Tag Archives: Penis

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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The naked mole rat: More than an ugly face

If you live in the UK, have you been watching the latest BBC Attenborough offering, Africa? If you answered “no”, WHY THE HELL NOT? and if you answered yes, OMIGODHOWGOODISIT??

“Sahara”, the episode that aired last night (30/1/13) has probably my personal highlight so far, not least because it featured one of my favourite funny looking creatures in the world ever: The naked mole rat.

Despite it’s appearance, this creature is not actually an old wrinkly penis with buck teeth growing from it’s nostrils.The naked mole rat is so ugly it is almost adorable. But not only does it look so goddamn horrendous that it becomes some sort of mythical being that you can’t quite drag your screaming yet sympathetic eyeballs away from, it is also a very interesting little creature, for several really cool reasons. Let’s start with its physical appearance, since we’re all judging it already. DON’T PRETEND YOU’RE NOT.

It’s hard to miss the fact that naked mole rats are, well, naked. But if you look closely you will see that they are not completely hairless, oh no. Natural selection has given them the mole-rat equivalent of a fig leaf and provided them with some lovely long super-sensitive whiskers which help them feel around, which is handy when you live in narrow tunnels under the ground (they can also run backwards just as effectively as forwards, which is also handy for the environment they live in). Because they are perfectly adapted to living in underground burrows, the naked mole rats have practically lost the need for their eyes, which are present but are located underneath their skin and are essentially useless. And who can ignore those gnashers? Like all rodents, the incisor teeth of the naked mole rat never stop growing. They protrude in front of the lips (which are sealed behind the teeth) in order to stop the critters swallowing soil when they are using their wallies to dig through the soil – all of which I’m sure you will agree just adds to their sex appeal. 

Although they are mammals, naked mole rats do not regulate their body temperature the same way most mammals (including ourselves) do, through a process called thermoregulation. Sweating, shivering and panting are examples of thermoregulation, whereby an organism keeps it’s core temperature at a stable level, regardless of the temperature surrounding it. Rather, naked mole rats are cold-blooded thermoconformers, meaning that their core body temperature changes depending on how hot or cold their environment is. But because they live underground in the African desert, the habitat of the naked mole rat has a more-or-less constant temperature which is comfortable for them.

Perhaps the most peculiar thing about naked mole rats concerns their social structure, as they are one of the very few mammals which are described as “eusocial”: Eusociality is a hierarchical social structure more commonly found in social insects like ants and bees, wherein each colony (for a naked mole rat this is around 80 individuals) has a reproductive “queen” who produces all the offspring in the group, and all the other individuals are “workers” who are sterile. When the queen rat dies, some of the other females in the colony begin to develop eggs within a week of her death. Older females in the colony develop eggs first, and often fight to the death for the right to become the breeding female.

Hopefully I have convinced you that naked mole rats are more than just a pretty face. But the interesting facts do not end there, oh no. It turns out that these creatures may hold a very important secret which could lead to huge breakthroughs in cancer research: Mole rats are thought to be the only mammals who never develop cancer, and scientists have possibly found out why. Cancer is caused by unregulated cell growth, and the cells of naked mole rats poison and kill themselves when they multiply too much, thus cutting out cancer.

So there you have it. Cold-blooded, eusocial, cancer-resistant, bald mammals. Naked mole rats are awesome.



BBC Nature:

Wired: Why Blind Mole Rats Don’t Get Cancer:



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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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