Non-linear latitudinal cline of egg size and its consequence for larval survival in the rhinoceros beetle

The egg size of insects often exhibits latitudinal trends, which are attributed to local adaptations to temperature or temperature-related ecological factors. Although increased egg size improves offspring fitness, the relationship between size and fitness is influenced largely by environmental variables. Additionally, there is a trade-off between the size and number of eggs if the quantity of reproductive resources is constant. Despite the key associations among offspring size, number and performance, relatively few studies have jointly considered these traits when studying the genetic differentiation of egg size. Here, we examined the effects of latitude on egg size using 14 populations of the univoltine rhinoceros beetle Trypoxylus dichotomus (Coleoptera, Scarabaeidae, Dynastinae) along a 1200-km latitudinal gradient under common garden laboratory conditions. We found that egg size decreased with increasing latitude in a non-linear manner. Geographical variation in egg size was independent of maternal body size, and there was no difference in female lifetime fecundity between the low-latitude (large eggs) and high-latitude (small eggs) populations. Therefore, low-latitude females invest more resources in reproduction compared with high-latitude females. We further found that the larvae of the high-latitude population showed higher mortality than those of the low-latitude population under both nutrient-poor and nutrient-rich conditions. Although the ecological factors that shape the non-linear cline of egg size were not identified in our study, geographical variation in local environments or in female reproductive strategies could have caused the latitudinal trend.