Color my world

When you think about it, color vision is kind of strange.  Our eyes -- unless you have a genetic or physical inability to do so -- are able to sort out the frequencies of light, and each range in the visible light spectrum looks different to us.  But why do we have the ability to distinguish between, for example, light with a wavelength of 570 nanometers (which looks yellow) and that with a wavelength of 470 nanometers (which looks blue)?  It's a small shift in wavelength, but triggers a completely different response in our eyes and brain -- so it must be important, right?

Color perception in the natural world seems to serve a fairly small number of functions.  There's sexual signaling -- the (often) brighter colors of male birds, for example, is most likely a cue for females signaling fitness (and thus good genes, worthy of producing young with).  It can be a sign that food is ready to eat, such as fruits changing from the blend-with-the-foliage shades of green to something more eye-catching.  It can also be a danger signal, as with the brilliant warning colorations of coral snakes, the foul-tasting bright orange and black monarch butterfly, and Central and South America's dart poison frogs.

So our ability to sense colors, an ability shared with many other mammals, birds, reptiles, amphibians, fish, and some arthropods, seems to have evolved as a way of distinguishing things that need to stand out from the background, for purposes of reproduction or survival.  There's a reason, for example, that stop signs are red; our dim-light vision is poorest in the red region of the spectrum, so when car headlights catch a bright red stop sign at night, it immediately grabs our attention.  (The flipside of this phenomenon is why snow under moonlight looks blue.  It's not that snow preferentially reflects blue light; it's simply that our eyes are better at picking up the blue region of the spectrum in low light levels, so it's almost as if our eyes are subtracting the red frequencies from the white light reflected from snowbanks, resulting in it appearing blue.)

What this means, of course, is that pigment production has to have evolved in tandem with color perception.  There are undoubtedly exceptions, where colorful chemicals have evolved for other purposes, and their hues are accidental byproducts of their molecular structure; but otherwise, the evolution of bright pigments must have coevolved with the ability to perceive them.  The brilliantly-colored organic compounds produced in the petals of many flowers, for example, are generally for the purpose of attracting pollinators, and the reds, oranges, and yellows of ripe fruit attract animals to consume the fruits and then disperse the seeds.

Scarlet passion flower (Passiflora coccinea) [Image licensed under the Creative Commons gailhampshire from Cradley, Malvern, U.K, Scarlet Passion Flower - Flickr - gailhampshire, CC BY 2.0]

What's curious about this, and why the topic comes up today, are the findings of a study out of the University of Arizona that appeared in the journal Biological Review last week.  It showed that based on genetic studies of distantly-related animal groups, color vision evolved a very long time ago -- on the order of five hundred million years ago, so the middle of the Cambrian Period -- while the first fruits didn't show up for another 150 million years, and the first flowers 150 million years after that.

So the earliest production of functional color (and the ability to perceive it) almost certainly was driven by sexual signaling and warnings.  Then, once animals were able to see in color, it became an evolutionary driver in plants to ride the coattails of that capacity in order to facilitate cross-pollination and seed dispersal.

And once that back-and-forth coevolutionary relationship was in place, it was off to the races.  Give it another couple hundred million years, and we have the rainbow hues of the natural world today.

One thing I still find hard to explain -- from an evolutionary standpoint, at least -- is why we find brightly-colored things beautiful.  Having our attention caught by a bright red apple, or the wild stripes and spots of the venomous lionfish -- sure, those make sense.  But why is it almost universal to find a daffodil or a wild rose beautiful?

Ah, well, maybe it's just one of those accidental things that is a consequence of other, more vital, evolutionarily-derived traits.  Whatever it is, we can certainly still enjoy it, and not let our wondering why it occurs interfere with our appreciation.

But it's still kind of cool that the ability that allows us to have that experience goes back at least five hundred million years.

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Pieces of the mosaic

The diversity you find among birds is really remarkable.

There are differences in bill shape, from the weird angled beaks of flamingos, to the longer-on-the-bottom fish skewers of skimmers, to the absurd (and aptly-named) spoonbills and shoebills, to the pelicans -- about whom my dad taught me a limerick when I was little:

A wonderful bird is the pelican.
His bill can hold more than his bellican.
He can stash in his beak
All his food for the week,
But I really don't see how the hellican.

Yeah, it's kind of obvious where I got my sense of humor from.

Of course, it doesn't end there.  The impossibly long toes of the South American jacanas (called "lilytrotters" because they can walk on the floating leaves of waterlilies).  The phenomenal wingspan of the albatross.  The insane plumage of the birds-of-paradise.

And the colors.  Man, the colors!  Even in my decidedly non-tropical home we have some pretty amazing birds.  The first time I saw an Indigo Bunting, I was certain that one of my sons had put a blue plastic bird on the bird feeder just to rattle my chain.  There couldn't be a real bird that was that fluorescent shade of cobalt.

Then... it moved.

