Colorful Communication

Transcript from the interview with ASU School of Life Sciences Professor Kevin McGraw.
Science Studio Podcast Vol 12

Transcript - [Printable PDF format]

Peggy Coulombe: Hi, this is Peggy Coulombe from the School of Life Sciences at Arizona State University, and welcome to "Science Studio".

It may come as a surprise for those of you who don't live here, but the Sonoran Desert isn't a pile of sand dunes. In fact, right now lots of things are blooming--troya, hedgehog and prickly pear cacti have peachy, hot pink and yellow flowers and the long, whip like branches of the ocotillo have leaves and red inflorescences at the ends that make the hummingbirds just go wild.

As you might imagine, all this blooming has both the bees and the birds abuzz abroad, on campus and in people's yards. The flowers' scent and color are not just pleasing, they are signals. In fact, color in nature is used in lots of different ways to give information to those who see it, like a warning--for example, like the banded body of a wasp--or to say, "Hey, I'm cute! Let's have babies!"

Our guest today is Kevin McGraw, an assistant professor in the School of Life Sciences. Kevin studies animal behavior and coloration in birds. His interest is in unlocking the mysteries behind the meaning and purpose of color in birds, what information might be contained in color, and breaking the secret code for us.

Welcome, Kevin.

Kevin McGraw: Thanks, Peggy.

Peggy: Before we go further here, I'd like to talk a bit about color and feathers. I read in a magazine, "Birder's World", that there are like 30 uses for feathers. Can you tell me basically what a feather is?

Kevin: Feathers are interesting. They are actually, technically, exaggerated scales, like on a reptile. So a scale on the skin of a lizard or a snake is quite firm, dead tissue, surrounding the outside of the skin.

A feather does just that, but in a more elaborate way, obviously, of having a greater length, separated from the body, and sort of providing more of a hair like appearance and function around the skin of birds.

Peggy: So color arises from a manipulation of light, where there are some wavelengths absorbed and some reflected; and there are different ways that color can be produced--pigment-based and structurally-based. Kevin, can you explain the way that color can be produced in birds?

Kevin: Yeah, birds produce colors just like any other animal out there, or even like any artist would on a painting. A variety of pigments are present in feathers and their skin or legs that absorb light--so they essentially are subtractive colorants. Just like by adding red paint to a white canvas, you are subtracting light out--namely all other wavelengths--and reflecting the red light back. So there are a variety of those biochemicals that animals can consume in their food or make themselves.

They can also produce color by reflection as well, and so there are a series of structures that animals can use in their feathers, or other parts of their bodies, that truly act in a mirror like sense, by taking that light harnessed from the sun and bouncing it back into the eyes of the receiver to create different colors, actually, up in the blues and the iridescent and the ultraviolet types of colors, tend to be these more reflective-like structures.

That is sort of based on the architecture of the tissue. It has to have a certain index of refraction to allow light to come in and then bounce back at a certain angle and at certain wavelengths.

In reality, though, both types of wave-generating color are present. In other words, you can't have a pigment that is present in a black environment and get any light coming back, because you don't have any true reflection. So in essence, this artificial designation of "absorbing pigments" and "reflecting structures" is in fact artificial. You have to have both present in order to generate some true color.

Peggy: Are there many different kinds of pigments or nanostructures in birds?

Kevin: Yeah. I briefly hinted at that, but you can sort of subdivide the pigments that are out there into two general classes: the types that animals consume, which primarily include the carotenoid pigments. A variety of bird feathers are brightly colored red, orange, or yellow, because the red, orange and yellow pigments that are present in plants or plant parts, from juice to fruit, carrots, etc., make these pigments themselves. They actually play a role in the photosynthetic nature of the organism, so they act as accessory light harvester to facilitate photosynthesis in the plant or plant part.

Birds, then, by consuming plants, plant parts, or other animals that have eaten those plants, can in turn get carotenoids from their food, and then basically transport them through the bloodstream, deposit them in their tissues like feathers.

There are also pigments that are not dietarily acquired, but are synthesized by the animals. Probably the most common and classic example of that are the melanin pigments, which are the blacks, the browns, the grays, that we deposit in our hair and in our skin, and that birds similarly do.

