Sexual size dimorphism and male reproductive traits vary across
populations of a tropical rainforest dung beetle species
(Onthophagus babirussa)
Abstract
Sexual size dimorphism (SSD) arises when natural selection and sexual
selection act differently on males and females. Rensch’s rule predicts
that SSD increases with body size in species when males are the larger
sex. However, male-biased SSD is rare in insects and the rule does not
always hold between species and even among populations. Here, we
investigate intra-specific variation in SSD as well as relative
investment in precopulatory (horn length) and postcopulatory traits
(sperm length and testes weight), in a tropical rainforest dung beetle
Onthophagus babirussa across Singapore and Peninsular Malaysia. Overall,
three out of four populations displayed significant male-biased SSD but
contrary to Rensch’s rule, SSD was greater in populations with smaller
overall body size. Average male body size was similar across all
populations, but female body size differed significantly suggesting that
the pronounced SSD may also be due to weaker fecundity selection on
female body size. Across all populations, horn length showed a strong
positive static allometry while postcopulatory traits showed negative
allometry (in all but one population), which suggests an evolutionary
trade-off between precopulatory and postcopulatory traits in this
species.