5. Conclusions
This study is the first report of male-biased SSD accompanied by intraspecific reproductive trait variation in the widespread, tropical dung beetle, Onthophagus babirussa . Contrary to the predictions of Rensch’s rule, increased SSD is observed in populations with a lower average body size than those with a higher average body size. We suggest that the pronounced male-biased SSD may be due to weaker fecundity selection acting on females in the Singapore populations and not just than strong sexual selection on males. In order to resolve this, future studies should estimate fecundity selection on females as function of body size as well as sexual selection on male body size, under common garden experiments using more widespread populations across the region. In addition, population level differences in reproductive traits are likely affected by environmental conditions such as diet quality and developmental temperature (i.e. phenotypic plasticity). Phenotypic plasticity can differ between sexes and may even generate variation in SSD among populations. Hence, future studies could investigate the condition-dependence of reproductive traits in this species to better understand the observed population level differences in SSD.