They make these pigments from some basic dietary precursors. So in that sense they can be dietarily linked. But the process of forming those actual dark molecules actually occurs inside of particular cells that are dedicated to enzymes that make those particular bio-molecules.

Peggy: And is the color just found in the feathers?

Kevin: No, of course you see various bird beaks that are brightly colored, spanning the spectrum from blues through reds, oranges and yellows, like you see in the classic zebra finch--a domesticated pet bird that you can see at any local pet store.

The legs of birds can be brightly colored. Some, again, bright blue, brilliant green, through yellow, oranges and reds. There are combs and wattles like the chicken or the rooster comb, which can be brightly colored.

There are even secretions of birds, that they can basically take these endogenous pigments and secrete through various oils, through their glands, like the pineal gland, and in fact rub all over their feathers to become sort of tinged red or pink.

Peggy: So how do birds see color? Is it much like we do?

Kevin: Birds actually see a much more rich version of color in the world than we do. It's for two main reasons. The first of which is they simply have a sheer higher density of the photoreceptors, the cells in the eye that perceive light and perceive variation in true color. Those are called the "cones", and of course birds in turn have higher density of those. They also have more different types of cones.

So each different photoreceptor cell is sensitive to a particular wavelength of light. So in humans we have red-sensitive cones and green-sensitive cones and blue-sensitive cones, and they overlap in their sensitivities to some degree so that we can see intermediate colors like yellows and oranges.

Birds in fact, again, not only have more densities of those three types, but they also have a fourth type of cone, and that is ultraviolet sensitivity. And thus a world in which we protect ourselves from the ultraviolet, protect our skins using the sunscreen to prevent damage, birds actually see those wavelengths.

They see them emitted from the sun, and as a result they see the world in much different ways than we do because of the fact that a lot of things in nature are ultravioletly reflecting. Thus, patches of feathers on birds, or berries, or insects, are UV-reflective, and thus serve as signals, as things animals can pay attention to that for centuries we had no idea existed.

Peggy: What kind of messages are birds trying to tell each other?

Kevin: Boy, I've tried to categorize that in some sort of formal sense, and the best I can recall is about eight different categories of messages, I guess, that we have tried to agree upon as scientists. Rather than go through all of those intricately, I figure I'll review some of the basic ones.

It is pretty important to know who males and females are, and so there are often very dramatic sex differences in color. The males often being brightly colored and the females tend to be a little less or drab. So sending a message of gender or sex is one key criterion.

Sending a message about species. "Am I a mallard duck or a black duck?" I mean, that's an important distinction to make when you're trying to choose a mate and decide how best to pass on your genes.

There is this category that we call sort of quality, which is, some birds maybe within the male sex are a little more brightly colored than the next. And thus females may look to them to gain some information about how good they are--are they a good survivor, are they a good father, do they have great genes?

So those are sort of three, sort of examples of the types of things that birds can share with each other about their colors.

Peggy: And then they're used in things like territoriality as well.

Kevin: That's right, yes. We tend to think of those in, I guess, the same sense, that territorial defense or aggressive ability is one of these attributes of quality.

So you could think of it in terms of the age of the individual, the health of the individual. Even the diet of the individual might be reflected in their color, and thus aspects of these different qualities that could be valuable in an evolutionary sense.

Peggy: We talked a little bit about what controls pigment color in birds. But my understanding is that you have looked at how the intensity of color might be linked to health, and the type of food that a bird eats. Can you tell me something about this?

Kevin: Yeah. So we dedicated quite a few years of research into these carotenoid based colors. Again, the carotene-containing components that birds that eat fruits and seeds tend to get a lot of in their foods. By taking those foods in high quantities, you can display those pigments that you got from the foods in feathers, and thus honestly reflect what you ate.

In essence, this is a good example of "you are what you eat", in a sense. So females can pick up on that and detect who the best foragers are, and maybe get a sense of who the best mate would be for paying attention to the offspring.

The other part of the story is interesting as well, in that these same pigments that they get from the diet and for coloration are actually antioxidants. So they are the things like lutein or beta-carotene, which they tell you to eat your carrots because it's good for your vision, or eat your corn because it's good for your health, and this is true for birds as well.

