Differential Response of Ant Colonies to Intruders: Attack Strategies Correlate With Potential Threat

Animals are often threatened by predators, parasites, or competitors, and attacks against these enemies are a common response, which can help to remove the danger. The costs of defense are complex and involve the risk of injury, the loss of energy ⁄time, and the erroneous identification of a friend as a foe. Our goal was to study the specificity of defense strategies. We analyzed the aggressive responses of ant colonies by confronting them with workers of an unfamiliar congeneric species, a non-nestmate conspecific, a co-occurring congeneric competitor species, and a social parasite—a slave-making ant. As expected, the latter species, which can inflict dramatic fitness losses to the colony, was treated with most aggression. A co-occurring competitor was also attacked, but the ants used different behaviors in their responses to both enemies. While the slavemaker was attacked by biting and stinging and was approached with spread mandibles, the competitor was dragged, a behavioral strategy only possible if the defending ant is similar in size and strength to the opponent. Non-nestmate conspecifics were treated aggressively as well, but less than the slavemaker and the co-occurring competitor, presumably because they are less easily recognized as enemies. An unfamiliar congeneric species was rarely attacked. This first detailed study comparing the aggressive responses of ant colonies toward slave-making ants to other species posing different threats indicates that the responses of ant colonies are adjusted to the risk each opponent poses to the colony.