Bert Hölldobler and Hubert Markl.
Notes on the interspecific, mixed colonies in the Harvester ant genus Pogonomyrmex.
Psyche 96:237-238, 1989.

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NOTES ON INTERSPECIFIC, MIXED COLONIES IN THE HARVESTER ANT GENUS POGONOMYRMEX
BY BERT H~LLDOBLER* AND HUBERT MARKL**
During our long-term investigations on colony interactions and communication in the harvester ants (Pogonoymrmex) in the Chi- hahuan desert near Portal, Arizona and Rodeo, New Mexico, we occasionally observed interspecific, mixed colonies of P. rugosus and P. barbatus. In all cases it appeared that P. barbatus workers were incorporated into a P. rugosus nest, because P. barbatus were by far in the minority.
We first noted this phenomenon in July 1974, when we encoun- tered a P. rugosus nest, where approximately 5% of the workers emerging from the nest entrance were P. barbatus. Between 1974 and 1988 we observed 9 additional such incidents. The percentage of P. barbatus workers recorded at the nest entrance of P. rugosus nests varied from 3% to 11%. In most cases the P. barbatus workers were engaged in foraging, and no difference in behavior was noticed between the P. barbatus foragers and their P. rugosus nestmates. There was, however, one remarkable exception. In July 1988 we observed a large P. rugosus colony where workers of this species were carrying P. barbatus workers out of the nest and were releasing them a few centimeters to approximately 2 m from the nest entrance. During transportation the P. barbatus workers assumed a pupal posture, with the appendages tightly folded to the body. When released they often remained in this position for a few seconds until they "unfolded" and returned to the nest. From a total of 20 P. barbatus workers marked with fast drying enamel, we verified that individual P. barbatus workers may be carried out repeatedly. In one case we recorded a worker being evicted from the nest 19 times within 3 hours. We observed this behavior for 12 days, when it finally began to wane and within the following two days it ceased *Department of Organismic and Evolutionary Biology, Harvard University, Cambridge, Mass. 02138, USA. Current address: Zoologisches Institut der Uni- versitat Wurzburg, 8700 Wurzburg, Federal Republic of Germany. **Fachbereich Biologic, Universitat Konstanz, Konstanz, Federal Republic of Germany.
Manuscript received by the editor June 15, 1989.



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238 Psyche [vo~. 96
completely. From the appearance of the cuticle and the develop- ment of their ovaries and fat bodies we concluded that these P. barbatus workers were young ants. During the following 3 weeks we noticed only occasionally a few P. barbatus workers patrolling the vicinity of the nest entrance, but we did not see them foraging. How do these interspecific mixed colonies originate? During our investigations of intra- and interspecific interactions in Pogono- myrmex populations, we repeatedly observed mature P. rugosus colonies raid smaller colonies of other congeneric species. We re- corded such raids on conspecific nests (3 times), on nests of P. barbatus (4 times), on P. desertorum (once) and on P. maricopa (once). We also twice observed raids conducted by P. barbatus on small P. rugosus colonies that were approximately 1-2 years old. In several of these cases we noted that the raiders completely wiped out the raided colonies, pillaging live and dead workers, and brood. In one case we observed a mature P. rugosus colony raiding a P. barbatus nest 12 m away and capturing at least 46 pupae and 22 larvae. We checked this site two years later and did not find any sign of the raided colony, whereas the P. rugosus colony was still at the same location.
These observations suggest that interspecific mixed colonies in Pogonomyrmex might result from territorial raids. Although labo- ratory experiments indicate that a large portion of the captured brood of other Pogonomyrmex species is eaten by the raiders, P. rugosus readily adopts brood of P. barbatus (and vice versa), so that the foreign pupae may eclose in the raiders' nest and can subse- quently serve as slave workers.
Acknowledgment: We thank Stefan Cover for verifying the iden- tification of the ants and for his comments.



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