Encounter-induced hostility to neighbors in the ant Pristomyrmex pungens

Abstract

Field observations have demonstrated that internest hostility is negatively correlated with the distance between nests in the Japanese queenless ant, Pristomyrmex pungens. This runs counter to the “dear enemy” phenomenon. This result led us to hypothesize the existence of encounter-induced hostility in P. pungens. We created “neighbor(s)” and “stranger(s)” by an experimental method and tested the ant's ability to discriminate the neighbors from strangers. The results indicated that the ant could distinguish the neighbors from the strangers and displayed significantly stronger hostility toward the neighbors. The recognition of neighbors was quickly established after a few encounters and was maintained for at least 10 days after the most recent encounter. Nest mates without direct encounter experience with neighbors did not show hostile behavior toward neighbors. These findings suggest that information about neighbors (probably colony odor) is not transmitted to nest mates within the colonies. Aggressiveness toward the neighbors appears to be caused and maintained on the basis of individual memories from direct contact. It may be adaptive for a species that changes nest sites frequently to discriminate strangers from neighbors and selectively attack the latter.