Experimental allometry: The scaling of growth, nutrient assimilation, and metabolism in larval hawkmoths raised on natural and artificial diets

Background/Question/Methods Recent theory attempts to explain metabolic scaling based on the evolutionary optimization of the structure and function of resource supply networks and exchange surfaces. However, we know of no studies that have simultaneously examined the scaling of both metabolism and resource assimilation. We use the larvae of tobacco hawkmoths (Manduca sexta), which grow ca. 10,000-fold in mass in 18 days, to investigate whether metabolic scaling is related to developmental changes in nitrogen (N) and carbon (C) assimilation and excretion. To examine the effects of diet quality (C:N), and to understand whether laboratory results can be extended to field conditions, we raised caterpillars on either a stock laboratory diet or on the leaves of tobacco (Nicotiana tobacum) a natural hostplant. Results/Conclusions Animals fed tobacco grew more slowly, and though they grew for a longer period, they reached a smaller asymptotic size. At the same time, metabolic scaling exponents were indistinguishable from ¾ for animals on both diets, which contrasts with other recent studies documenting the effect of food quality on metabolic scaling. The scaling of excretion (frass production) was steeper than the scaling of metabolism, while the scaling of ingestion was curvilinear and substantially shallower. Together, these results lead to a relatively simple model of ontogenetic growth and point to the potential importance of the stoichiometry (C:N balance) of nutrient uptake for understanding the relationship between metabolic scaling and growth. This study is part of a larger NSF-funded undergraduate interdisciplinary research and training project (Manduca InSTaRs) assessing the role of midgut morphology and function in metabolic scaling.