Provisioning strategies and the evolution of morphological castes in nest-building Hymenoptera.

Abstract

This thesis is in two distinct parts. Part 1 focusses on the optimization of provisioning strategies and Part 2 on the evolution of morphological castes.

(1) In the Hymenoptera, the transition from mass to progressive provisioning, where mothers feed offspring gradually as they grow, is associated with sociality. Because progressively provisioning mothers usually feed more than one offspring simultaneously, they have to make decisions about which offspring to feed next. The genus Ammophila is especially tractable as a study system because different offspring are in spatially separate nests, so that maternal activities can be clearly associated with particular offspring. We use an individual based simulation in combination with field data from A. pubescens to investigate how provisioning strategies might be optimized. We find provisioning the oldest offspring first significantly reduces the risk of both conspecific and heterospecific parasitism and outperforms our other strategies. We also find that this strategy could have evolved without the use of memory to keep track of the relative ages of offspring, despite incurring heavy sampling costs through parasitism. We then go on to consider the impacts of poor weather on informing optimal provisioning strategies.

(2) The evolution of eusociality is regarded as one of the major evolutionary transitions (METs), where units that previously reproduced independently function as a single, more complex entity or group. Conflict and control are particularly important in understanding METs, since minimising within-group conflicts is critical for efficient functioning of the larger entity. Eusocial societies, characterised by a division of labour between worker and queen castes, represent one of the most studied METs. In advanced eusocial societies with morphological castes, queens and provisioners often influence the morphology of workers through a variety of mechanisms, thus potentially constraining worker behaviour. For morphology to constrain behaviour, offspring must then modify their behaviour in response to morphology, but the correlation between behaviour and morphology in reality might not be perfect. We explore some of the conditions under which morphological castes may arise and the factors constraining their evolution. We find that a mother’s certainty about her offsprings’ behaviour, including the ability for offspring to modify their behaviour in response to their morphology, are key in determining when we expect morphological castes to evolve.