Tag Archives: Genitals

Happy Valentine’s day! Testicle size and mating systems

I really despise Valentine’s day because I do not appreciate society telling me when I should feel romantic and loved-up and when I should not. HOWEVER as a bit of fun, I thought I would write a post about love, romance and sex but most importantly SCIENCE!

And testes*.

Testicles are funny things, and almost all healthy male vertebrates boast two of them. In many mammals, including ourselves, they hang from the body in a scrotum because the valuable sperm they contain are mighty fussy, and mammalian body temperature tends to be just a little too hot for them. So natural selection kindly began dangling the sperm of males from two sacs between the rear legs, nice work evolution. But it turns out that we can tell a lot about a species’ mating system (i.e. how monogamous or promiscuous they are) just from looking at the size of the male testis. The diagram below is both hilarious and helpful:

Here you can see the gonad size of various primate species in relation to body size. The top row are the males – the big circle represents their body size, the arrow is the penis and the balls are, well, the balls. (No laughing at the gorilla and yes guys, your penis is quite large compared to our primate cousins). The bottom row shows the female sexual organs of the same species – I can’t help but giggle at the human female. MASSIVE BOOBIES!

So the size of the testes can tell us a lot of information about the mating system adopted by various species. We have to remember that although in our culture monogamy is often (rightly or wrongly) seen as the norm, this is far from the case in most other mammalian species. There are lots of different mating systems: monogamy (one male one female), polygyny (one male, several females), polyandry (one female, several males) and promiscuity (basically a free-for-all orgy where it is completely acceptable for anyone to have sex with anyone).

Males of species with promiscuous mating systems (such as chimpanzees) tend to have the largest testes, and this makes sense because of something called sperm competition. In a promiscuous mating system, lots of males are having sex with lots of females, and everyone wants a good shot at fathering the most offspring, because this means passing on your genetic material and is a big fat evolutionary WIN. So for this reason it is advantageous to have a lot of sperm, and big old testes to store the little guys in.

However, the males of species with polygynous mating systems (e.g. gorillas)  tend to have smaller testes, because a single male has almost guaranteed access to at least a couple of females. So there is no need to waste extra energy on producing lots and lots of sperm in giant testicles, because his chances of impregnating a female is pretty high and he has no competition to wane off.

Now I know what you’re thinking. Where do we fit in all of this? Well guys, I have to say, your testicles are classified as “moderate”. Bigger than a gorilla, but shy of a chimp. This actually fits well into the mating system hypothesis – although humans are often socially monogamous, they do participate in moderate levels of non-monogamy (SHOCK HORROR KLAXON)!!

So there you have it. The bigger the balls, the more promiscuous the sexy-times. But smaller testicles aren’t for losers – it just means they don’t have to try so hard! Quality over quantity perhaps? Maybe that just means more time and energy can be spend on post-coital cuddles or, you know, child-rearing.

So whether you have testes or ovaries, and regardless of their size, I wish you all a very happy and sexy V-day.


A bonobo with large testicles chillin’ out.  Source

*I will leave it up to you to guess what type of mating system this squirrel may participate in.

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1 shaft, 4 heads: The echidna’s penis.

The echidna is an odd animal for many reasons. It looks like a cross between a hedgehog, a platypus and an anteater (and indeed it is commonly known as the “spiny anteater”). Additionally, the females of this mamalian species lay eggs, which is really rare – in fact, the only other mammal known to do this is the platypus (egg-laying mammals are called monotremes), so already we can see that these are special ickly cuties. But what is perhaps most bizarre about this little Australian waddler is its penis. Dare you look? GO ON SCROLL DOWN I DARES YE!

IT HAS FOUR HEADS! FOUR! I think it looks like the foot of a baby rhino or something. But be assured, it is in fact a penis. But WHY LAUREN?! I hear ye screech?! The short answer is that we don’t really know. What we do know that when a male echidna has sex with a female echidna, one side of the penis seems to shut down, leaving only two out of the four heads in use. The next time he mates, the heads on the other side will be used. In fact, if all four heads are active and grow in size, the penis would have great difficulty getting inside the reproductive tract of the female, which only has two canals.

