Longer exaggerated male genitalia confer defensive sperm-competitive benefits in an earwig |
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Authors: | Email author" target="_blank">Emile?van?LieshoutEmail author Mark?A?Elgar |
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Institution: | (1) Department of Zoology, University of Melbourne, Parkville Victoria, 3010, Australia |
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Abstract: | As evidence mounts that male genitalia can affect relative fertilisation success, the role that sexual selection has played
in the rapid and divergent evolution of genitalia is becoming increasingly recognized. Unfortunately, the limited functional
understanding of these complex structures and their interactions with the female reproductive tract often limit interpretation
regarding their evolution. Here, we address this issue using the earwig Euborellia brunneri, where both the male intromittent organ and the female spermatheca are highly exaggerated in length yet structurally simple.
In a double mating design, we use the sterile male technique to study how sperm precedence patterns are affected by male genital
length, male age, and the size of the male sperm storage organ, the seminal vesicle. Relative fertilisation success exhibited
considerable variation around modest last-male paternity. Only an interaction between first and second male genital length
affected paternity, where males gained reduced paternity when preceded by rivals with longer genitalia. Longer genitalia confer
defensive benefits in sperm competition by apparently depositing ejaculate deeper in the tubular spermatheca, safe from removal
by rivals. Paternity similarly depended on an interaction between the ages of both males, likely mediated by sperm traits
as testes size decreased with age. Seminal vesicle size showed positive allometry but did not affect paternity; instead, greater
seminal vesicle size in last males expedited oviposition. The exaggerated yet relatively simple genitalia of E. brunneri facilitate an unusually clear example of post-copulatory selection on phenotypic variation in multiple reproductive traits. |
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