Reclaiming Lost Territory: The Response of Owyhee Harvester Ants to Forager Intrusions by Neighboring Colonies

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Student Presentation

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Ian Robertson


Neighboring colonies of the Owyhee harvester ant, Pogonomyrmex salinus, often share non-overlapping foraging boundaries in the areas between their nests. Interactions between neighbors along these foraging boundaries were infrequent but peaceful. Exclusion experiments showed that when one colony of a pair situated 5-7 m apart was denied access to its foraging range, individuals from the other colony would usually (i.e., in 7 out of 10 cases) enter the unoccupied space within one day. In 6 of 7 cases the occupiers set up habitual foraging trails in the newly acquired area in 5 to 39 days (median = 13 days). When foragers from the excluded colony were subsequently allowed access to their original foraging area, they reclaimed the entire space within 11 days but did not extend beyond this range. In contrast to the previous encounters between neighbors, encounters between neighbors during the reacquisition period were often aggressive, and in 14 of the 57 encounters we recorded one or both of the combatants was killed. Non-lethal contests were much shorter duration than lethal contests. Results show that competition for foraging space in harvester ants is intense despite the seemingly peaceful relationship between neighboring colonies prior to perturbation of foraging boundaries.

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