I even found an academic paper on the echidna’s penis, published in one of America’s oldest academic journals nonetheless. Below is a direct quote from the article, describing how research on such matters is carried out (basically, it would seem, you train an animal to be okay with getting erections out of context, then you masturbate it until it ejaculates). YAY FOR SCIENCE!

In 2005, Currumbin Wildlife Sanctuary (Gold Coast,
Australia) came into possession of a 17-year-old captive
male echidna that had become habituated to human presence
as part of an interactive public display. Zookeepers
noted that, on handling, this animal would readily produce
an erection. Over a period of 2 weeks, zookeepers at Currumbin
Wildlife Sanctuary conditioned the echidna to develop
an erection to the point where it would ejaculate.
In preparation for semen collection, the echidna is placed
in lateral recumbency on a clean surface of the floor of
its enclosure. Using a closed fist, the zookeeper gently
pushes his hand into the lower abdomen of the animal,
at which time the echidna usually responds by pushing its
cloaca up against the zookeeper’s fist and developing an

What’s that you say, the imagery just isn’t strong enough? Well here is A VIDEO of the process! (You’re freakin’ welcome)

The scientists actually did some pretty cool research with the semen they collected. By examining the number of sperm in the ejaculate, they found that up to 100 individual sperm would bundle together, and this caused them to move very quickly in a very organised fashion- causing them to be much more effective working as a team than alone. What’ going on here? The most likely answer is sperm competition. Fertile females are never left alone for very long: Males will quite literally queue up to mate with them. With all this competition to be the one to fertilise the egg, males whose sperm behave in this “bundling” way will be more likely to father the offspring and therefore the team-building sperm would be favoured by natural selection, causing it to become widespread in the population.



Johnston, Steve D. and Smith, Brett and Pyne, Michael and Stenzel, Deborah J. and Holt, William V. (2007) One-Sided Ejaculation of Echidna Sperm Bundles.  The American Naturalist 170(6):E162-E164.

BoingBoing:  More than you maybe needed to know about the echidna

New Scientist: Exhibitionist spiny anteater reveals bizarre penis

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The barnacle’s penis.

This is a barnacle.

You have probably seen these little rocky warts on the beach attached to rocks and stuff, like the one above. You’d be forgiven (by most people, not by me) for thinking that these are just ugly stoney growths but all 1220 species of barnacle really are Living Things: Crustaceans in fact, related to lobsters and crabs. Barnacles cannot move about on their own and instead opt for a life of attaching onto things – whether that be rocks, boats, seashells, etc – although they do have legs. Legs and they can’t walk?! Those are pretty useless legs I hear ye cry! But nae: the legs of barnacles are modified in such a way that they facilitate feeding. Remaining in their shell, they use their little feathery legs to reach out into the water and beat them rhythmically, in order to swoosh tasty plankton into their shell and eat them all up.

You often find barnacles grouped together in clusters (as in the photo below) because they cannot move and so like to have other barnacles to hang out with*.
*Not scientific fact.

So far we have learned that barnacles can’t move and have useless legs. WHY ARE YOU WRITING ABOUT THEM, LAUREN? Oh I will tell you why, curious reader.


Most barnacles are actually hermaphrodites, owning both male and female reproductive organs, but although it is possible for them to fertilise their own eggs, this appears to happen rarely: They much prefer to mate with other individuals rather than themselves whenever it is possible to do so. But as we have already discovered, barnacles do not move, so finding a mate can be extremely difficult. It’s not like they can take each other out to an underwater bar and impress each other with their awesome moves on the seafloor.

But let me tell you. If there is one thing a barnacle can move, it is it’s penis. VIDEO EVIDENCE:

Mating barnacles from Casey Dunn on Vimeo.

That is not all. Researchers have discovered that barnacles’ penises can actually change shape to suit their environment. Apparently, barnacles living in gentle waters have long, thin penises which are best for getting reaching far away, whereas those living in rough waters have short, firm penises that are more likely to withstand strong waves. This is a really cool finding (at least it is for those of us obsessed with the genitalia of every species) because it suggests that a creature’s environment can shape its genitals as much as female choice or male competition.

There you have it. The barnacle. And it’s massive shlong. No longer an ugly boat-wart, is it?

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