The origin and maintenance of eusociality is a central problem in evolutionary biology. Eusocial groups contain individuals that forfeit their own reproduction in order to help others reproduce. In facultatively eusocial taxa, offspring can choose whether to found new nests or become helpers in their natal groups. In many facultatively eusocial insects, offspring need continuous care during development, but adult carers have life expectancies shorter than the developmental period. When a lone foundress dies, her partly reared brood are usually doomed. Here, we show that helpers in a tropical hover wasp (Liostenogaster flavolineata) have an insurance-based advantage over lone foundresses because after a helper dies, most of the brood that she has partly reared will be brought to maturity by surviving nest-mates. After some of the helpers are experimentally removed from a multi-female nest, the reduced group is left with more brood than it would normally rear. We found that larger, more valuable extra brood were reared through to maturity, but not smaller, less valuable brood. Smaller brood may be sacrificed to feed larger brood, and reduced groups probably benefited from increased short-term helper recruitment. Rearing extra brood did not increase adult mortality or brood development time.

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abstract:We explore the effects of group size on the direct reproductive success of subordinate helpers in eusocial animals where only a single, dominant individual reproduces at one time. Helpers can reproduce directly if they inherit dominance, but when dominance is age based, an individual born into a larger group has a longer wait to inherit. We show that this disincentive to help can potentially be offset by increased productivity, increased life span, and insurance-based benefits for helpers if they survive to inherit dominance in larger groups. We analyze a field experiment in which group size was manipulated in the hover wasp Liostenogaster flavolineata. Productivity increased linearly with group size, larger groups were less likely to fail, and dominants in larger groups may have lived longer. Combined with the probability of inheriting dominance, these effects led overall to a negative correlation between group size and expected direct fitness, mainly because group size decreased during our study period, so that helpers could not expect to inherit as large a group as they started queuing in. Our analysis suggests that the relationship between group size and productivity plays a central role in determining the fitness consequences of helping.

Recent explanations for the evolution of eusociality, focusing more on costs and benefits than relatedness, are largely untested. We validate one such model by showing that helpers in foundress groups of the paper wasp Polistes dominulus benefit from an insurance-based mechanism known as Assured Fitness Returns (AFRs). Experimental helper removals left remaining group members with more offspring than they would normally rear. Reduced groups succeeded in preserving the dead helpers' investment by rearing these extra offspring, even when helper removals occurred long before worker emergence. While helpers clearly gained from AFRs, offspring of lone foundresses failed after foundress death, so that AFRs represent a true advantage for helpers. Smaller, less valuable offspring were probably sacrificed to feed larger offspring, but reduced groups did not preferentially attract joiners or increase their foraging effort to compensate for their smaller workforce. We failed to detect a second insurance-based advantage, Survivorship Insurance, in which larger groups are less likely to fail than smaller groups. We suggest that through their use of small offspring as a food store to cope with temporary shortages, wasps may be less susceptible than vertebrates to offspring failure following the death of group members.

Recent ¢eld 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 £avolineata). 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 £oaters 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 £oaters, but a signi¢cant proportion of nests were never adopted (9 out of 20 in 1995, 7 out of 20 in 1996). Helpers that visited nests did not originate from particular kinds of social group. Nests containing older broods were more likely to be adopted, and adopting females rarely destroyed older brood. A general feature of social insect, but not vertebrate life histories, namely the long period of o¡spring dependency relative to the short life expectancy of adult carers, may be a key factor constraining independent nesting.

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