Colony age, neighborhood density and reproductive potential in harvester ants

At about age 5 years, colonies of the harvester ant, Pogonomyrmex barbatus, begin to produce winged, sexual forms (alates) that mate in large annual a ggregations. We examined how colony age and neighborhood density affect the numbers, body mass, and body fat of alates produced by 172 colonies rangin g in age from 4 to 17 years. Over one-third (36%) of all colonies produced no alates. Failure to reproduce was independent of colony age. Of those col onies that did produce alates, older colonies produced more alates than you nger colonies. Older colonies produced lighter female alates (in dry mass), but the total biomass of additional alates produced by older colonies far outweighed the reduced allocation to female alate body mass. Body fat conte nt was much higher in female alates (36.0% on average) than in males (3.7% on average). Alate body fat content was not related to colony age. The fitn ess of female alates may be related to their fresh body mass; that of femal es captured after mating and reared in the laboratory was positively correl ated with egg-laying rate, although not with the total number of eggs in th e first brood. Neighborhood density was not related to alate number, mass, or fat content, in contrast to the results of a 1995 study at the site, in which alate numbers were negatively related to neighborhood density. Thus t he influence of crowding on reproductive output appears to vary from year t o year, perhaps in response to variation in rainfall and food supply. Alate output by individual colonies was correlated among years. These results su ggest that a few, older colonies dominate the pool of reproductives year af ter year.