Latitudinal cline of larval growth rate and its proximate mechanisms in a rhinoceros beetle

Juvenile growth rate is a key life-history trait with a major effect on various fitness components. Duration of the growing season changes with latitude, often resulting in latitudinal variations in growth rate. While many studies have shown a latitudinal cline of growth rate, most have used linear measurements of growth rate without considering the nonlinear nature of growth trajectories. Furthermore, elucidation of the proximate mechanisms causing increased growth rates would provide a fundamental understanding of the evolutionary process of the latitudinal cline of growth rates, which are largely unknown. We used common garden experiments to examine the latitudinal variation of growth rate. We analysed the larval growth curve of 14 populations of the univoltine Japanese rhinoceros beetle Trypoxylus dichotomus (Coleoptera: Scarabaeidae) found along a 2,000-km latitudinal gradient. To clarify the mechanisms responsible for the latitudinal cline of growth rate, we compared food consumption and growth efficiency (i.e. the capacity to convert food into biomass) using three populations at high, middle and low latitudes. There was a strong positive correlation between growth rate and latitude and a strong negative correlation between age at the inflexion point of the growth curve and latitude, indicating that larvae from northern populations grew more rapidly and reached the inflexion point earlier than those from southern populations. The food intake increased linearly towards higher latitude. The growth efficiency of larvae from middle latitude was greater than that of larvae from low latitude, but there was no difference in the growth efficiency between high- and middle-latitude populations. The increased growth rate at higher latitude is probably a local adaptation to complete development and growth during the short growth season. The rapid growth rate at higher latitudes could be explained by both the increased food consumption and the growth efficiency. However, the growth efficiency is probably maximized in the middle-latitude population, suggesting that some physiological costs prevent further increase in growth efficiency at higher latitudes. This study provides compelling evidence for latitudinal cline of juvenile growth rate and disentangles the contributions of food intake and growth efficiency to the cline. A free Plain Language Summary can be found within the Supporting Information of this article.