Tomorrow, Friday August 22, is the deadline for submitting your bad ad hoc (bah) hypotheses for consideration to be presented at BahFest 2014.
I've been chewing a bit on an idea about mandrill coloration and it's just not at all ready, so I'm not going to submit it despite my desperate desire to participate. And I'll just hope that by next year's submission deadline I'll have a much better idea, one that I've had time to actually and properly research and to build strong with data, one that's funnier, one that doesn't require me to read Freud, and one that's wackier since plausibility is not permitted.
But since I've got the outline for this mandrill idea, of course I want to share what I've got here. Who knows, maybe it will spark someone to submit their own bahfest hypothesis or to think about why mandrills are colored like they are or how we could ever know.
So here it is... my bad, bad ad hoc hypothesis... an infantile hypothesis for mandrill coloration... which 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. (Maybe don't watch those amazing ones until after you've suffered through mine?)
An infantile hypothesis for
Mandrillus sphinx coloration
Holly M. Dunsworth, B.A, M.A., Ph.D.
“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
|Male mandrill at Singapore Zoo (Robert Young, Wikipedia)|
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?
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 that you can now enjoy all to yourself.
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.
Or we could go with the Cyrano de Bergerac Principle: His nose looks so much like his junk. He’s so cute! Again, if males are choosy, this goes both ways.
It's highly possible that having one rear-end and another end that looks like a rear-end causes confusion, on the part of the male, during copulation, that can accidentally lead to some innovative, pleasurable positions that strengthen social bonds (@ElroyBeefstu, personal communication).
Relatedly, having a face like this could be a nice way for females to test male intelligence and choose intercourse partners accordingly: If he can't distinguish which end is the business end, then no way am I making this transaction.
What about lowering standards? Wouldn't a decrease in mating restrictions increase reproductive success for those who've had less of it? This might entail something along the lines of, wow I totally expected his face which looks like a butt to smell like a butt, but it doesn't! I guess I'll make a baby with him. Unfortunately, to my knowledge, no one has published on the relevant odor preferences of mandrills.
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 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.
The Perinatal Imprinting Hypothesis
When it comes to infants, selection pressures are on hyperdrive, 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 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.
|Simulation of mandrill birth.|
|Female of breeding age. Look closely and you'll see the same color pattern of the male rump is there, just muted. (captured from Arkive film)|
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 abysmal end of their source of food and social development, and these, of course, are their requirements for life.
The colorful bum, alone, is just too distracting. So, mothers with color faces to match their butts have more surviving offspring, that go on to have surviving offspring, than others. They can even get away with those boring whitish nipples because their faces are so pretty.
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 these individuals born literally to color 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.
|Young male grooming adult male. (source)|
So that first splash of color that neonatal mandrills experienced was 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 hairs of all the mandrills to match the rest of their olive-colored bodies. All future mandrill babies will be born to a dull mother's 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 hairs 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 hairs of pregnant drills (the boring-looking cousins of mandrills) to match female mandrills' and see if (a) drill neonates spend too much time hanging around mom’s distractingly colorful butt, (b) there are negative consequences of this, and (c) mother drill's faces evolve coloration in future generations and, also, coloration evolves in drill males too. Easy.
|Drill. Mandrillus leucophaeus (source)|
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. Everyone, especially Darwin, was thinking about this all wrong!
***Note: 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.