Evolution of social behaviour in the primitively eusocial wasp Ropalidia marginata

What is it about?

Ropalidia marginata is a primitively eusocial wasp widely distributed in peninsular India. Although solitary females found a small proportion of nests, the vast majority of new nests are founded by small groups of females. In such multiple foundress nests, a single dominant female functions as the queen and lays eggs, while the rest function as sterile workers and care for the queen's brood. Previous attempts to understand the evolution of social behaviour and altruism in this species have employed inclusive fitness theory (kin selection) as a guiding framework. Although inclusive fitness theory is quite successful in explaining the high propensity of the wasps to found nests in groups, several features of their social organization suggest that forces other than kin selection may also have played a significant role in the evolution of this species. These features include lowering of genetic relatedness owing to polyandry and serial polygyny, nest foundation by unrelated individuals, acceptance of young non-nest-mates, a combination of well-developed nest-mate recognition and lack of intra-colony kin recognition, a combination of meek and docile queens and a decentralized self-organized work force, long reproductive queues with cryptic heir designates and conflict-free queen succession, all resulting in extreme intra-colony cooperation and inter-colony conflict.

Featured Image

Why is it important?

Social insects are classified as primitively or highly eusocial based primarily on whether or not there is morphological differentiation between queens and workers. The absence or the presence of such morphological caste differentiation is correlated with a suite of contrasting traits. In primitively eusocial species, caste determination is largely post-imaginal, adults have flexible roles and can switch from worker to queen roles, colony sizes are usually small, and physical aggression—especially by the queen—is important for the maintenance of social organization. In highly eusocial species, on the other hand, caste determination is pre-imaginal, adults often have fixed roles making it impossible for adult workers to become queens, colonies are usually very large, and chemical regulation by pheromones has largely replaced physical aggression as a key mechanism for the regulation of social organization [1]. Because workers can potentially switch to queen roles, implying that workers are behaving altruistically even though they are potentially capable of behaving selfishly, primitively eusocial species offer attractive model systems for testing theories of social evolution. In addition to all the features of primitively eusocial species mentioned above, Ropalidia marginata has the additional advantage of being in the tropics, making it possible for the species to have a nesting cycle that is largely aseasonal, perennial and indeterminate—new nests can be founded at all times and do not have a fixed lifespan. This additional feature makes it possible for female wasps to have several different options available to them—(1) founding new solitary nests, (2) founding new multi-female nests, (3) staying back and working at their natal nests for all of their lives, and (4) staying back to work for some time and eventually take over the role of queen in their natal nests (figure 1). Male wasps leave their natal nest about a week after eclosion to lead a nomadic life and will therefore be ignored in the rest of this discussion. We have contrasted the two options for female wasps, of leaving to found solitary nest (option 1 = selfish, single foundress strategy) on the one hand and staying back to work for their entire lives (option 3 = altruistic worker strategy) on the other, to ask why some wasps pursue the altruistic strategy while others do not. In doing so we have often neglected the fourth option of working first and then taking over as queens (mixed altruistic and selfish strategy). This neglect, necessitated by technical difficulties, is at least partly responsible for the open question in the title—do we need to look beyond kin selection? [2].