Sociality Along an Altitudinal Gradient in the Newly Discovered Silk Wasp (Microstigmus sp.)

Abstract

The origin and maintenance of eusociality is a long-standing question in evolutionary biology. In this thesis, I utilise the extensive environmental variation found across an altitudinal gradient in the tropical montane forests of the Ecuadorian Andes to explore the adaptive value of sociality in a newly discovered species of the wasp genus Microstigmus (a species of ‘silk wasp’). As these investigations must be founded on a clear understanding of the key behavioural features of my study system, I begin by presenting a comprehensive description of the nesting biology of M. sp. ‘darkred’. Using a combination of genetic analyses and behavioural observations, I explicitly focus on defining the nature and extent of sociality occurring. Progressive provisioning of young, due to the apparent extension to the duration of parental care, is suggested to be an important factor promoting the evolution of sociality. I go on to examine this theory by using experimental manipulation to test the importance of adult presence for offspring survival in the mass provisioning M. sp. ‘darkred’. Experimentally orphaned nests were found to have an average of 67% fewer healthy offspring than control nests, thereby determining that extended parental care is critical for brood survival in M. sp. ‘darkred’. These findings have important implications for the potential adaptive value of group living in mass- provisioning species. Owing to the large altitudinal range that M. sp. ‘darkred’ inhabits, this species offers an excellent opportunity to examine intraspecific social variation in response to environmental conditions. I next investigate the adaptive significance of sociality in this species through consideration of altitudinal patterns of social behaviour and productivity. I find a clear trend of increasing group size at higher altitudes, where environmental conditions are significantly cooler, drier and have more variation in rainfall relative to lower altitudes. Moreover, an observed decrease in the rate of brood production in these areas indicates that this group size trend is likely to be a result of adaptive changes in dispersal decisions - potentially owing to the environmentally mediated payoffs associated with independent vs group living. Finally, I explore sex allocation in M. sp. ‘darkred’, a trait thought to be both significantly influenced by, and have important implications for, the evolution of sociality. I use molecular techniques to investigate sex ratio variation across the altitudinal gradient in relation to trends in social group formation and localised levels of inbreeding. The brood sex ratio in this species is found to be significantly female-biased in a ratio of 1:2 M:F with a clearly defined male-first precise sex allocation strategy (M-F-F). These results, together with the high levels of inbreeding and sib-mating found, provide a rare example of local mate competition occurring in a social aculeate hymenopteran.