WORKER SIZE POLYMORPHISM CONDITIONING SIZE MATCHING IN 2 SYMPATRIC SEED-HARVESTING ANTS

The relationship between size matching and degree of polymorphism in t wo sympatric seed-harvesting ants, Messor bouvieri and M. capitatus is discussed. The two species converged in the use of seeds as their mai n food resource, but they collected different-sized items, those of M. capitatus being larger than those of M. bouvieri. Size matching of fo rager size and seed size was also greater in M. capitatus than in M. b ouvieri. It was related to the specific worker caste polymorphism of e ach species: M. bouvieri showed a monophasic allometry, while M. capit atus displayed a diphasic allometry with two segments of different slo pe meeting at an intermediate point. Small M. capitatus workers, in th e same size range as M. bouvieri workers, showed a similar weak size m atching, while there was greater correlation between worker size and s eed size for bigger M. capitatus individuals. This indicates that size matching is only rewarding when the loads carried are quite large. Th e role of major M. capitatus workers in increasing the range of sizes carried to the nest was evaluated by means of cafeteria experiments: M . capitatus colonies with major workers were able to transport large s eeds (20-30 mm length), while M. bouvieri colonies and M. capitatus co lonies without majors were only able to collect smaller seeds.