Tuesday, November 18, 2008

Mountain Biking, Moonlight, Color Vision & Tetrachromatic Women

OK, so this post covers a lot of ground and gets a bit technical, but it all connects and it’s got a great ending, so stick with it.

Watcher on JEMThe weekend was a blast. The point of the weekend was of course mountain biking, so let’s talk about that first. The pic of me here is on JEM trail, a fast desert singletrack on the bench below Gooseberry Mesa. It was taken right around Sunset Saturday; we completed the ride as a night ride, using lights, which-like pretty much all night-rides- was fantabulous.

During the day we rode on Little Creek and Gooseberry Mesas. If you’ve never ridden there, these videos will give you a quick flavor (all on YouTube, clink on the links):

Action Videos!

Video 1: Me climbing a ramp. The slickrock provides excellent traction, allowing you to ride up inclines you couldn’t clean on a dirt trail.

Video 2: Colin doing a nice wheelie-lift. Here’s a nice little video of ”Clean Colin” cleaning a tricky section on Gooseberry called, “Wheelie World”.

Video 3: Here’s Organic-Chemistry-Rick (guy who never reads my blog) cleaning a section called The Staircase.

Human Candy Cane The trip was a success, both mechanical and injury-free for all 4 of us, and I returned home without so much as even a sunburn. (Unlike my last trip back in April, where I forgot sunscreen and wound up looking like The Human Candy Cane- pic right. And yes, that is a Thomas the Tank Engine Towel I’m wearing, immediately following a sunshower. When you have small children, their past obsessions inevitably make their way into your camping gear-box.)

But although mountain biking is the primary goal of a trip to Hurricane, there’s so much to love about the place I’d have had a great time even if I’d never swung a leg over the bike. First is the botany, which I blogged about several times back in the spring. IMG_7359 Our rides took us through everything from Creosote in China Wash to the relic Ponderosas on Little Creek, and seeing so many of these trees and shrubs for the first time in more than 6 months felt a little like a reunion with old friends. Second was the landscape; the topography of this part of Utah is fantastic (pic left = Rainbow-Sprit Paul riding on Little Creek, with relic Ponderosas in near background and Zion NP in distant background), and something I hope to go on about in a future post. But what I’m going to talk about in this post is color.

Color in Moonlight

I have a favorite campsite on Little Creek Mountain It sits off a side road/track at about 5,600 feet on the rim of a side canyon, set in an open woodland of Singleleaf Pinon and Utah Juniper. Directly across from the campsite, to the West is the slickrock area with the Relic Ponderosas, and in the mornings I can sit up in my sleeping bag and watch the dawn sun light up the rock and scattered trees.

Tangent: I almost never camp in a tent. I always prefer to sleep under the stars. If rain is an issue, I use this. If bugs are the problem, I use this. And if wind is the problem, I know of windproof overhangs at several of the campsites I frequent.

About 3:00 in the morning Saturday morning I briefly woke and sat up to look around. It was the night after the full moon, and the moonlight was incredibly, almost annoyingly bright. People have different benchmarks for a “really bright” moon. Some will say it’s bright if you can hike or even ski without a flashlight. Some will say it’s bright if you can read by the moonlight. For me, a “really bright” moon means this: you can see color. And when I sat up in my bag, the first thing I noticed was the bag itself, bright red in the moonlight.

Camper Moonlight Color Most of us who know anything about how the eye works know that we usually can’t see colors at night because there are 2 types of light-receptive cells on the retinas of our eyes: cones and rods. Cones allow us to see in color, but require lots of light. Rods require less light, and allow us to see when it’s dim or dark out, but they don’t pick up color. Only a really bright moon will bring out a bright color, like red, at night.

rodcone When we compare our color vision to that of other animals, people tend to sound a little proud. Many of us have heard that dogs, for example, are “colorblind”, as are lots and lots of other animals, to greater or lesser degree. But comparing ourselves to other mammals is really giving ourselves a pretty big handicap, because here’s the stark truth: Compared to most other animals, and especially reptiles and birds, the color vision of mammals sucks.

Color Vision in Humans - Lame

Humans have 3 types of cone cells, commonly referred to as “Red”, “Green” and “Blue”, each of which is optimized to receive light of a specific wavelength. The strength of the relative signals coming from each type of cone cells is combined by the brain to produce the color images that we see.

Human Eye Cones Side note: This is a misnomer. The “Red” cones in our eyes are actually optimized to receive greenish-yellow light, and the other 2 types are optimized for higher frequencies. So it’s more accurate to designate the 3 types of cone cells as L (for Long wavelengths), M (for Medium wavelengths) and S (for Short wavelengths.) But the principle is the same, so I’ll stick with “Red”, “Green” and “Blue” for simplicity.

