Odor-Mediated Aggregations of the Aposematic Coccinellid Beetle, Hippodamia convergens: Supplementary Functions in Chemical Communication
- Author(s): Wheeler, Christopher Andrew
- Advisor(s): Carde, Ring T
- et al.
Preexisting benefits to the signaler and receiver of a candidate semiochemical, facilitates the coevolution necessary to transition that chemical cue into an evolved signal. This natural selection framework would operate within a species in the evolution of multi-functional pheromones, and between species in forms of inter-species communication such as aposematism. The evolution of aposematism depends on a strong coupling between an indicator trait and the defensive trait, yet there is little empirical evidence that supports this relationship. Complex signal theory, also lacking empirical support, predicts specific correlations in the investments among the different multi-modal aposematic displays and the toxicity they advertise.
This dissertation identifies the aggregation pheromones of an aposematic coccinellid, Hippodamia convergens, and highlights their supplemental functions, as well as variation in their production and the behavioral responses they evoke. Such an approach is rarely used, but it can enhance our understanding of the evolution of aggregations, aposematism, and communication in general.
Through laboratory and field assays, I demonstrate that a H. convergens aposematic odor (2-isobutyl-3-methoxypyrazine) also functions as an aggregation pheromone among diapausing beetles. In conjunction with this volatile warning signal, H. convergens also orient to a cuticular hydrocarbon (tricosane) left behind by walking and aggregating conspecifics. These two aggregation signals interact to mediate the formation and persistence of aggregations at specific hibernacula. Additionally, both have supplementary functions that would transfer additional benefits to aggregating individuals, offsetting the costs of producing them as aggregation pheromones alone: 2-isobutyl-3-methoxypyrazine as an aposematic odor, and tricosane as a component of oviposition deterrent and species-specific signals.
I also quantified the visual and olfactory aposematic displays of H. convergens (via spectral analysis of elytra and gas chromatography of whole-body extracts respectively), and demonstrate that they both vary predictably with defensive alkaloids (hippodamine and convergine). Overall, the production of methoxypyrazine aposematic odor is negatively correlated with the degree of alkaloid toxicity. Methoxypyrazines alone do not honestly signal the extent of an individual's toxicity. It is through their synergistic interaction with the honest signal of color, that they exhibit a more nuanced relationship and together function as multi-modal aposematic signals.