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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
Abstract
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.
Results: 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
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