Cristina Ottocento will defend her doctoral thesis titled “The Evolution and Ecological Drivers
of Variation in Chemical Defences in the Wood Tiger Moth (Arctia plantaginis)” on September 15th, 2023 at 12:00 Finnish time. The public defence will take place in YAA303, Ambiotica, University of Jyväskylä. The Opponent is Professor Mike Speed from the University of Liverpool, United Kingdom, and the Custos will be Dr Ossi Nokelainen from the University of Jyväskylä.
In her dissertation, Cristina studied variation in chemical defences of the wood tiger moth and tested how said variation influences predators' behaviour. These moths produce two strongly smelling compounds SBMP (2-sec-butyl-3-methoxypyrazine) and IBMP (2-isobutyl-3-methoxypyrazine) and advertise their chemical defence to predators with vivid warning colours on their wings. Birds find those compounds repellent and often desert the moth because of its smell. This work shows that birds found moths from high predation pressure populations more repellent than moths from populations that face low birds’ predation. These results suggest the evolutionary arms race between predators’ pressure and prey defences. Surprisingly, this research also discovered that those highly repellent moths were not those that produced the highest total amounts of pyrazines but rather the relative amounts of pyrazine compounds mattered more. Thus, the efficacy of the two methoxypyrazines (SBMP and IBMP) is not purely additive. This result highlights the importance of using natural predators to evaluate the efficacy of prey defence rather than relying only on the analysis of compounds and their amounts.
Defence is costly
Defence comes at a cost and Cristina’s work found that the early life nutrients are key to shaping the armor. By manipulating specific nutrients in the diet, she was able to investigate how resources are distributed between growth, chemical defence, and colour pigmentation, and whether trade-offs between those traits occur. This study discovered that moths reared on high-resource diets had more efficient defensive fluids than those raised on low-resource diets. The pigment components of the warning signals were only marginally influenced by nutrients’ availability, but the study found a positive correlation between the intensity of visual warning signals and chemical defence. This phenomenon, known as “honest signal” points to a strategy where animals effectively communicate the efficacy of their deterrent defences to predators through their visual signal.
The online version of this thesis can be found here