Open and closed economies as possible alternative strategies to resource heterogeneity in ants

Ant colonies have either a single nest (monodomy) or multiple nests (polydomy). A challenge is to explain their adaptive significance, specifying costs and benefits of each colony type. An explanation for ant polydomy is adaptation to spatially heterogeneous environments. With polydomy a colony can exchange complementary nutrition among nests within the entire colony occupying a wide territory. We tested this resource redistribution hypothesis using two closely related species, i.e. the polydomous ant Pheidole megacephala and the monodomous ant Pheidole noda. We put each colony in an artificially polydomous setting with two nests connected by tubes. We provided liquid food lacking protein to one nest and that lacking carbohydrates to the other nest. P. megacephala almost totally failed to produce brood when the connecting tubes were clipped, whereas it improved reproductive performance when the tubes were open. In marked contrast, P. noda managed to maintain high performance for a long period even when only nutritionally biased food was provided, most likely by relying on stored provisions that compensated for the missing nutrients. Based on these data, we propose the hypothesis that ant polydomy is an open economic strategy to counter heterogeneity in resource distribution 'spatially' by trading between nests and extending the resource searching area, whereas monodomy may be a closed economic strategy to cope with resource heterogeneity 'temporally' by withstanding food-depressed periods with stored nutrition and by efficient utilization of frugal diets. More empirical data in other ant taxa are necessary to test generality of this idea.