Nestmate recognition in the unicolonial ant Formica paralugubris

In unicolonial populations of ants, individuals can mix freelywithin large networks of nests that contain many queens. Ithas been proposed that the absence of aggression in unicolonialpopulations stems from a loss of nest mate recognition, butfew studies have tested this hypothesis. We investigated patternsof aggression and nest mate recognition in the unicolonial woodant, Formica paralugubris. Little aggression occurred, evenbetween workers from nests separated by up to 5 km. However,when aggression took place, it was directed toward non–nestmates rather than nest mates. Trophallaxis (exchange of liquidfood) occurred very frequently, and surprisingly, workers performedsignificantly more trophallaxis with non–nest mates thanwith nest mates (bias 2.4:1). Hence, workers are able to discriminatenest mates from non–nest mates. Higher rates of trophallaxisbetween non–nest mates may serve to homogenize the colonyodor or may be an appeasement mechanism. Trophallaxis rate andaggression level were not correlated with geographical distanceand did not differ within and between two populations separatedby several kilometers. Hence, these populations do not representdifferentiated supercolonies with clear-cut behavioral boundaries.Overall, the data demonstrate that unicoloniality can evolvedespite well-developed nest mate recognition. Reduced levelsof aggression might have been favored by the low rate of interactionswith foreign workers, high cost of erroneously rejecting nestmates, and low cost of accepting foreign workers.