Evolutionary transition to a semelparous life history in the socially parasitic ant Acromyrmex insinuator

The recently discovered social parasite Acromyrmex insinuator (Schultz, Bek kevold & Boomsma 1998) exploits colonies of the leafcutter ant A. echinatio r. We document that A. insinuator represents a rare early stage in the evol ution of social parasitism, because a worker caste is still partially prese nt and mating phenology has remained at least partially similar to that of the host. A. insinuator is tolerant of host queens, and sexual offspring pr oduced in parasitized colonies can be either exclusively A. insinuator or a mix of A. insinuator and A. echinatior. The remarkably high abundance of A . insinuator in nests of the investigated Panamanian host population and th e fact that A. insinuator colonies readily reproduce under laboratory condi tions allowed us to test evolutionary predictions on reproductive life hist ory evolution that are not possible in most other socially parasitic ants. We show that (1) A. insinuator has a semelparous 'big bang' reproductive li fe history which exploits host colonies without leaving reserves for surviv al; (2) social parasite sexuals are significantly smaller than A. echinatio r host sexuals, but still large compared to host workers, confirming an evo lutionary scenario of gradual size reduction and loss of the worker caste a fter transition towards a socially parasitic life history; (3) major change s in the life history of ants can evolve relatively quickly compared to ada ptations in morphology, caste differentiation and mating phenology.