FOOD FROM SEED-DISPERSAL MUTUALISM SHIFTS SEX-RATIOS IN COLONIES OF THE ANT APHAENOGASTER RUDIS

Workers of Aphaenogaster rudis collect seeds of many species of spring -flowering, perennial herbs. This is part of a seed-dispersal system ( myrmecochory) for which benefits to plants are documented, but consequ ences to ants have not been quantified. To test the predictions that c olony size or reproductive output will be enhanced as a consequence of ants collecting seeds, we conducted a field experiment in a forest ne ar Gambler, Ohio, in June and July 1993. Experimental colonies receivi ng seeds of Sanguinaria canadensis (N = 24 colonies) had similar to 3. 5 times as many gynes as control colonies (N = 27). Only 25% of contro l colonies produced any gynes, whereas 65% of experimental colonies di d so. Control colonies produced as many males as experimental colonies . Access to seeds shifted the mass and numerical investment ratio in c olony reproductive output toward female bias but did not affect the nu mber of workers or queen size. These data support the hypothesis that myrmecochory is a true mutualism. Additionally, the experiment indicat es that resource levels influence investment ratios in reproductive an ts.