This study aimed to determine differences in activities between two male morphs of the dung beetle Copris acutidens Motschulsky (Coleoptera: Scarabaeidae) during the reproductive period and to examine the size distribution of reproductive males that stayed in nests. The activities of two male morphs distinguished by a threshold value of body size were compared with those by horn length. Regardless of body size or horn length, earlier activity of minor males was observed during the reproductive period. The sex ratio showed the greatest female bias when minor males were the more abundant than major males, indicating that minor males were the most active when competition was the weakest and these could avoid direct combat with major males. In morphs distinguished by horn length, more major males than minor males stayed in nests with females although the major males became the most active from the middle of the reproductive period. Thus, longer horns may directly confer a competitive advantage to males, enabling them to stay in nests with females, whereas early activity of minor males does not always indicate the effect of horn length directly. Therefore, this behaviour may occur regardless of whether the morphs differ in body size or horn length.

The physical mechanism of cuticular color in Phelotrupes auratus was investigated by polarized inspection, spectrophotometry and transmission electron microscopy (TEM). No color change was observed when viewed through either a right-or left-handed circular polarizer. Further, under the incidence of linearly polarized light, the reflected intensity was markedly reduced when observed through a linear polarizer set with its optical axis perpendicular to that of the incident light. These results indicate that P. auratus does not possess any circularly polarizing reflectors. TEM observations revealed a total of ten or twelve thin layers (about 60-120 nm in thickness) of two types of material (electron-dense and electron-lucent) alternately stacked in the epicuticle. The thickness of the layers in the different color forms of the beetle corresponded to the peak wavelengths in the reflectance spectra, lmax(a), with thicker layers found in beetles exhibiting reflectance peaks at longer wavelengths and vice versa. Based on these findings, we concluded that all the cuticular color forms of P. auratus were not produced by a circularly polarizing reflector but by a simple multilayer reflector.

Allometric scaling of male horn morphologies of Copris acutidens, including the head and prothoracic horns, was analyzed. The analyses of scaling relationships indicated a discontinuous increase in head horn length and prothoracic horn height but a linear increase in prothoracic horn length relative to body size. The different results for length and height of the prothoracic horn suggest that the height is functionally more important for ensuring mates in the nests. Furthermore, both the head and prothoracic horns were dimorphic, and this characteristic was not found in the scarabaeoid beetle Onthophagus. Similarity in the switch point values for the head and prothoracic horns suggests that the dimorphism of the two kinds of horns may result from the same developmental threshold mechanism.

We investigated geographic color variation of the beetle Phelotrupes auratus in the Kinki region of central Japan using a spectrophotometer. The reflectance spectrum of the dorsal surface of the elytron was measured for beetles collected from 23 sites. We focused on lmax(a), the wavelength at the peak (a peak) between 400 and 700 nm (human visual sensitivity range), for analyses of color variation. In populations distributed on the Kii Peninsula, in the southern part of the Kinki region, average lmax(a) values were lower (480-497 nm) than in populations distributed in the area west of Lake Biwa, the western part of the Kinki region (618-633 nm). For populations distributed in the areas south and southeast of Lake Biwa, a geographic cline of lmax(a) was observed approximately along an east-west transect, with average lmax values varying continuously from 624 to 557 nm. The easternmost populations along this cline had almost the same lmax(a) values as the populations distributed west of Lake Biwa. The coefficient of variation for lmax(a) tended to be larger in populations with intermediate averages than those with lower or higher average values.