SEX INVESTMENT IN A SOCIAL INSECT - THE PROXIMATE ROLE OF FOOD

In eusocial Hymenoptera, relatedness asymmetries lead to conflict betw een parents and offspring over sex investment; workers strive for a 3: 1 female-biased ratio of sex allocation, and queens strive for a 1:1 r atio. Many studies support this genetic relatedness hypothesis, but th e variation in allocation ratios in natural populations is great and r emains mostly unexplained. In this paper, we examined whether food sup ply determines sex investment of the ant Formica podzolica. We compare reproductive parameters of colonies and populations across habitats, and compare sex allocation of fed and unfed colonies. Nest density, wo rker size, sex ratio, and sex allocation were all greater along forest edges than in meadows, and these patterns were associated with natura l food abundance. Furthermore, there was a strong tendency for individ ual colonies to produce either all-male or all-female sexuals, and wor ker size was greatest in colonies producing all females and smallest i n those producing all males. Most important, sex investment was greatl y affected by a supplemented diet, as the population investment ratio, R (i.e., ratio of males to males plus females), was female biased (0. 36) for fed colonies and male biased (0.62) for unfed, control colonie s. Finally, investment ratios were more male biased in polygynous than in monogynous colonies as predicted by a genetic relatedness hypothes is. These results demonstrate that food supply has an important proxim ate influence on sex investment, and may explain much of the natural v ariation in sex investment in populations of eusocial Hymenoptera.