Size really does matter!  Well endowed male earwigs have their cake and eat it too…

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Many animal species employ a polyandrous sexual system, where one female mates with many males and stores sperm in a specialized storage organ.  Since fertilization doesn’t take place immediately (in some cases females can store viable sperm for several weeks), males actively compete to contribute to her sperm bank, regardless of whether it requires another deposit.  So what does it take to be reproductively successful as a male in a polyandrous sexual system?  There is a vast diversity of answers to this question, although one can discuss a few major themes.  First, be big.  Females generally prefer dominant males, who are generally associated with dominant genes (representing a large degree of biological fitness).  Second, be forceful. Males of many species have evolved complex structures for grasping, holding and subduing females in order to successfully transfer his seed. Third, be quick.  Some males have evolved strategies to be reproductively successful in a competitive environment by getting the deed done before any competing males realize he’s done it.  In addition, females that are busy mating have less time for other activities like feeding, finding shelter, and watching for predators.

So what is the best strategy if you are an Australian earwig (Euborellia brunneri)?  It turns out that none of the aforementioned strategies is involved.  Successful male earwigs are neither the biggest nor the most forceful.  They are also the ones who take the longest to achieve successful sperm transfer.  What is going on here?

It turns out that there is great disparity between males of this species with respect to the size of their genitalia.  Some males have extremely exaggerated sperm-transfer appendages (longer than their entire bodies), and some do not.  It’s important to clarify that just because a male has a longer penis, he isn’t necessarily bigger overall.  Genital size may be simply a heritable trait (hence the ‘sexy son’ hypothesis), if you’re dad had a great big schlong then it’s likely that you will too.  In a series of experiments designed to test the preferences of females for well endowed vs not-so-well-endowed males, researchers found that females overwhelmingly chose to mate with the former.

Evolutionarily speaking, this situation represents an interesting conundrum.  Here’s why:

1. As I mentioned above, males with longer appendages (virgae) are NOT necessarily bigger than males with shorter ones, so traditional arguments about females preferring dominant (larger) males do not apply here.
2. Males do not mate forcefully with females through coersion, so she has an active choice in her sexual partnerships (regardless of the endowment of her partner).  She is free to walk away (and end copulation) at any time.
3. Males with longer appendages deposit their sperm more deeply into a females’ reproductive tract, which means it takes them longer to reproduce than a male with a shorter member.

So why do females prefer the males with the largest genitalia?  The authors suggest that the elongation is an antagonistic adaptation that increases male control through sensory exploitation.  And I quote:

“During copulation, the genitalia of certain males may elicit more favorable female responses through superior mechanical or stimulatory interaction with the female reproductive tract.”

Do you really need a translation here?  Probably, since it’s kind of hard to believe:  male earwigs with giant members are titilating their partners into submission while they take their sweet time depositing their sperm.  Kind of like earwig porn really…Ok perhaps that’s an exaggeration, but the authors do reiterate that the male genital phenotypes that are most successful in sperm competition (those that can reach further into the females’ reproductive tract) are also mediated by mating benefits from the female.  Females allow well-endowed males to occupy a greater amount of time during mating, perhaps due to some form of sensory exploitation from his giant, stimulating sexual organ.

Just when you think you can generalize about sexual strategies in the animal kingdom, one comes along that seems to fly in the face of clearly defined categories.

Awesome.

Lieshout, E. (2010). Male genital length and mating status differentially affect mating behaviour in an earwig Behavioral Ecology and Sociobiology DOI: 10.1007/s00265-010-1021-1