Zoological Studies

Vol. 53, 2014

Male-male mounting and the unreliability of body size as a character for mate choice in male Japanese beetles (Popillia japonica Newman)

Paul V Switzer1*, Patrick S Forsythe2 and Kipp C Kruse1

1Department of Biological Sciences, Eastern Illinois University, Charleston, IL 61920, USA
2Current address: Department of Natural and Applied Sciences, University of Wisconsin-Green Bay, Green Bay, WI 54311, USA

Background: Same-sex pairing is common in many animal species. In many insects, same-sex pairing is often thought to be a result of poor sexual discrimination (i.e., a mistake), but few detailed studies of the mechanisms underlying the mistaken pairing have been conducted. Previous studies have found that in the field, a small proportion of Japanese beetle (Popillia japonica) mating pairs consist of two males instead of a male and a female. In the current study, we investigated the relationship between body size, the tendency to mount other males, and the duration of these mounts, in laboratory experiments on male Japanese beetles.
In the first experiment, we observed male-male mounting in all-male groups in which each male had been uniquely marked. Males of all sizes were likely to mount other males and extend their aedeagus (copulatory organ), but the mounts were longer, and aedeagus extension was more likely to occur, if the mounted beetle (in the ‘female’ position) was larger than the mounting beetle (in the ‘male’ position). In the second experiment, we observed male-female behavior in mixed-sex groups. Females did not immediately copulate with males that had mounted them. If copulation did occur, males tended to remain on the back of females for an extended period of time. Males that mounted other males in mixed-sex groups tended to mate subsequently with a female and then stay with her.
Conclusions: We propose that the minimal physical difference between the sexes, in combination with benefits to the males of rapidly attempting to pair with any available female, explains the tendency for males to mount other males. Extended mounts may occur because larger individuals are more likely to be female and because of selection on males to persist in a copulation att empt when females do not immediately copulate with a male.

Key words: Mate choice; Japanese beetle; Sexual discrimination; Homosexual behavior; Insect; Body size.

*Correspondence: E-mail: pvswitzer@eiu.edu