I’m supposed to be turning an abstract I wrote into a paper and I’m having some troubles beginning. So rather than staring at the computer screen all day wondering how to begin, I’m going to try banging out my ideas here and see if that gives me a kernel to begin my “real” paper.
I’m presenting at the upcoming American Association of Geographer’s meeting as a panelist in a session called Lay Science and the Environment. Lay science, two terms I’ve hardly ever heard used together ((More typical for me would be Citizen Science.)), means the work done by non-professional scientists. Since they’re non-professional, scientist doesn’t really work, so I would suggest observer. Observer suggests that they only watch (which isn’t the case), so I’m going to use the term “naturalist.”
So, I was uploading this photograph taken in Peru of a Turkey Vulture. Now, with its red head and the fact that it was on the beach in Mancora, I kind of thought that it looked like it had a sunburn. The thing is, TVs have red heads all the time, so I’ve just assumed that its not really a symptom of a sunburn in the same way it is for us (mammals). This left me wondering, though: can birds get sunburns ((I’m guessing that exposure to the sun shouldn’t be an issue for most mature birds because they’re covered in feathers, which would act as some kind of physical barrier to sunlight.))?
And the answer is?
The answer to that simple question was waaay more complicated than I anticipated. So here is the short answer: I don’t know. The medium answer: It depends on what you mean by sunburn. And the long answer? Read on…
Now, there are a couple of things that I need to know in order to answer the question. I’ll begin with what I already know: bird skin is different than mammalian skin. Why? The simplest answer is that we’re different kinds of animals. Zoologists and taxonomists would say that humans are vertebrates (that would be animals with backbones) part of the class Mammalia. Birds, vertebrates too, part of the class Aves. Again, providing the simple answer, we have different evolutionary histories. These different pasts mean that today, while we may share the common trait of having a backbone, we also have lots of traits in un-common (feathers, anyone?). It just happens to be that the make-up of our skins has important enough differences that to assume that since we burn in the sun, birds burn too.
The similarities are only skin-deep
Let’s start with commonalities: all vertebrate’s skin is made up of (at least) two layers: the top, thin epidermis (source of the nerd’s schoolyard taunt “Your epidermis is showing”) and the dermis that lies beneath. While the epidermis is the barrier between you and the outside world, your dermis contains things like connective tissue, nerves, smooth muscle and blood vessels. This is where, in the case of mammals and birds, the commonalities end.
Your epidermis is showing
We’re interested in the epidermis because this is where the action takes place when we get a tan or a burn. Located in and around the epidermis are pigment cells called chromatophores ((The location of which can be in different places in different vertebrates)). The chromatophores that we’re interested in are called melanophores. These cells contain melanin, a protein that produces black, brown or reddish colours.
When I’m out with unprotected skin in the sun, ultraviolet light (UVA rays to be specific) causes my melanophores to start to produce melanin. This melanin then acts as a barrier to UVB rays, which cause the problems we have with sunburns. It takes a while for the melanophores to amp up their melanin production to a level that is protective, so in the meantime if I spend too much time in the sun, I begin to burn.
A sunburn then, is the body’s reaction to the damage caused by UVB rays. The redness that occurs when I’m burned is an erythema—a medical term that describes the redness of my skin caused by capillary (little blood vessels in the dermis) congestion (meaning they’re embiggened).
Skin vs. skin
Now that I’ve got a better idea what is making me red, I need to know if that same reaction can occur in birds ((Interestingly, this changes the nature of the question. Rather than asking “Do birds get sunburns?” I’m now asking “Do birds get embiggened blood vessels in their dermis in reaction to UVB damage?” It hardly rolls of the tongue though, so we’ll just keep pretending that my original question still applies)). So, let’s have a look at bird skin: while there are similarities, I suggest that differences rule the day ((Birds, for example lack sweat glands, or glands of any sort for that matter, in their skin. They do have one uropygial gland at the base of their tail which secretes oils used to waterproof feathers. Ah, if only I had a gland that could carry my rain-gear)). One significant difference is the lack of melanocytes in the dermis. It seems that the pigmentation is injected directly into the developing feather.
For our question, this seems significant because it then raises the question: what produces skin colouration in birds? To try and answer this, I turn to the article The Integumentary Morphology of Modern Birds—An Overview by Peter Stettenheim. In it, he suggests that in bare skin, “coloring, which often contrasts with the adjacent plumage, is due either to intrinsic pigments or structural mechanisms in the epidermis, or to blood in the superficial capillary network” and, later, that “colors in bare skin and epidermal outgrowths are produced by ordered arrays of collagen macrofibrils.”
