Evaluation of the "dear enemy hypothesis" in the ant, Acromyrmex lobicornis, through behavioral and genetic studies

Background/Question/Methods

Territory defense through aggressive behavior is a phenomenon well known in animals. Some territorial animal species respond less aggressively to intrusions by their neighbors (which may have an established territory) than to intrusions by non-neighbors (strangers), to minimize the losses that continuous fights produce. This difference in the intensity of the response has been termed the ‘dear enemy phenomenon.’ One mechanism that could explain the ‘dear enemy phenomenon’ is the degree of genetic relatedness (i.e., less genetic differentiation) between near neighbors compared with distant neighbors. We tested this hypothesis, and the possible mechanism in the leaf-cutter ant Acromyrmex lobicornis in the East boundary of the Nahuel Huapi National Park, Argentina. In the field we ran individual encounters of ants from nests at different distances. Our experimental units were trios that consisted of one focal nest, a close neighbor nest (less than 50 m from the focal nest), and a distant neighbor nest (90-200 m from the focal nest). In 17 trios we quantified the level of aggression in encounters between individuals from the focal nests vs. their close neighbor nests, focal nests vs. their distant neighbor nests, and the focal nests vs. the focal nests (control). We ran 15 trials per trio (5 replicates per treatment).

Results/Conclusions

Most of the encounters (61%), between ants of the focal nest and their near neighbor presented intermediate levels of aggression, whereas encounters between the focal nest and the distant neighbor presented the highest levels of aggression (57%). These results show that individuals from near colonies are less aggressive among themselves than to individuals of distant colonies. Using isozyme electrophoresis we determined the level of genetic differentiation between nests. We did not find that individuals from near nests were more related between each other, than individuals from distant nests. This result suggests that the different aggressive behavior between distant and near colonies does not have a genetic explanation. The results of this study support the ‘dear enemy hypothesis’ and suggest that the differential aggressive behavior is acquired and is not related to the degree of genetic relatedness.