Social evolution in melittobia
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Date
2009Author
Innocent, Tabitha M.
Metadata
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.