Those that get a very carotenoid rich diet are more colorful. They have a better nutritional ability. And they also have a higher health status too, which gives females another reason to pay attention to how brightly colored a potential mate is.

Peggy: Do the antioxidants then protect them against UV?

Kevin: Well the feathers themselves are dead tissues. So if the antioxidants, like the carotenoid pigments, are present in feathers they're not going to give you a ton of protection because, again, those are dead tissue. What it might protect from, potentially, is the skin from underlying UV exposure and thus damage.

There's another place that birds are possibly using carotenoids, which is an interesting sort of link between color and vision. And that is they put these carotenoids not just in feathers but in their eyes. So they actually deposit these carotenoids in their retinal cells to act as sort of screening filters to get rid of any of the damaging light that might reach the eye, to keep those retinal cells really vibrant and active and protected. But it also helps them see color.

So it's one of these really neat ideas where the more carotenoids you eat in a food, the more colorful and healthy you are, but also the better you can see those colors that carotenoids help you see, which are the reds, oranges, and yellows.

And so thus you can get even more carotenoids or better mates by having a carotenoid-rich diet. So it's a really interesting co-evolutionary sort of feedback mechanism all based on this single molecule--these magic carotenoids.

Peggy: Most of the most colorful birds are male. Why is that? And why do females see the most colorful birds as the best choice?

Kevin: When you sort of think about it in humans in a similar sense, and actually not in just humans, the animal kingdom at large, you tend to see that males tend to be bigger, males tend to be more ornamented or exaggerated. The females don't tend to be as much. So color isn't unique in that sense.

But the reason is that this general phenomenon of what we call "sexual dimorphism"-- sex differences in shape or color, or size has a basic sort of root in gametes. It's based on the idea that females produce sort of large, single eggs, compared to males, which produce large amounts of fairly cheap and small sperm.

As a result, the males tend to not be as choosy with mates, whereas the females, that very sort of large investment and a very infrequent reproductive opportunity means that they've got to extra sure that the get a really good mate. And so they sort of put the males to the test, in an evolutionary sense, to try and convince them that they're worthy of a mate. And thus, males have, in turn, evolved signals that say "Hey, I'm better than the next guy, and thus worthy of getting you as a mate."

Now why females see the most colorful bird as the best choice, I think goes back to this idea of that they have a good diet and that's truly reflected in their color. They have a good health and that's truly reflected in their color.

There is other information that can be encoded, but things like hormones might affect the coloration. And a female might want a big, bad, aggressive guy to help protect her and defend a good territory.

And it can also even go back as far as sort of the genes. The genes an animal has might predispose it to be a good forager, to be healthy, to get a good diet, and thus to be colorful.

So there's a whole suite of what we think of as sort of direct benefits that females can get, how she can actually help herself as an individual, as well as indirect benefits, the genes that she can help pass on to her offspring and future generations, that are encoded in the colors of various birds.

Peggy: Does the behavior play a role in what color means and how it is used? Say, like the shaking of a peacock's unfurled tail.

Kevin: This is sort of a really under-studied part of color. We think of color as a very sort of permanent thing. You grow feathers, they're dead, they're on the bird and they're displayed for months at a time.

Peggy: Static.

Kevin: Right. But in reality, behavior though is quite dynamic. And the degree to which you can maybe exaggerate your color by behaving and flashing it or waving it, wagging it in some cases, might certainly get the attention of certain individuals from a distance.

And then maybe even seduce them, if you can think of behavior as sort of yet another challenge that the males are having to perform. A female looks at a bunch of males, sees them as varying in color, but they also see one that really dances vigorously and flashes and sort of shows their energetic abilities, and thus their true potential as a mate.

So I think people, by and large, have thought about that behavioral component as well. Maybe they do it in sort of a short-term sense, or maybe everyone does it just to sort of flash their cape once they've got the cape. But we've sort of thought of that cape as being the more costly part.

I don't necessarily think that's fair. So I think it's definitely an opportune time now to start to think about the dynamics of color and how it can potentially change in concert with an individual's behavior and affect mating and signaling.

Peggy: You spoke a little bit about this earlier, and I'm curious, does color change with birds as they age?

Kevin: Yeah, they can. I wouldn't say that they always do. Certain signals, the ones I've spoken about that are sort of condition-dependent that are reflective of an animal's quality definitely can change with age.

