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Friday, March 4, 2011
Earwig genitals: Size really does matter!
People spend a lot of time talking about the size of male genitals. One has only to type "jokes about penis size" in Google to discover that there are no fewer than 387,000 pages about the topic!
"So this sailor walks into a bar with an earwig perched on his shoulder..."
Wait a minute. This blog is about animal behavior. Let me focus on the earwig!
Copulation presents any number of challenges for earwig males. They need to copulate often enough - and long enough - to ensure that sperm are transferred successfully. If a female has previously mated, it may take even more (or lengthier) copulation to remove or overwhelm sperm left by the previous suitor.
As I have argued in previous posts, extra sex may not be in the best interests of a female. She may only need to mate once to have all of her eggs fertilized. This sets the stage for behavioral and evolutionary antagonism between the sexes: what is good for the goose is not good for the gander and vice versa. That is a common theme in this blog!
Enter male genitals. In some animals, males have evolved penis shapes that make it difficult for females to disengage. For example, male genitals may swell in size or protrude anchors while inserted, making it difficult for the female to end copulation. Longer male genitals may be more effective in removing the sperm of rivals from the female reproductive tract. Hence male genital shape evolves in response to both females and other males.
But this is not the case in earwigs. Females can end copulation any time just by walking away.
Emile van Lieshout of the University of Melbourne published a study in the February issue of Behavioral Ecology and Sociobiology about genital size and reproductive success in the earwig Euborellia brunneri. Males have unusually long ejaculatory ducts (by "unusual" I mean longer than the entire body) with a structure at the tip for displacing rival sperm. Males with the longest ducts therefore have the greatest advantage in sperm competition because they can place sperm beyond the reach of less-endowed rivals.
But wait: females of the species each have an extremely long spermatheca ("sperm receptacle") that matches and complements male genitals. Females clearly have not been neutral bystanders in the evolution of genitals in this species!
So who is winning the battle of the sexes? The study addressed this question by examining how often males and females copulate -- and for how long. If males are winning, they should copulate longer with females that have already mated with other males (sperm competition). If females are in control, however, copulation time should be shorter after they have already mated and obtained sperm.
Copulation diminished after mating experience in both sexes: virgin males and virgin females copulated more often than males and females that had already mated. However, the duration of each copulation did not differ with mating experience in either sex. The battle of the sexes seems to have ended in an evolutionary standoff.
But things are not always what they seem. Duration of copulation varied among males with different sized genitals! Males with lengthier genitals copulated significantly longer (but not more often) than less-endowed males.
There seems little doubt that in earwigs, at least, females prefer males with bigger genitals. But the female preference is "cryptic" because well-endowed males do not win more copulations than those with smaller genitals. They may, however, fertilize more eggs because they have more time (and reach) to remove the sperm of competitors and replace it with their own.