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 consequences to ants have not been quantified. To test the predictions that colony size or reproductive output will be enhanced as a consequence of ants collecting seeds, we conducted a field experiment in a forest near Gambler, Ohio, in June and July 1993. Experimental colonies receiving seeds of Sanguinaria canadensis (N = 24 colonies) had similar to 3.5 times as many gynes as control colonies (N = 27). Only 25% of control colonies produced any gynes, whereas 65% of experimental colonies did so. Control colonies produced as many males as experimental colonies. Access to seeds shifted the mass and numerical investment ratio in colony reproductive output toward female bias but did not affect the number of workers or queen size. These data support the hypothesis that myrmecochory is a true mutualism. Additionally, the experiment indicates that resource levels influence investment ratios in reproductive ants.
