Abstract: To elucidate the sexual and male horn dimorphism in adultXylotrupes socrates tonkinensis, multivariate statistical analysis was used to compare the differences of eight morphological traits between male and female, while allometry models of head horn length and thoracic horn length were used to determine the existence of horn dimorphism. The male-male competition behaviors of large and small males for females were observed in the laboratory. The results showed that the distributions of all traits, except tarsal length, overlapped considerably between males and females. The traits for males were generally larger than females (P<0.05). The coefficient variations of head horn length and thoracic horn length were 25.0% and 30.8% respectively, which were significantly larger than those of other measured traits. The values of head horn length and thoracic horn length followed normal distribution (P>0.05). The allometric relationships of head horn length and thoracic horn length to body size were fit with positive and simple linear models y=1.796x-2.8 (R²=0.765, P<0.05) and y=2.059x-2.628 (R²=0.951, P<0.05), respectively. Both small (head horn length <6.56 mm) and large males (head horn length>11.92 mm) that classified by probability grade could use horns to compete for females. Therefore, there is a sexual dimorphism. The head horn length and thoracic horn length of males had great variation, but without obvious horn dimorphism through allometric analysis. The dependence of reproductive strategies on body sizes of males needs to be further evaluated in the field.

Key words: Xylotrupes socrates tonkinensis, sexual size dimorphism, male horn dimorphism, allometry, mate competition.