Altruism in its extreme form is seen in social insects where most individuals give up their own reproduction and work to rear the offspring of their queen. The origin and evolution of such sterile worker castes remains a major unsolved problem in evolutionary biology. Primitively eusocial polistine wasps are an attractive model system for investigating this phenomenon. Ropalidia marginata (Lep.) (Hymenoptera: Vespidae) is one such tropical primitively eusocial wasp, in which new nests are initiated either by a single foundress or by a group of female wasps. Worker behaviour in Ropalidia marginata cannot be satisfactorily explained by the haplodiploidy hypothesis due to the existence of polyandry and serial polygyny which reduce intra-colony genetic relatedness to levels lower than the value expected between a solitary foundress and her offspring. Besides, wasps appear to move frequently between newly initiated nests, perhaps further reducing intracolony genetic relatedness. To study social organization and examine the possibility of kin recognition and task specialization under conditions of low intra-colony relatedness, genetically mixed colonies were created by introducing alien one-day old wasps onto recipient nests. As a first step I have tried to determine the factors that influence the acceptance of foreign wasps onto established colonies. I have introduced wasps between 1 to 20 days of age from donor colonies located at least 10 km away onto 12 different recipient colonies, observed these wasps for a period of 10 hours and later dissected them to examine their ovarian condition. Observations were carried out in the blind i.e. the observer was unaware of the identity of the wasps. Wasps upto 6 days of age were accepted by the alien nests. Older wasps may have been rejected because their relatively better ovarian condition may have been perceived as a reproductive threat to the recipient nest. Alternatively, younger wasps may have been accepted because they may be more easily moulded to the desired roles or due to some other correlate of age per se independent of ovarian condition. Although ovarian condition appeared to influence the probability of acceptance, it was not statistically significant in the presence of age in multiple regression models, making a favourable case for the 'ease of moulding hypothesis' or 'age per se hypothesis' over the 'reproductive threat hypothesis'. In any case these findings gave me a method to create genetically mixed colonies. On 12 different nests Ropalidia marginata, I similarly introduced one-day old wasps and thus created genetically mixed colonies. Such an introduction simulates the eclosion of distantly related individuals which is quite common on nests of R. marginata due to the presence of serial polygyny. About 7 such wasps were introduced per colony and the introductions were so arranged as to matched with natural eclosions on the recipient nest. After 7 days following the last introduction, colonies were observed for 20 hours each. Alien wasps became well integrated and performed most of the behaviours and tasks shown by the natal wasps. There was no evidence of kin recognition or task specialization between natal and introduced wasps. The introduced wasps also sometimes became replacement queens. In an attempt to test the costs in terms of brood rearing efficiency, of living in such genetically variable groups, I created kin and non-kin pairs of wasps in plastic containers. They were provided with ad libitum food, water and building material. The nests initiated were monitored till an adult offspring eclosed. There were no detectable differences in either the productivities or the developmental periods of immature stages in the kin and nonkin pairs suggesting that there is no apparent cost of living with unrelated or distantly related individuals. To compare the extent of cooperation between the two wasps in kin and non-kin pairs, I conducted behavioural observations on 12 pairs each of kin and nonkin wasps. I found no difference in the rates at which the non-egg layers brought food and pulp, fed larvae and built the nest in the kin and nonkin pairs suggesting that cooperative nest building and brood rearing was common to the kin as well as non-kin pairs. The results reported here strengthen the idea that factors other than genetic relatedness must play a prominent role in the maintenance of worker behaviour in Ropalidia marginata.

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.