases.

The true extent of the Polyergus ants' dependence on the Formica becomes apparent when the worker population grows too large for the existing nest. Formica scouts locate a new nesting site, return to the mixed-species colony, and recruit additional Formica nest mates. During a period that may last seven days, the Formica workers carry to the new nest all the Polyergus eggs, larvae, and pupae, every Polyergus adult, and even the Polyergus queen.

Of the approximately 8,000 species of ants in the world, all 5 species of Polyergus and some 200 species in other genera have evolved some degree of parasitic relationship with other ants.

Which of the following statements best represents the main idea of the passage?