And I think the pattern there tends to be more of a statistical one. In other words, you have young birds, they can vary in color, some of them can get as red as any other bird ever. But if you compare that group to, say, the older, more experienced birds, the older birds tend to be, on average, a little bit more colorful.

And again, that tends to be linked to this sort of experience. You're more healthy, you just have a better sense of the world, where to forage. Thus, you see birds tending to become a little bit more colorful, even on an individual basis, with age. And that's for certain types of colors.

There are other types of colors though that are wholly shifted with respect to age class. In other words, in your first year of life you develop a color that's kind of not very attractive; it's sort of a little bit more attractive than a female but definitely not as attractive as the best male in your species. And that phenomenon then transitions to, in the next year, those individuals all gain this attractive super sort of ultimate older age appearance.

This phenomenon is called "delayed plumage maturation" where there's a very systematic way in which all individuals of a species go through this transition. It's thought to be one of these sorts of evolutionary strategies for kind of maximizing mating.

The first-year birds, if challenged, they may not be able to make the best plumage, and so they're going to lose out to the best males. So what they evolutionarily may have been favored to do is to sort of almost look a little bit like a female so they can sort of just blend in and not look like a rival.

And then maybe when a female chooses a real stud male, real bright, he sort of even isn't viewed of as a male, he's sort of viewed as a female that's just sort of hanging around, but then swoops in and maybe gets some fertilizations or some mating opportunities.

So, yes, there are these interesting subtleties with which age can be related to color.

Peggy: How much variation can exist in color within a species before the message gets kind of blurred? And is that variation the same for all species?

Kevin: It's funny because I've thought about it in this way: There's a lot of variation in variation. In other words, within a species you can get finches, like House Finches on the ASU campus, which vary from bright red to drab yellow. However, if you look at a cardinal in the desert, you'd see the cardinal looks kind of pink or a little more red.

And so there are these ranges of variation within species that differ, and that probably depend on things like, maybe, our eyes, which we've established aren't as good as a bird's. And thus we may be underestimating the capacity with which they can judge their own colors and their variations.

But you can't ignore things like crows are black, ravens are black, and they're all black. Then you can measure them with a spectrophotometer, which is one of these handy little gadgets that, independent of humans, quantifies objectively how color is reflected or how light is reflected as a color. And black is black is black.

And so in their species, something different is going on. They're not signaling with their color anything about their attractiveness or their health or their diet. They're probably favored to do other things like stay camouflaged maybe in some species or in others maybe for thermal properties.

It's actually been shown that black tends to help dissipate heat more so than you think it would. You tend to think black is sort of absorbing and real dark isn't a good strategy in the desert, when in reality in the world you actually see a ton of black birds in the desert. And it's linked, again, to this sort of heat dissipation.

So, yes, there's this tremendous spectrum of variation that is sort of linked to the challenges or the pressure to becoming colorful. Finches face a lot of pressure to be red, it's challenging, not everyone can do it, whereas crows, you sort of have a black pigment, you make it, you just put in your feathers and there's other reasons that favor that. Like I said predation or thermal conditions.

Peggy: How or why do you think the coloration evolved in birds? Do you think it's solely linked to sexual fitness?

Kevin: No, by no means. Again, camouflage is a form of coloration where birds are intending to blend in. I think of that as a signal still; it's more of an anti-signal though. If you think of signals in a spectrum of you really want to stand out, or you really want to not stand out and blend in, yes, there is that range of variation.

So I definitely see there are certain predictors of how it would evolve or why it would evolve in species. If you're diurnal you get a lot of light, you live out in the day, bright sunlight, now you're going to be more likely to use things like bright colors to communicate.

Whereas if you're sort of nocturnal or you're not very mobile maybe, you just going to sort of stay put, you want to blend in, so whether you're a predator or a prey is going to matter with respect to what color you evolve.

I even think, and this is sort of a little bit of a stretch or maybe one of these sort of umbrella encompassing sort of hypotheses, but I actually think that the sheer mobility of birds, the fact that they simply move more on average and thus can interact more with individuals in a diurnal setting sort of raises the opportunity more for them to use colors and sort of signal to one another.

