POLYMORPHIC GROWTH IN LARVAE OF THE BUTTERFLY MACULINEA-REBELI, A SOCIAL PARASITE OF MYRMICA ANT COLONIES

Caterpillars of Maculinea rebeli have two growth strategies for living underground as social parasites of Myrmica ant colonies. Laboratory e xperiments and field data show that 25% of caterpillars live ten month s with ants before pupating, whereas 75% grow slowly, parasitizing the ir hosts for 22 months. Both types of caterpillar form apparently iden tical similar-sized pupae. This may be the first description in the an imal kingdom of polymorphic growth rates spanning different years with in the same population, yet without resulting (as in salmonid fish) in two morphologically distinct adult types with obvious differences in behaviour. We suggest that a balanced polymorphism has evolved in M. r ebeli growth rates, representing the most efficient way of exploiting the limited, yet steady, daily supply of food available to cuckoo-feed ing parasites of long-lived ant societies. Bet-hedging benefits would also accrue to adult butterflies producing a mixture of annual and bie nnial offspring. Despite ergonomic and other benefits, partial biennia lism is unlikely to evolve unless slow-growing individuals have enhanc ed survival and can remain attached to their mobile hosts. We show tha t caterpillars become so closely protected by, and integrated with, th eir host colonies that slow growers experience no greater mortality ov er two years than fast growers over one, and are transported in prefer ence to the ants' own larvae when the host colony moves nest site.