Social evolution in melittobia

Abstract

Interactions between individuals can range from peaceful cooperation, through mediated contest, to escalated conflict. Understanding such diversity of interactions between individuals requires an understanding of the costs and benefits involved with these behaviours, and the influence of relatedness between interacting individuals. Species in the parasitoid wasp genus Melittobia display social behaviours at both extremes of this spectrum, from the potentially cooperative traits of the ratio of male to female offspring that they produce, and the dispersal of females to new habitats, to the extreme conflict of violent contests between males. In this thesis, I examine a number of aspects of social evolution in Melittobia. First, I consider the pattern of sex allocation – the division of resources between male and female offspring - where local mate competition theory predicts that females will adjust their offspring sex ratio (proportion of males) conditionally, with females laying increasingly female biased sex ratios as the number of other females laying eggs on the same patch increases. In Chapter 2, I show that M. acasta females always lay an extremely female biased sex ratio, and that this may be explained in part by the fact that male Melittobia engage in violent lethal combat in competition for mates. Early emerging males have a competitive advantage and thus there is a limited advantage for later laying females to produce a less female biased sex ratio. However, I also demonstrate that the advantage of early emergence can be reduced when we consider male body size, which is linked to fighting ability, suggesting that the occurrence of this extreme conflict does not fully explain the unusual pattern of sex allocation in Mellitobia. In Chapter 3, I examine whether the level of dispersal varies in response to the extent of local competition for resources, and the relatedness between competitors. I use the species M. australica, which readily produces two distinct female dispersing morphs, to show that the production of dispersing females increases with the competition for resources. I consider the parallels between the evolution of dispersal and of sex ratio. In Chapter 4, I examine male fighting in more detail and explore theory that predicts that when extreme conflict does evolve, the incidence of fighting varies with resource value, number of competitors, and the level of relatedness between males. I show that mating opportunities are sufficiently valuable that male Melittobia will always engage in fighting irrespective of relatedness, that there is no evidence of opponent assessment prior to fighting, and that the intensity of fights increases with the number of competitors. This thesis highlights the importance of considering combinations of social traits and the interactions between them, to understand the evolution of social characters.