Colony Founding And The Evolution Of Eusociality In Primitively Eusocial Wasp, Ropalidia Marginata

Abstract

Many animals live in societies of varying degrees of organization. Some individuals in these societies seem to sacrifice their own fitness to increase the fitness of some others. Understanding the forces that mould the evolution of such altruistic behaviour has become a dominant theme in modern evolutionary biology. Primitively eusocial polistine wasps provide excellent model systems to study the evolution of altruism as they show high degrees of plasticity in their behaviour. Different individuals in the same population pursue different social strategies such as nesting alpne or nesting in groups. When wasps nest in groups, usually only one individual becomes the egg layer, while die rest assume the role of sterile workers. Why do the workers not become solitary foundresses and rear their own offspring instead of working to rear the brood of another individual? Here I have used the tropical primitively eusocial wasp Ropalidia marginata to explore some factors that might potentially favour the worker strategy over the solitary founding strategy. Workers in multiple foundress nests may benefit by rearing brood more closely related to them than their own offspring would be. However, from previous work on this species it is known that relatedness between sisters is rather low and that workers therefore rear quite distantly related brood. Therefore, I have concentrated on factors other than genetic relatedness that might potentially favour the worker strategy. A total of 145 naturally initiated nests with different numbers of foundresses was monitored over a period of 16 months, and their productivities were compared. Although the total colony productivity increased, the per capita productivity did not increase with increasing foundress numbers. Colonies with larger foundress numbers did not produce significantly heavier progeny and did not produce them significantly faster than colonies with fewer individuals. The conspecific usurpers preferred to usurp single foundress colonies more often than multiple foundress colonies. Therefore, protection from conspecific usurpers might be an advantage of multiple foundress associations. About 10% of the multiple foundress nests experienced queen turnovers. This provides a finite chance to reproduce and gain some individual fitness for workers, at some future point of time. Wasps may not be similar in their reproductive abilities and those who are less fertile might be joining others who are more fertile. Testing such a hypothesis would require that individuals who have chosen to be subordinate cofoundresses in multiple foundress associations are forced to nest alone. During this study a total of 77 nests was monitored. Cofoundresses forced to nest alone had significantly lower productivity than natural solitary foundresses and also queens of multiple foundress nests who were forced to nest alone. This suggested that wasps are not similar either in their reproductive ability or brood rearing ability or both. To ascertain which of the factors was responsible for lower productivity in cofoundresses, productivity of wasps isolated into laboratory cages was compared. There was no significant difference in the productivity of isolated cofoundresses and isolated queens. This suggests that wasps are not subfertile per se but probably differ in their foraging and brood rearing abilities. The certainty with which resources are brought into the nest and therefore, the certainty with which the mean per capita productivity is attained, provides an automatic benefit of group living according to the central limit theorem. This prediction was also tested. The coefficient of variation of mean per capita productivity decreased significantly with increasing foundress numbers. Behavioural observations on another 36 colonies, with different number of adults, showed that the coefficient of variation of food brought to the nest and the rate at which larvae were fed, decreased significantly with increasing number of adults. A computer simulation was used to find out the effect of group size on the variance in feed larva. Assuming that larvae cannot be starved for too long and cannot utilize more than a certain amount of food at a time, the fitness of larvae was found to increase with an increase in the number of adults attending the nest.

Previous work on R. marginata has been largely confined to postemergence colonies. An attempt was made to look at and compare social organization in preemergence colonies with that of postemergence colonies. It was found that the egg layer was not the most dominant animal in the well-established preemergence colonies. There were no detectable differences in the social organization of the preemergence colonies (of this study) with that of postemergence colonies of the earlier studies. Perhaps my conclusions drawn from studying preemergence colonies are therefore applicable more widely to the species. It can be concluded that the apparent increased fitness of the worker strategy over solitary foundress strategy does not come from any increase in per capita productivity, but comes instead from (i) the greater predictability with which the mean per capita productivity is attained in larger colonies, (ii) the lower probabilities of usurpation of larger colonies, (iii) queen turnovers that provide opportunities for workers in multiple foundress colonies to gain some direct individual fitness and (iv) the lower brood rearing abilities of workers in multiple foundress nests that make the worker strategy the best of a bad job.