But nothing prepared me for the colors I saw on my visits to Ecuador, especially amongst the birds of the tanager family.  There are hundreds of species of tanagers in that tiny little country, and because they often travel in mixed foraging flocks, you can sometimes see twenty or thirty different species in the same tree.  These include the Green-headed Tanager:

[Image licensed under the Creative Commons Lars Falkdalen Lindahl (User:Njaelkies Lea), Green-headed Tanager Ubatuba, CC BY-SA 3.0]

The Black-capped Tanager:

[Image licensed under the Creative Commons Joseph C Boone, Black-capped Tanager JCB, CC BY-SA 4.0]

And the Flame-faced Tanager:

[Image licensed under the Creative Commons Eleanor Briccetti, Flame-faced Tanager (4851596008), CC BY-SA 2.0]

Being a biologist, of course the question of how these birds evolved such extravagant colors is bound to come up, and my assumption was always that it was sexual selection -- the females choosing the most brightly-colored males as mates (in this group, as with many bird species, the males are usually vividly decked out and the females are drab-colored).  If over time, the showiest males are the most likely to get lucky, then you get sexual dimorphism -- the evolution of different outward appearances between males and females.  (This isn't always so, by the way.  Most species of sparrows, for example, have little sexual dimorphism, and even experienced birders can't tell a male from a female sparrow by looking.)

The truth is, however, this is an oversimplified explanation, which I suppose I should expect given how long I've been in science.  Nature is both way more complex and way more interesting than we usually expect.  Just this week a paper was released in the journal BMC Evolutionary Biology that looks at another group of birds that look like someone went nuts with a paint-by-number set -- the Australasian lorikeets.

Lorikeets are in the parrot family, and even by comparison to other parrot species they're ridiculously flamboyant.  Take a look, for example, at the aptly-named Rainbow Lorikeet:

[Image licensed under the Creative Commons Dick Daniels (http://carolinabirds.org/), Rainbow Lorikeet RWD, CC BY-SA 3.0]

The researchers, Brian Smith, Glenn Seeholzer, and Jon Merwin of the American Museum of Natural History's Department of Ornithology, were curious about how lorikeets balance being bright enough to attract mates while not being so showy they attract the attention of predators -- the latter being in no short supply in Australia and New Guinea, where the birds are found.  Using spectral analyses of museum specimens encompassing nearly the entire diversity of lorikeets, Smith, Seeholzer, and Merwin found out a few things that were absolutely fascinating:

• Virtually all the color diversity in lorikeets is on the underside -- breast, abdomen, and front of the face.  The backs of almost all species are plain green -- making them camouflaged from above and less visible to predators like hawks.
• Some of the range of colors they do have is invisible to the human eye.  A number of species have pigments that reflect strongly in the ultraviolet region of the spectrum, which is visible to birds but not to us -- and presumably, not to many non-avian predators either.  So they can be as flashy as they want in the ultraviolet and still not attract attention from hungry carnivores.
• Each of the patches is under the control of a different set of genes and thus can be selected independently, meaning different species of lorikeets can diverge in terms of the facial color while remaining similar in the coloration on the back and abdomen -- something called "mosaic evolution."

"The range of colors exhibited by lorikeets adds up to a third of the colors birds can theoretically observe," Merwin said.  "We were able to capture variation in this study that isn't even visible to the human eye.  The idea that you can take color data from museum specimens, infer patterns, and gain a larger understanding of how these birds evolved is really amazing."

Of course, I wondered if the same forces might be involved in the evolution of two groups I've actually seen in the wild, hummingbirds and the aforementioned tanagers.  It certainly seems to fit the same pattern -- a wide range of eye-catching colors on the front of the body, and -- especially in the hummingbirds -- largely green on top.

But that's just a guess.  It certainly opens up an interesting line of inquiry into the evolution of other bird groups.  And -- perhaps -- will end up explaining a great many of the other pieces of the biodiversity mosaic.

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One of my favorite people is the indefatigable British science historian James Burke.  First gaining fame from his immensely entertaining book and television series Connections, in which he showed the links between various historical events that (seen as a whole) play out like a centuries-long game of telephone, he went on to wow his fans with The Day the Universe Changed and a terrifyingly prescient analysis of where global climate change was headed, filmed in 1989, called After the Warming.

One of my favorites of his is the brilliant book The Pinball Effect.  It's dedicated to the role of chaos in scientific discovery, and shows the interconnections between twenty different threads of inquiry.  He's posted page-number links at various points in his book that you can jump to, where the different threads cross -- so if you like, you can read this as a scientific Choose Your Own Adventure, leaping from one point in the web to another, in the process truly gaining a sense of how interconnected and complex the history of science has been.

However you choose to approach it -- in a straight line, or following a pinball course through the book -- it's a fantastic read.  So pick up a copy of this week's Skeptophilia book of the week.  You won't be able to put it down.

[Note: if you purchase this book using the image/link below, part of the proceeds goes to support Skeptophilia!]