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.