Whereas if you're a ground-dwelling sort of large maul home-range animal, you're just not going to interact with that many animals and just not going to be maybe as interested in or get as much out of these intricate forms of communication.

Peggy: Why do you think there's such diversity in colors expressed?

Kevin: Maybe in an ecological level you can sort of think about all the variety of places you can live in the world and all the different light environments you could think of. Even as a sheer observer taking pictures in a forest versus a grassland versus in the night, there's just a lot of different light environments. And light, of course, is color.

And thus, I think it's useful to think about how an open grassland bird would have the opportunity to signal with brilliant colors, as opposed to a deep interior forest bird, which is going to sort of be left with a lot of green light and sort of dark, and thus probably not tend to be as brilliant.

Or, there's an alternative, and this has been shown in a couple of cases, that if they want to communicate in color, and some animals have evolved this capability, they have to actually be extra bright in order to reflect more light and capitalize on the little light that's available.

So, yes, there's habitats sort of associated, pressures linked to color. Things like whether they have to recognize their own species or not is important. If you live on your own, you look like nothing else; you don't have to worry much about that. But if you're a bunch of ducks pooling together in a little group, a little raft of ducks, you have to tell who's who, and thus you may have to develop different colors just to stick out as a species.

There can be different overall pressures. If you want to think about in a sense of, do you want to hang out with others and thus stick out so that individual birds can find you? Sort of an analogy there might be parrots, which are often bright red and they stand out against a deep green background in a forest.

Or are you happy living alone and don't want to socialize, and in fact, choose to try to blend in and thus be camouflaged. So I think all of these sorts of ecological and evolutionary pressures definitely play in.

Peggy: That's really interesting because when you have things like flocks of flamingos and flocks of blackbirds, the expression of color is completely different than their level of, I guess, socialization.

Kevin: I mean if we're thinking of them as flocks, I guess we can probably think of them as somewhat similar social groups.

But I guess I would think there that the difference in their colors is the difference in their backgrounds. So, what they're reflecting light against. You can be brilliant green if your sitting against a nice blue sea, however if you're brilliant green against a green background of trees, you're not going to stand out the same.

So I think pink/red is a very uncommon color in nature, and thus, animals that want to stand out tend to really converge on using red as a standing out vibrant color.

Peggy: The color red has great significance in human systems as well as in birds. It's been used in wealth and status in the Middle Ages, and to confer good luck in Chinese culture, among other things. You just mentioned red. So why does red pigment have special importance to you?

Kevin: Well, you know, it's actually used in sports too. Actually a study came out last year showing that the last Olympics, those individuals or teams that won more often than others tended to wear red.

Peggy: How bizarre.

Kevin: Which is a really bizarre link. I don't know if there's a cause or effect sort of relationship. You almost wonder if the more confident or aggressive individuals were choosing to wear red, which is actually known in a sport I follow quite closely, golf. Tiger Woods, one of the best golfers in the sport, wears red always on his final Sunday of golf in the tournament.

But I think, yes, the red pigment really is of interest to me because of its many roles. Again, you can create brilliant color with it. You can be healthy with it. It comes from plants and it has a real ecological association, which sort of reflects, again, no pun intended there, but the true intricacy with which an animal has to be associated with its environment in order to survive and reproduce.

Peggy: And you discovered a new pigment?

Kevin: Yes! So we, I wouldn't say got bored with the traditional pigments, I've certainly studied that for a number of years. But you have to then wonder, "If I've spent all this time trying to study how carotenoid signals work or melanin signals or structural colors, what else is out there? Have we really even begun to understand the range of pigments that are found?"

And so just casual preliminary studies, we delved into work on parrot feathers and penguin feathers, and have discovered that in just those two groups, they've both stumbled upon what appear to be wholly unique pigments that they use to become colorful.

So it sort of does speak to this interesting idea that you can't judge a pigment by its color, that there's a lot of different opportunities that animals have, by eating things or making things themselves, to become colorful.

Parrots, they make their color on their own. So if you've got a parrot for a pet, like a scarlet Macaw, you can try to feed it as many carrots or orange juice or what have you, and they're not really going to change that red much, because it's not carotenoid based. It's actually fairly similar in nature, it's a lipid soluble pigment, but that doesn't come from the diet.

