Ecological constraints on independent nesting in facultatively eusocial hover wasps

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Abstract
Recent field experiments suggest that cooperative breeding in vertebrates can be driven by a shortage of breeding territories. We did analogous experiments on facultatively eusocial hover wasps (Stenogastrinae: Liostenogaster flavolineata). We provided nesting opportunities by removing residents from 39 nests within a large aggregation (1995), and by glueing 20 nests obtained from a distant site into a second aggregation (1996). We prevented nest–less floaters from competing for these opportunities in 1995 but not in 1996. In both years, helpers in unmanipulated groups were given opportunities to nest independently without having to incur nest–building costs and with a reduced wait before potential helpers emerged. Helpers visited the nests we provided, but adopted only a small proportion (5% of 111 vacancies created in 1995). Others were adopted by floaters, but a significant proportion of nests were never adopted (nine out of 20 in 1995, seven out of 20 in 1996). Helpers that visited nests did not originate from particular kinds of social group. Nests containing older brood were more likely to be adopted, and adopting females rarely destroyed older brood. A general feature of social insect but not vertebrate life–histories, the long period of offspring dependency relative to the short life expectancy of adult carers, may be a key factor constraining independent nesting.