Most mammals have only 1 or 2 types of cone cells, and are therefore monochromats or dichromats, as opposed to humans and Old World primates, which are trichromats.

Color Vision in Birds - Rocks

But virtually all reptiles and birds are either tri- or tetrachromats, and some birds, such as the pigeon, and possibly the Black-Capped Chickadee, are even pentachromatic, with 5 different types of cone cells. In other words, reptiles and birds are seeing colors we can’t see, or maybe even imagine.

Bird Eye Cones But the color vision of birds doesn’t just beat ours by virtue of more cone cell types; the eyes of birds are engineered to a level completely beyond ours through the production and utilization of specialized oil droplets.

The retinas of birds have a tiny oil droplets sitting atop of many/most of the cone cells, and these oil droplets are colored, typically some shade of red, orange or yellow. The colored droplets effectively shift the frequency of light reaching the underlying cone cells, and in working in tandem with those cells, act to create effective cone receptors optimized for yet additional frequencies.

Bird Eye Cones Droplets So while a pigeon may have but 5 actual cone cell types, it may have say, 3 more cone-droplet pair-types, for a total of 8 effective types of receptors, each tuned for a different optimal frequency. In other words, many birds may well be seeing a world with 2 or 3 times as many colors as the world we see.

Tangent: Guess what the coloring “agents” are in the little oil droplets? Carotenoids. That’s right, the same type of pigment-chemicals we looked at when talking about how leaves changed color are at work in the eyeballs of birds. Scientists have identified at least 5 distinct carotenoids at work in the eyes of birds (though how many are in a specific bird, such as a pigeon, I don’t know.)

velociraptor So why is mammal vision so sucky? The prevailing theory is focused on our nocturnal origins. A couple of hundred million years ago, when mammals first started showing up, the world was dominated by reptiles (dinosaurs.) For more than a hundred million years, the mammals that lived and thrived were mainly small, shrew1rat or shrew-sized critters, and these critters- like rats and shrews today- made their living largely by staying out of the way of big predators, and that probably meant being nocturnal. Presumably we mammals lost our reptilian-ancestral color vision during this long dark night of our evolution.

After the extinction of the dinosaurs, mammals rapidly evolved into all sorts of biological niches formerly dominated by reptiles. And for some of these, diurnal, niches, color vision was an asset. fruit One example was Old World primates, who lived largely in trees, eating fruit. Finding fruit is much easier if you can distinguish colors. And so Old World primates re-evolved color vision, but using different genes and a different part of the brain for visual processing.

Tangent: You may have noticed that I keep saying “Old World Primates”. The evolution of color vision in New World Primates appears to be a separate, and utterly fantastic story, but unfortunately way out of the scope of this post. I recommend Richard Dawkins’ “The Ancestor’s Tale” for a wonderful telling of the tale.

The Part About Women

So wonderful as seeing in color is, we really see just a faint echo of the rainbows seen by birds. But I’ve saved the best part of the story for last, and that part is about women.

Pretty much everybody knows that not all people see the colors the same way because we all probably know someone who is “colorblind”. “Colorblind” is actually a really broad, vague and overused term. Some people really are “blind” to color- they don’t see any colors. But most people whom we call “colorblind” are dichromatic, lacking the ability to synthesize either the critical protein for “Red” cone cells, a condition called Pronatopia, or the critical protein for “Green” cone cells, a condition called Deuteranopia. And the overwhelming majority of these dichromatic humans are males.

I mentioned a moment ago that when Old World primates re-evolved color vision, they used different genes to do so. And 2 of those critical genes- that for the “Red”-cone-protein and that for the Green-cone-protein- lie on the “X” chromosome, one of the 2 mammalian sex-determining chromosomes. Males have 1 “X” and 1 “Y” chromosome; females have 2 “X”s and no “Y”s. If there’s a defect or mutation in the critical “Red” or “Green” cone-making protein in a man’s X chromosome, he’ll be a dichromat. But if that same defect is present on a woman’s “X” chromosome, her other “X” has a copy that will provide the correct instructions. Only if the cone-making gene on both of her X chromosomes is messed up- a very unlikely scenario- will the woman be “colorblind”, and that’s why colorblindness is so much rarer in women.

But here’s where things get weird. It turns out that there are 2 different proteins which can be synthesized to make “Red” cones work. Some “X” chromosomes carry a gene that makes the first protein, and some carry a gene that makes the second protein. And while both of these proteins work, it appears that they don’t work exactly the same, meaning that some colors may appear slightly different to the possessor of one protein/”Red” cone cell vs. the other. hzygous femaleAnd where it gets really weird is that some women carry both genes, 1 on each of their 2 “X” chromosomes, and that some portion of these women have 2 types of “Red” cone cells- one with each protein- giving them 4 cone cell types total, making them tetrachromatic. And these women may see colors we don’t.