It seems like parrots make them themselves, actually, red at the feather follicle. You don't find those pigments anywhere else in the parrot's body other than in its feathers. And you don't find the pigments anywhere else in the world, in the universe, to the extent that we know.

So why parrots have come about this little color expression scheme is really kind of a question that we're trying to pursue and that we don't, unfortunately, have great answers for right now.

Peggy: So just out of curiosity, why isn't a cardinal blue?

Kevin: Evolution doesn't tackle the "why not" questions very well.

Peggy: [laughs]

Kevin: I mean the mutations that arise clearly get favored and weeded out, and that accumulates over time. But I guess I can answer that question in two ways.

A cardinal isn't blue because it doesn't have the blue pigments to become blue. In other words, it can eat raspberries or blackberries, which tend to be a good component of their diet, but which vertebrate bodies tend to use more as antioxidants and break them down into sort of colorless byproducts. So you'll never see an animal that you would feed it blueberries, turning blue.

You could also answer it evolutionarily and say "why isn't a cardinal blue?" I think you would have to involve arguments like, "Well, blue might stick out more to predators and thus be detrimental to them. Blue might not be as good of a signal as far as the perceptual abilities of the animals in their environment."

So I think those are probably the best hypotheses we can propose for answering one of these classic "why not" types of questions.

Peggy: Can you shift a bird's color by altering its diet?

Kevin: You can. I did some of this work early in my career. I was sort of toiling with this idea of how conserved are color systems. Can you sort of push animals, dietarily or genetically, into weird sort of expression patterns, which lends some insight into how they sort of evolve and come to be.

And the good poultry literature on all sorts of domestication, and all these genetic crosses you can do with chicken breeds, and you can even do it with pigeons, shows that genes can definitely tinker with the color expression. You can actually change what you feed them and change the color.

So you could make a canary turn white if you do not feed it carotenoid pigments. Or you can make a yellow goldfinch turn orangeish or red if you feed it bright orange or red pigments. Now presumably they don't have those colors in nature because they can't include those dietary foods in their range.

But it has actually happened in a couple cases. The most notable one happened here in the United States. The cedar waxwing, a fairly common sort of neo-tropical migrant, for years and years had a sort of red patch on a sort of waxy wing, and a yellow tail, and that was their expression pattern. People always wondered "Well why don't they have red in their tail? They clearly have the capability to be red on their wing."

Well what happened was, and this doesn't necessarily shed light on the difference between the wing and the tail, but that an introduced honeysuckle became available as a berries food source to them in the 1960s. And so over the time, in the late '60s, '70s, 80's, more and more people started reporting cedar waxwings with orange tails, never seen it before.

And it was simply just this human change in the environment, this addition of a food source, which not only affected their color patterns, and surprisingly, no one really studied this, but it may even have changed their pattern of evolution, their course.

Maybe those animals became more attractive, and thus were going to see an accumulation of more and more orange birds, and thus completely rendering the yellow ones I wouldn't say extinct, but certainly in a less favored variant in the species.

So, yes, the diet can do some pretty crazy things to bird colors in that sense.

Peggy: If you were to become a bird, which one would you be?

Kevin: You know, someone asked me that once. I don't get that question often. And the context they asked it to me was, "I'm an athlete, I coach soccer. I have a couple favorite soccer players. What's your favorite bird?"

So I told the person I would be a Galapagos finch. And a Galapagos finch, not because necessarily I like finches and the majority of work that I've done has been on colorful finches. But the answer I gave him was "They lead a pretty simple life, island, no complexities, no iPods or cell phones. Sort of free of human disturbance, a nice sort of uninvited tropical island habitat, and they spend their whole life with their kids. The kids never leave. There's no place to go, so you basically stay with your family."

And the fourth reason I told him was that I would have gotten to meet Darwin had I been around in the 1860s or '50s when he explored the Galapagos. So I guess that's my bird.

Peggy: Well it sounds like a great reason to be a finch to me. Well, Kevin, I want to thank you so much for joining us today.

Kevin: Appreciate it.

Peggy: This is Peggy Coulombe, and you've been listening to the School of Life Sciences podcast "Science Studio".

The School of Life Science is in the College of Liberal Arts and Sciences on the Tempe campus of Arizona State University.

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