The Evolution of Buttfaces Explained

While I was very very pregnant about two years ago, I posted something on this blog and then took it down the same day.

It was a labor of love and admiration for BAHfest. I didn't believe it was worthy of submission to the judges, but I thought it was worth sharing here.

But then, even given all the silliness I've posted here over seven years of mermaid-hood, seeing my b.a.h. in print was too much for me to bear and bare. Hence the embarrassment and why I took it down.

Butt now, I have good reason to post it again and for good!

It's all thanks to this news about a recent primatological study:

"Chimpanzees recognize rear ends like people recognize faces"

Here's the rub:

Because rear ends serve a big purpose in the chimp world. Female chimps’ buttocks grow redder and swollen when they are ovulating, signaling to males that it’s business time. And it’s important to know whose bottom it is, in part to prevent inbreeding. The buttocks have, in scientific parlance, a “high socio-sexual signaling function.” 

But when we began walking upright, our bottoms became fleshier and no longer broadcast our ovulation status, possibly to discourage casual hookups in favor of pairing up and sticking together for the children’s sake. On the other hand, humans — “especially females,” the researchers write — developed ruddier and thicker lips, as well as fattier faces.

So not only are chimpanzees better at recognizing butts and worse at recognizing faces than we are, which is interesting in its own right. But this suggests that our faces function like our ancestors' butts! 

Bummer? Yes and no.  On the one hand, this makes my "bad ad hoc hypothesis," re-posted below, worthy of sharing without any more embarrassment. Butt on the other hand, it means it's no longer bad enough to make BAHfest. So, instead of working on this one some more, I need to come up with an entirely new one from scratch if I'm going to have a shot at ever participating.

Butt before I go back to the drawing board (with my hot glue gun, see below), here's that old post. Like that recent news story, it's about butts driving the evolution of primate faces. In this case we're focusing on rainbow-colored monkey butts, but the theories may be liberally applied to this idea that human faces are functionally ancestral hominin butts. OK! Enjoy?

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 “No other member in the whole class of mammals is coloured in so extraordinary a manner as the adult male mandrill.”  ~  Charles Darwin, The Descent of Man, 1874

Darwin was famously astonished by the extraordinary coloration of the mandrill monkey, Mandrillus sphinx. Because males are more striking than females, evolutionary explanations have focused on the adult male. And, as the thinking goes, it's the adult male face that's been the primary focus of selection, with duller female faces and the colorful rumps of both sexes being secondary, in evolutionary terms. 

One explanation for the colorful male mandrill face is sexual selection. Males with healthy, robust physiologies capable of building and maintaining that rainbow visage are the sexiest. And because coloration isn't as pronounced in females, that's an indication that it's less crucial for their reproductive success. But their ability to choose male mates based on good looks is, and the particular genetic mechanism which beautifies the male carries some of that beauty along in females. So that sufficiently, albeit vaguely, explains the mandrill face.

But for many of us an even more urgent question is, Why did the mandrill rump evolve to resemble the face? 

And there are a few possible answers.

There's the more-is-better explanation: those with colorful faces are seen, socially and sexually, as all right, but those with colorful faces and butts are all that. They're the real peacocks of the troop. 

There's also a potential social benefit to being visible and, better yet, identifiable, both coming and going in the dark dense forests where mandrills live. 

Then there's a strength-in-numbers sort of idea, where other groups or predators, even, will see twice as many of you. 

Alternatively, the development of rump color could be genetically linked to face color, so it could simply be an accidental byproduct of selection on the face. 

But what if we flip our view around and assume that the monkeys' rainbow hinies are the primary focus of selection? After all, we find colorful bums and privates across the primates, and in both males and females, and in species without much to match on the face. (Yet.)  This alternative perspective could free us to arrive at the real explanation for mandrill coloration. 

And this means we should ask, Why did the mandrill face evolve to resemble the rump?

Dear Reader, I'm sure you can think up all sorts of advantages to having a face that looks like a butt. 

For instance, by appearing to groom your ass, rather than eat food, you might not attract competitors to your precious food source.

And there's always the Handicap Principle:  He’s got a face like a butt, but he’s still got it going on. And if males are choosy (it's possible!) it could go the other way too.  

It's possible that having basically two rear-ends causes confusion, on the part of the male, during copulation, that can accidentally lead to some innovative, pleasurable positions that strengthen social bonds.

Relatedly, having a face like a bum could be a nice way for females to test male intelligence and choose procreative partners accordingly: If he can't distinguish which end is the business end, then no way am I making this transaction. 

There's great possibility that this coloration is a sort of menage-a-trompe-de-l'oeil. Females are more attractive if they're not one but two! And to any onlookers, this threesome is quite impressive. 

