Evaluating the role and measures of juvenile growth rate: latitudinal variation in insect life histories

Latitudinal and elevational trends in body size are found in numerous animal taxa, with various adaptive explanations proposed. It is however debatable whether geographic trends in adult body size are accompanied by corresponding differences in juvenile growth rate (= mass gain per unit time). Respective studies have been complicated by conceptual and methodological problems related to defining and measuring this variable, particularly in organisms with discontinuous growth like arthropods. Using an original method for estimating differential (instantaneous) juvenile growth rates, we compared geographically distant European populations of six insect species in a common garden experiment. We found no among population differences in differential growth rate in any of the species. This result is in concert with concurrent increase in both adult size and developmental time towards the south. While opposite examples exist, we interpret our results as challenging the view that growth rate is a trait that readily responds to environmentally based selective pressures. Our results thus advocate the more classical view of growth rate maximisation within its physiological limits. We discuss the advantages of using differential (rather than integral) measures of growth rate in evolutionary ecological studies and evaluate the reasons for the detected latitudinal trends in life history traits.