Driven by hunger and guided by the sun, swarms of red western ants indugle in daring kidnapping raids against a neighboring species. So reports Howard topoff, a psychologist at Hunter College in New York City, who has been analyzing the behavior of the western slave-making ant, Polyergus breviceps, in an Arizona mountain desert. These ants are dependent for survival on their slaves, a related species Formica gnava. The slaves are captured young, as pupae, and they emerge as adults in the slave-makers’ nest. The slaves forage, defend the nest and feed and groom their masters.
If the colony relocates, they carry the slave-makers one by one to the new site. A colony of 3,000 slave-making ants may have more than 6,000 slaves. Topoff has observed the events leading to the capture of pupae. First, scouts search for Formica nests. A successful scout returns to its colony and uses tactile and chemical means to recruit raiders.
The swarm of raiders may number 2,500, and advance in a phalanx 3 feet wide and 16 feet long. Topoff finds that the scout and its followers rely on sun position, as well as on a chemical trail previously left by the scout, to find their way. With a mirror held to change the apparent position of the sun, Topoff can make the raiding party reverse direction. When the slave-makers reach the target nest, they spray it with a chemical that forces the Formica adults to flee, leaving behind most of the young. The raiders then capture thousands of pupae, some to eat and others to raise as slaves.
A single Polyergus colony may steal as many as 30,000 Formica pupae each year. Topoff finds that need for food is a stimulus for the raids. If he overfeeds a colony, kidnapping raids become less frequent.