Abstract
Eusocial insects are characterised by a reproductive caste differentiation such that egg-laying is restricted to a small number of queens. The majority of the colony members function as non-reproducing workers and gain indirect fitness by rearing the offspring of the reproductives. In primitively eusocial species, some workers can also get direct fitness by sneaking in some eggs in the presence of the queens, replacing the queens and becoming new queens, or initiating new nests. Here we aimed to understand the factors that permit some workers to gain direct fitness and alter the number of workers doing so, using the primitively eusocial wasp Ropalidia marginata. We transplanted 12 naturally occurring colonies into large laboratory cages where there was adequate space for the workers to leave their natal nests and initiate new colonies. We compared six control colonies that we provided with ad libitum food placed near the nest to six test colonies in which we hand-fed the wasps in addition to the ad libitum food. Wasps in test colonies consumed more food, showed more aggression, replaced their queens, and initiated new nests significantly more often than those in control colonies. When considering all 12 colonies, the rates of queen replacement and nest foundation were significantly positively correlated with food consumption rates. The additional nutrition gained by hand-fed wasps appears to help workers in test colonies to develop their ovaries and lay eggs, implying that they are nutritionally castrated in control colonies.
Highlights
The differentiation of colony members into reproductive and non-reproductive castes is one of the defining features of eusociality (Wilson 1971)
It is widely accepted that the evolution of the non-reproductive worker caste is facilitated by the indirect fitness gained by helping closely related reproductive castes and their offspring (Gadau and Fewell 2009)
Nutrition induced direct fitness for workers in a primitively eusocial wasp. Colonies until it refused to take any more, it was impossible to have any kind of sham control. In both control and test nests, we counted the number of C. cephalonica larvae remaining unconsumed every day at the feeding station and calculated the number consumed as the difference between this and the number provided on the previous day and added this to the number of hand-fed larvae in the test nests
Summary
The differentiation of colony members into reproductive and non-reproductive castes is one of the defining features of eusociality (Wilson 1971). It is widely accepted that the evolution of the non-reproductive worker caste is facilitated by the indirect fitness gained by helping closely related reproductive castes and their offspring (Gadau and Fewell 2009). Workers can reproduce in the future, either by replacing the present queens of their colonies or by leaving their nests to found.
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