Group-living and vibrational communication in larvae of an Australian sawfly, Perga affinis

Abstract

Group-living occurs throughout the Animal Kingdom. Its ubiquitous nature raises two fundamental questions: why do organisms do it, and how do they maintain their group structures? Answers to these questions shed light on the evolution of social living and from them, two principles emerge. The first principle relates to why organisms live with others. In order to evolve, grouping must provide a net fitness benefit to the individual members. In this way, group members attain a higher level of reproductive success than those living alone. The second principle relates to how organisms live in groups and here, communication is key. Groups are composed of separate entities and as such they require a mechanism to form and maintain a cohesive unit; communication provides such a mechanism. Additionally, communication provides a medium of exchange during competitive and cooperative interactions, both of which occur in group settings. It serves to minimize costs of association from competition while enhancing benefits that stem from cooperation. For my dissertation, I investigated why and how larvae of the Australian sawfly, Perga affinis, live in groups. In light of the two principles, I examined both the benefits of group-living and the role of vibrational communication in their gregarious lifestyle. Chapter 4 contains experiments testing for benefits related to predation protection, thermoregulation, immune function, feeding efficiency, and pupation success. While grouping provided an overall survivorship advantage, the main benefits stemmed from thermoregulation and feeding facilitation. Chapters 1-3 provide detailed studies of P. affinis? vibrational communication. In chapter 1, I characterized two signals, contractions and tapping, and tested whether the signals were competitive or cooperative in nature; both have cooperative functions. In Chapter 2, I examined tapping as a mechanism for cohesion between groups and separated larvae while also investigating differential levels of investment in the signal exchange. The exchange was analogous to the Raise-the-Stakes model of cooperation where groups gradually increased their investment according to the time spent signaling by the single larva. In chapter 3, I tested whether or not different tapping rates encoded for alternate signal meanings via a playback experiment