[chris (OP)]

On the radio in South Africa the other day a caller phoned in to ask why, having glanced surreptitiously around at the other occupants of the male changing room at his swimming pool, he'd noticed that bums (for the benefit of the Americans, that's arses as opposed to homeless people) appear to have darker skin than the rest of the body. He wondered why.

I wasn't sure whether it was fact or fiction, so we elected to "ask the audience", and the presenter declared "the bum poll open". I'd like to report back later this week, if we have some perspectives.

So what do people here think? And input from any females, to get their views too so to speak, would be nice.

Also, please feel free to post (decent) photos to illustrate your answer. (This is the biological equivalent of "show your working").

Chris

[RD]

Maybe acanthosis nigricans in buttock cleft ?.

[chris (OP)]

Well the caller was implying that all the bums he stealthily surveyed were definitely darker than the skin elsewhere on the body...

[RD]

Not true of white Australians ...
http://www.fiveaa.com.au/photo_didn-t-we-have-the-full-moon-already-this-month_115079

[neilep]

I can't really post an image to support the hypothesis but I can post a piccy of a phone sticking out of my arse !


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Glad I could help !

[Geezer]

What happened to that sheep tattoo you used to have?

[neilep]

What happened to that sheep tattoo you used to have?

On the other side....cheeky !!  []

[chris (OP)]

From the link above. Highlights the situation at Mardi Gras, Sydney. No darker bums there...

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However, I wonder whether the situation presented here is slightly confounded by the fact that many of the individuals pictured (being Aussies and hence having access to this thing called sunshine, a foreign concept to us here in Britain) have suntans. Maybe, therefore, their arses look falsely paler than their more exposed body parts, which have been made artificially darker by sun exposure...?

Just throwing that out there...

[Variola]

I can't really post an image to support the hypothesis but I can post a piccy of a phone sticking out of my arse !

Oh Neil.....

There isn't that much to tell your bum from your head, apart from that your bum has more hair on it...  []

[chris (OP)]

Thank you, everyone, for your critical ANALysis of this important research question.

Having subjected this fARSEical issue to further careful thought and discussion involving other leading thinkers in my local pub - this is called BEER review - we conclude that the enhanced darkness of the bottom is secondary to the presence of bumfluff - hair. That is, the presence of dark hair on a bottom makes the skin look artifically darker. The effect is therefore an optical - or should that be BOTical - illusion.

chris

[Jane Johannesburg]

Chris, there's a better explanation than hairy bottoms.  The research sample so far (judging by the pictures) consists largely of pale people.  Yes go on, blame the South African for bringing race into the issue - but it's important here because I think your sample is biased towards white males with hairy backsides.  Go and look at some black bottoms.  They are indeed darker than the rest of the body, especially where the bottom meets the top of the leg, and generally there isn't much hair there.  Then, get a black person, who is standing, to lift one leg as high as he or she can, and what happens?  The darker skin on that bum cheek disappears.  The reason is simple - skin with some natural pigment looks darker when it is bunched up.  Stretch it, and it is lighter.  So, when someone black stretches their bum skin to the full extent the skin is no longer bunched up so looks lighter. The lift-one-leg method is the best demonstration because you can compare the one bum cheek to the other (for as long as the person can balance).  How do I know this?  It was demonstrated to me by a friend, a black South African scientist (incidentally, whose work, published in Nature, was reported by your Naked Scientist team) - now that's really naked science, and you have to tell Redi this next week!   

[chris (OP)]

Dear Jane

BRILLIANT; thank you so much for the superb suggestion - but do please reassure me that your Nature-publishing scientist friend didn't get his or her bottom out to demonstrate this to you?!

Meanwhile who is the said scientist whose work we covered?

Thank you, once again, for some super input.

Chris

[Jane Johannesburg]

Er, Chris, now you may be used to being a Naked Scientist.  My source is not.  Given that I have already divulged that my source got his (yes, his - not her) kit off to demonstrate, I am not about to divulge his identity so publicly - even in the name of science.  But, his explanation, like any other good science, is independently verifiable.  Should you have any doubts, best YOU persuade someone to do what he did.  I am confident that his explanation will stand up to your rigorous testing.... 

[chris (OP)]

Fair enough. I'd still like to know what the research topic was we covered though...

Chris

[Don_1]

You can doubtless imagine, that upon seeing this question, I became much interested in matters concerning the old gluteus maximus.

I have decided to research this phenomenon for the purpose of scientific furtherment and have equipped myself with a suitable pair of spectacles for the purpose.



I am now off to watch the synchronised swimming practice session at our local pool and will report back ASAP.

Now, where's my dirty old mac?

[Jane Johannesburg]

Chris, you covered some of his work in the area of evolutionary genetics.  Cheers!  J

[Don_1]

Hello folks, here I am, back from my first sorté in my quest to establish bum darkness.

Not wishing to waste any time, I fixed a mirror to my right shoe to enable me to look up skirts without the need to trouble the wearer with the details of my research. It was rather dark up those skirts, but I did manage to establish that bums have not only a shade difference, but also a colour difference to other parts of the skin.

For example, a Caucasian girl had a black bum, which was quite shiny. I assume she must clean her bum with Cherry Blossom. By contrast, an Afro-Caribbean girl had a white bum. Other girls had blue bum’s (presumably they were cold) some had red bum’s and some even had polka dotted bum’s.

Alas, these observations were cut short, when a girl turned around and trod on my mirror, I assume by accident, and in doing so, slipped and swinging her arms to regain her balance, knocked my glasses off. I may have quite a shiner later today as a result of this accident.

I arrived at the swimming pool and immediately began my observations. Unfortunately, where my glasses had been knocked to the ground earlier, they had become rather dirty. Taking out my handkerchief, I began to breathe heavily on the lenses to clean them. This sound resonated around the pool and I was asked to leave. Perhaps I had put off the young ladies in training for their latest synchronised swimming competition.

Undeterred, I went to the local school playing field, where the 6th form girls were playing netball and recommenced my study.

I am due in court in the morning.