It could be as simple as mandrills getting along better with mandrills with faces that look like butts because that's just, pure and simple, the very best part of a mandrill, to a mandrill. This applies beyond the sexual and into the general social realm.

One, some, or all of the above hypotheses, and many others that I'm sure you've already thought of, could easily explain mandrill face coloration. But I now offer what I think is the best rump-first-then-face explanation of them all. 

When it comes to infants, selection pressures are on hyper-drive, so our adaptive hypotheses about babies are essentially iron-clad. Nature’s got to get infancy right for evolution to continue and nature’s got a genius way to get it right in mandrills and it’s why mandrills are colored the way they are.  

Mandrill face coloration is an adaptation to infant perception.

As mandrill neonates slowly emerge from their mothers’ bodies during parturition, they are gobsmacked by the electric coloration of her rump.

Photo of mandrill birth was unavailable.

Sure, female mandrill rump coloration is not as striking as males', but imagine if it's the first real color you ever saw... ever. So, from a neonate's perspective, this welcome to the world is probably as striking as the healthiest mandrill males' tookus is to other mandrill adults, and to us.

Look closely and you'll see the same color pattern of the male rump is there, just muted.
(captured from Arkive film)

Think about how much we as primates love colors. If you saw that booty upon your earthly arrival, you'd be enchanted. You'd want to keep looking at it, wouldn't you? 

And if it weren’t for the mother’s colorful face proximal to her teats, mandrill infants would be dangerously inclined to literally hang around at the gorgeous yet abysmal end of their only source of food and social development. Food and social boding are, of course, requirements for primate life. 

The colorful bum, alone, is just too distracting. So, mothers with colorful faces to match their butts have more success nursing their infants, and thus have more surviving offspring, that go on to have surviving offspring, than others. They can even get away with those plain whitish nipples because their faces are so enticing.

(source for pic on left)

So that explains mandrill female faces but what about the male rumps and faces? Especially since they’re even more colorful?

This crucial and intense early experience, which selects for colorful mother’s faces, affects mandrill phenotypic preference throughout their lives. 

All social and sexual realms are better with color because of the experiences of these individuals born  to colorful bums and raised by moms with colorful faces.  Colorful males are adaptive in this situation because youngsters fall in love with how they look too, ingratiating themselves with what could be a killing machine, softening his heart and preventing him from ending lineages of mothers with colorful faces who birth babies through their colorful places. 

And this could explain, in turn, why male faces look so much like male genitalia but also why male faces look so much like female genitalia, especially at their peak attractiveness.  (See photo of fertile female's rump, above.) Males with these features are attractive to other males, which promotes group cohesion and reduces tension and competition. Likewise males with these features are attractive to females because it makes them more like their mothers and sisters, that is, not just beautiful but less threatening. 

So that first splash of color that neonatal mandrills experienced is such a technicolor Oz, that they grow up preferring not just color but the most electric adults out there… Runaway selection at its finest! 

To test whether the rump or the face is the driving phenotype…

Dye the butt fur of all the mandrills to match the rest of their olive-colored bodies. All future mandrill babies will be born to a mother's dull rump. And then if selection is relaxed on the face coloration, as predicted by the rump-first approach, mutations should take over and remove the color from the face. Then next, stop dying the butt fur of the mandrills and selection should bring back the colorful face again. Unfortunately this will only answer the question as to which end, the face or the bum, is driving the appearance of the other. 

To test the Perinatal Imprinting hypothesis….

Dye the butt fur of pregnant drills (the rainbow-free cousins of mandrills) to match female mandrills' and see if (a) drill neonates spend too much time hanging around mom’s distractingly colorful butt and, thus, not enough time nursing and bonding with mother’s eyes and face, (b) mother drill's faces evolve coloration in future generations and, also, coloration evolves in drill males too. Easy.

Drill. Mandrillus leucophaeus (source)

But remember, one of the most compelling aspects of the Perinatal Imprinting hypothesis is that it cannot be proven wrong, even if other explanations are better supported. 

Concluding Remarks

Not only is adaptive coloration of the mandrill face secondary to the primary adaptive value of the coloration of the butt, but the adaptive coloration of the males is actually secondary to the primary adaptive value of the coloration in the females!  

Colorful female rumps, and the infants who love them, are responsible for the extraordinary coloration of mandrills, not competitions for sexiest male. Everyone, especially Darwin, was thinking about this all wrong!

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P.S.

I recently donated to Arkive because I heavily rely on it for teaching, writing, and learning. I hope that if you use it like I do, that you'll do the same so that it continues to thrive as a resource. 

My infantile hypothesis  follows in the tradition of the wonderfully infantile ones to be born at Bahfest exemplified by this one from organizer Zach Weinersmith and also last year's winning hypothesis from Tomer Ullman. (2016 note: Dates are off because this note was written in 2014)