Abstract
Sexual size dimorphism (SSD) is widespread among animals, but its developmental mechanisms are not fully undestood. We investigated the proximate causes of SSD in three male-larger and one monomorphic scarab beetles using detailed monitoring of growth in individual instars. Apart from the finding that SSD in all three male-larger species started to develop already in the first larval instar, we generally found a high variability in SSD formation among the species as well as among instars. Overall, sexual differences in developmental time, average growth rate, as well as in the shape of the growth trajectory seem to be the mechanisms responsible for SSD ontogeny in scarab beetles. In the third instar, when the larvae attain most of their mass, the males had a similar or even lower instantaneous growth rate than females and SSD largely developed as a consequence of a longer period of rapid growth in males even in cases when the sexes did not differ in the total duration of this instar. Our results demonstrate that a detailed approach, examining not only the average growth rate and developmental time, but also the shape of the growth trajectory, is necessary to elucidate the complex development of SSD.
Highlights
Female-biased sexual size dimorphism (SSD), i.e. situation when females are the larger sex, is prevalent in poikilothermic vertebrates[1,2] and most arthropods, including insects[3,4]
Because at least in the third instar the growth is highly asymptotic and there is a long period of considerably retarded growth (Fig. 2), the average growth rate is highly biased by the period of the retarded growth and it considerably underestimates the true growth rate[32]
We tested if there are any sexual differences in the instantaneous growth rate estimated from the asymptotic growth models
Summary
Female-biased sexual size dimorphism (SSD), i.e. situation when females are the larger sex, is prevalent in poikilothermic vertebrates[1,2] and most arthropods, including insects[3,4]. Growth trajectories of holometabolous insects are typically not linear but rather sigmoidal[31,32], well described by asymptotic curves, which is true for scarab beetles[12,30] This holds especially for their final larval instar, where the variability in the duration of a period of very slow growth contributes to inter-, as well as intra-instar differences in body size and developmental time. It is reasonable to assume that there may be differences in growth between the last and the previous larval instars as well as between sexes in the last instar[24,33] For all these reasons, investigation of the ontogeny of SSD may be incomplete and inaccurate if larval development is monitored integrally (for example, if growth rate is expressed as adult size divided by egg-to-adult development time), a correct identification of the proximate causes of SSD requires continuous recording of larval growth[7]
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