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

1. 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. 2. 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 2000 km latitudinal gradient. In order 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. 3. 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. 4. 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. 5. 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
maximised 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.