Exaggerated horns are a characteristic feature of many male rhinoceros beetles. We surveyed and
compared the scaling relationships of these sexually selected weapons for 31 Dynastinae species with
different degrees of horn exaggeration. We found that nearly all rhinoceros beetle species were male
dimorphic, that the allometric slope of major males was consistently shallower than the slope of minor
males, and that the decrease in slope was greatest among species with the most exaggerated horns.
These patterns are consistent with the curved allometries of stag beetle mandibles and giraffe weevil
rostra, and suggest that the depletion of developmental resources is a general phenomenon limiting the
continued exaggeration of insect weapons. The dimorphisms in horn morphology are expected to
correspond to behavioural differences between major and minor males, but little is still known about the
mating tactics of most rhinoceros beetle species. Future studies on the relative benefits and performance
of horns during maleemale combat are needed to fully understand the diversity of horn allometries and
the evolution of exaggerated structures.
© 2015 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd. All rights reserved.