IMG_5837 Tangent: This concept is way super-hard to get one’s head around, because of course you can’t visualize a color you’ve never seen. I think about it like this: suppose you’d never seen anything red in your whole life, and then one day you were walking and came across a big red flower, or a woman walked past in a bright red dress. And of course for millions of dichromatic men, this is of course exactly what would happen if they could somehow take a pill or protein supplement that suddenly made their “Red” cone cells functional…

gilligan_wideweb__430x313Nested Tangent: The closest I came to this experience was watching Gilligan’s Island reruns on a black & white TV as a kid in the 1970’s. Back then I always assumed Gilligan’s shirt was green. When I finally came across an episode a decade or so later on a color TV, I was blown away to learn his shirt was red.

gilligan colorThis isn’t just conjecture; there’s some fairly significant research to support this. Think about it: some proportion of human women are seeing more colors than you are. (unless you’re one of those lucky women.)

fem eye The obvious question is what portion of women are tetrachromatic, to which the answer is we just don’t know. In my own research I’ve found estimates that anywhere between 2% to possibly as many as 50% of women may carry both protein-making genes, but even if a women carries both genes, it doesn’t mean they’ll both be expressed, and she may well make only one or the other.

Tangent: There’s decent evidence to support this. In one study, gene analysis was done on the X chromosomes of a bunch of women that identified a portion as being genetically tetrachromatic. Then color-perception tests were run on both the genetically tetrachromatic and the genetically trichromatic women.

Nested Tangent: What was the test? (This is a reasonable question, since virtually all "standard" color tests are designed by trichromats to test trichromatic vision.) The women looked at a spectrum of light- basically an artificial rainbow- and were asked to count the number of distinct color bands they saw. Tetrachromats see more bands than trichromats, who in turn see more bands than dichromats…

Some portion of the genetically tetrachromatic women saw significantly more color bands than the trichromatic women, while the rest tested like trichromats. So presumably the genetic tetrachromats who saw decidedly more color bands were phenotypically tetrachromatic, meaning both protein-making genes were expressed, while the others were expressing just one gene or the other, making them phenotypically trichromatic.

woman street1 So let’s be conservative and say that 1 out of maybe every 50 women is phenotypically (and therefore functionally) tetrachromatic. Think about that the next time you walk down the street. Look at the women passing by. Out of every 50, 1 of them is seeing a world of color you’re not, and from where she’s seeing things, you might as well be watching the world through a black & white TV set.

8 comments:

Enel said...

All right, this blog rocks. Love this stuff.

Found you through
Fatty and am bookmarking you.

Keep up the good work.

Mark Vande Kamp said...

I like this post a lot. Scientific American had an article about color color vision in birds a while back. One cool thing they noted is that some of the birds (like starlings) that we see as drab, with no difference between males and females, are much different in the wavelengths that the birds themselves see. If we saw the way starlings do, male starlings would be bright and gaudy. For some reason, I really like knowing that.

Watcher said...

Mark- Thanks for the pointer! I managed to find the article following your comment, here.

I wish I'd found this when I was researching the post; it's got great additional info (specially about UV vision in birds) and is very readable. For anyone interested in color vision and/or birds, I recommend you check it out.

Lisa said...

Thanks ,mountain bike riding and science . I am in love . great blog .

snowgecko said...

Wow -- awesome post! I ended up here researching remote sensing and spectral bands, and look how much I've learned. Did you create the "Cone Cells in Human Eye" figure? May I use it?

Watcher said...

Snowgecko- Glad you liked it. Yes, the Cone cells/human eye graphic is mine, and you’re welcome to use it. Thanks for asking.

As a rule of thumb, anybody is welcome to use any of my graphics. When doing so, I’d appreciate it if you could give credit to WTWWU. But if you don’t, or forget or whatever, no worries. I won’t sue you or send you nasty-grams or anything. This isn’t a commercial venture.

snowgecko said...

WTWWU, well, I'm stumped on something, but perhaps this is more of a physics question than a biology question. If you could only see in one color, like if you could only use your red receptor cells, do you actually see in black and white? In Photoshop, clicking on only one of the RGB bands ("channels") results in a grayscale image - albeit brighter in the selected band.

This suggests that a single band only shows you the intensity, while only two or more bands are needed to see color. I haven't yet found the answer to this question, and wonder if you've thought about it? Thanks, Lena

Watcher said...

Lena/Snowgecko- Yes, if you could only see in one color, you’d effectively be seeing in black & white. I say effectively, because I guess it could look like red & white, or blue & white, but it would only be 1 color. And for all I know, what looks like gray to me may look like red to you.