Estonian Research Council (PRG741): Phylogenetic comparative analyses of body size evolution in Northern European moths (Geometridae).
Herbs rely more heavily on qualitative defenses, expressed as secondary metabolites that can be toxic to herbivorous insects (ref). However, some insects evolved as herb feeders, as is the case with many moth species. To do so, they had to develop detoxification mechanisms (DMs), which are believed to be physiologically costly (ref). Therefore, there must be an evolutionary trade-off in which the evolution of DMs took place at the expense of resources that would be allocated to other traits, notably to body size. In this setting, woody-plant feeders would be less affected by such a trade-off since woody plants do not commonly rely on qualitative defenses (ref). We formulated and tested four initial hypotheses addressing the evolutionary association between body size and host-plant preferences:
- Species whose caterpillars feed strictly on woody plants evolved towards a larger body size optimum compared to herb feeders.
- Considering that evolving larger body sizes are believed to be beneficial in terms of fecundity and (add other benefits, but see ref), the rate of body size evolution is higher among wood-plant feeders than for herb feeders.
- Body size in woody-plant feeders is more conserved through time compared to herb feeders.
- Evolutionary transitions from feeding on herbs to consuming woody plants occur at a higher rate than in the reverse scenario.
In a subsequent step, we refined the previous hypotheses to test whether dietary specialization influenced the joint evolution of body size and host-plant preferences. Our motivation is based on the lack of evidence suggesting a consistent association between body size and dietary specialization. This is materialized by previous studies that have provided mixed results supporting (or not) the body size vs. dietary specialization relationship in moths (refs) and other insects (refs). More precisely, we tested the following four additional hypotheses:
- Body size in herb feeders and woody-plant feeders evolved towards similar optimal values as long as the species are specialists.
- Within generalist species, woody-plant feeders evolved towards a larger optimal body size compared to herb feeders.
- Specialists and generalist species evolved towards similar optimal body size values among herb feeders.
- Specialist woody-plant feeders evolved towards smaller body size optimum compared to generalist woody-plant feeders.
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