Helping behaviour in facultatively eusocial hover wasps: an experimental test of the subfertility hypothesis

A candidate explanation for the evolution of eusociality is that helpers ar e physiologically constrained such that helping is their only realistic opt ion. We tested this subfertility hypothesis in a species of facultatively e usocial hover wasp (Hymenoptera, Stenogastrinae: Liostenogaster flavolineat a) by seeing whether helpers that were forced to nest on their own were abl e to mature their own eggs. One focal helper was left alone on each of 22 n ests, from which all other adult wasps (including the dominant) were perman ently removed. After 18 days, all but one of the 19 focal helpers that rema ined on their nests had ovarian development and insemination status charact eristic of dominants, and the majority had probably laid eggs. This was in striking contrast to the reproductive status of other helpers removed from the same nests at the start of the experiment. These results provide convin cing experimental evidence that females do not become helpers because of so me unconditional physiological constraint. There is currently no unequivoca l support for the subfertility hypothesis in facultatively eusocial Hymenop tera lacking morphological castes. (C) 1999 The Association for the Study o f Animal Behaviour.