The palm pest Scapanes australis has been the object of considerable recent research to develop control measures that target male reproductive behaviour. This study investigated horn size scaling relationships and frequency distributions in two subspecies of S. australis for evidence of alternative male reproductive strategies. Indeed, a sigmoidal horn size allometry in both populations was detected as a significant vertical discontinuity by a piece-wise regression model, which indicates that distinct minor and major male phenotypes are expressed in both taxa. A non-linear regression algorithm showed that these subspecies differ significantly in two modelled parameters of horn allometry. A likelihood model was used for probabilistic discrimination of the alternative male phenotypes by estimating their ratios across the bimodal frequency distribution. The latter is intended to aid field investigation of the reproductive biology of S. australis by statistically discriminating the alternative male phenotypes on the basis of easily measured parameters in live samples.