Genital Morphology and Social System

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In primates, it has been noticed that if you live in a competitive mating system and you’re a male, you’ll have a very fancy penis.  Maybe some spines, or a few ridges here and there. These embellishments are keratinous structures and act to promote rapid ejaculation, which is useful if you’ve got nine other males lining up behind you, impatiently waiting for their turn.

In fact, the argument was made in the Ardi papers that humans have rather plain penises (except for their bigness).  They have no fancy spines.  Not even an os baculum.  In fact, the only remarkable thing about the human penis is that it’s so darn big!  But humans are another story for another blog post.  This one is about Mole Rats from Africa.

Mole rats are the only mammal that can be said to have a true eusocial system- that is, there is a queen and a few reproductive males, and all of the rest of the animals are non-reproductive “subordinates.”  In the female, the vagina remains imperforate until she becomes queen, so it’s impossible for one of these subordinate gals to have a family of her own.  Instead, she takes care of her sister’s babies by going out and finding them food, or defending the colony, or whatever else it is that baby naked mole rats need to be healthy.

Geintal Morphology in male and female mole rats.  Males on the right, females on the left.  From top to bottom: Naked mole rats, Damaraland mole rats, and Silvery mole rats.

Geintal Morphology in male and female mole rats. Males on the left, females on the right. From top to bottom: Naked mole rats, Damaraland mole rats, and Silvery mole rats.

In the naked mole rats (and in their cousins, the Damaraland mole rats), most individuals never become reproductive.  Wouldn’t it be a waste, then, to invest in maintaining elaborate genital morphology if you’re never going to use it?

As it turns out, yes.  If you’re a male naked mole rat, you barely even have a penis at all!  Your penis is merely a “genital mound” and can barely be distinguished from the genital mounds of the females. What’s more is that on the inside, you lack the typical mammalian “penile bulb,” which is a ventral expansion of the vascularized corpus spongiosum tissue that is typical for the shaft of the penis.

If you’re a male Damarland mole rat, who is eusocial, but slightly less so than the naked mole rats (there are smaller groups, and new colonies are started more frequently), things are a little better.  Your genital mound is a little longer than those in females.  You have a penile bulb, but it’s tiny.

So there are two mole rats who are eusocial.  The Silvery mole rat is a solitary animal.  If you’re a male and you’re wandering around the African savannah and you happen to bump into a female, you take the chance to mate with her.  There’s plenty of sexual dimorphism.  Your genital mound could now be rightly called a “phallus.”  Your penile bulb is three times as large as those in your Damaraland cousins.  And the muscles that attach all of those things to you are much bigger in volume.

Even though there are only three species represented here, and only two or two and a half social systems represented, these mole rats are an interesting illustration of the correlation between genital morphology and social/mating system in mammals.

Seney, M., Kelly, D., Goldman, B., Šumbera, R., & Forger, N. (2009). Social Structure Predicts Genital Morphology in African Mole-Rats PLoS ONE, 4 (10) DOI: 10.1371/journal.pone.0007477

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9 responses to “Genital Morphology and Social System

  1. Doctor Science October 16, 2009 at 3:30 pm

    In fact, the only remarkable thing about the human penis is that it’s so darn big!

    Don’t get an inflated sense of your own … inflation …, guys. Relative to body size, the bonobo’s penis is larger: if a typical male bonobo was human-sized, he’d have a penis 8 inches long — which is, despite what women are told, *not* the case for the average human male.

  2. Luna_the_cat October 16, 2009 at 6:00 pm

    You have your label of the picture above gender-reversed; it is actually males on the left, females on the right.

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  5. Christopher Ryan November 11, 2009 at 5:25 pm

    You might be interested in Errol Morris’ film “Fast, Cheap, and Out of Control” which has a section on the African mole rat. Interesting film, as with all Morris’ work.

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