Stereotyped seed preferences of the harvester ant Pogonomyrmex mendozanus in the central Monte desert

Abstract

The harvester ant Pogonomyrmex mendozanus is endemic to the central Monte desert. In this region, extensive grazing reduces grass cover and seed abundance in the soil bank. In previous studies carried out in ungrazed areas, this ant species consumes and prefers seeds of perennial grasses over forbs and shrubs. However, in grazed habitats, P. mendozanus expands its diet by including seeds from other plant groups along with fruits, dead arthropods and other non-seed items. The broader diet could be due to a flexible foraging strategy or could reflect intrinsic, context-dependent food preferences of the populations inhabiting heavy grazed areas, different from those populations studied under ungrazed conditions. We carried out food preference experiments in heavy grazed areas in the central Monte desert in order to discriminate between both hypotheses. We performed choice experiments using a paired comparison design, offering seeds and non-seed food items present in the diet of P. mendozanus. Ants preferred large- and medium-sized perennial grass seeds and, among the non-seed food items, arthropods were preferred. Forb and shrub seeds along with fruits showed intermediate preference levels, whereas the least preferred items were small grass seeds and flowers. Food preferences were similar to those reported previously in ungrazed conditions, supporting the hypothesis that seed preferences are stereotyped in this species and strongly suggesting that it behaves as an expansive specialist by including less preferred items in its diet in heavy grazed habitats where the most preferred seeds are scarce.