Collagen macrofibrils, eh? This is getting waaay too complicated. Following this lead, however, I track down Structural color production by constructive reflection from ordered collagen arrays in a bird (Philepitta castanea: Eurylaimidae), which long story short suggests that ornithologists know little about the process of colouration in bird skin. Published in 1994, ages ago as far as research data is concerned, I can’t assume that what wasn’t known in the mid-90’s still applies today. This 2003 article seems to suggest that collagen has something to with the production of ultraviolet, dark blue, light blue, green and yellow colours. But what about the red colour of the Turkey Vulture’s head? Are we to assume that it is not from these collagen arrays but due to the bird’s superficial capillary network?
A momentary interlude
Interestingly, and wholly unrelated to my original question, the authors of the 2003 article write that this mechanism of colouration has separately emerged at least fifty times in birds. Fifty times! That this has appeared so many times, seemingly spontaneously, boggles my mind.
Again, totally unrelated to my original question, but totally fascinating, this article suggests that the colour of male Blue-footed Boobies’ feet change intensity in as little as 48 hours as their diet change. When fed fresh fish, their blue feet get brighter. The researchers found that the dietary changes that make Boobies’ feet blue-er also improve the state of the Boobies’ immune system. Female Blue Footed Boobies, in digging a male’s blue feet select a mate that has a stronger immune system, thus more likely to pass on those good genes and, perhaps more concretely, a mate healthy enough to help with the incubation and chick rearing.
So, do bird get sunburns?
Is the red colour on the head of a TV a symptom of a burn or just normal colouration? Given that TVs have a red head all year-long, it would seem that the colouration is un-related to sun exposure. So where does the colour come from? The book Bird Coloration makes no specific mention of Turkey Vultures, but does suggest that carotenoids have been found in the skin of 150 bird species. Carotenoids give bird skin a yellow, orange or red hue. Turkey Vultures are not listed as one of those 150 species and the “closest ((Though they’re both vultures, the TV is a new world vulture and the Egyptian Vulture is an old world vulture–they’re superficially similar, but are not as closely related as their common names would suggest.))” relative listed is the Egyptian Vulture.
I know it doesn’t feel like it, but I’m getting closer to some sort of answer. I know that birds don’t produce colour in their skin the same way that we do. So that means that birds don’t tan. It also means that they won’t have the same kind of protection against UVB rays as we would. This means that I now need to find out if UVB radiation has the same effect on birds as it does on mammals, what a bird skin’s reaction is to UVB radiation and see if birds have blood vessels in their dermis. Piece of cake, right?
UVB Bad? Unknown.
Now I need to find out if UVB radiation effects birds the same way it does humans—messes with our DNA. If it does, then I would assume that birds would have developed some kind of defence for it, like we have. Surprisingly, answering this question is proving difficult. Searching for UVB and birds doesn’t help because birds, again unlike us, can see into that range of the spectrum. Research has been conducted on the implications of this—for example it is suggested that female European Starlings select mates based on the male’s UV colouration. Another tact: searching for “skin cancer” and “aves.” And I can’t find any relevant peer-reviewed papers! Searching the INTERWEBS, I find some supporting evidence for skin cancer in birds ((For example, at a website for keeping birds as pets. This makes sense to me because when you’re living with and caring for a bird it’s likely the only time a human would notice something like cancer. Wild birds just don’t get that kind of attention. BUT, the skin cancer just gets a passing mention. Other kinds of tumours get the billing.)), but nothing that counts as strong evidence in my mind. I’m stymied here. But I’ll press on.
Dermal blood vessels? Check.
In a deek of logic, I’m going to ignore the UVB & skin cancer issue as it would appear that what I’m really interested in is whether birds could exhibit the symptoms of a burn—skin erythemas. According to the article The Integumentary Morphology of Modern Birds—An Overview, a bird’s dermis contain blood vessels, similar to us. For me, this means that—because birds share the same structure as we do—erythemas are at least a plausible possibility. But yet again to try and find any kind of peer-reviewed literature that even mentions this, I strike out.
So what’s the answer?
Ah, good question. Let’s see. I actually need to answer a bunch of questions:
Q: Do bird’s skin change colour when exposed to sunlight? A: No, if you mean in the same way our (pale) skin changes colour when exposed to sunlight.
Q: Do birds suffer damage to their DNA from UVB rays? A: There is no definitive answer. Anecdotal evidence that birds suffer from skin cancer; I couldn’t determine what kind of skin cancer it would be ((It wouldn’t be a melanoma, however, because birds don’t have melanocytes)).
Q: Could sunlight cause damage, the symptom of which is an erythema? A: My educated guess is yes, since the capillaries are in the same layers in bird skin and our own.
Q: So where does the red colour come from in a Turkey Vulture’s head? A: Again, I couldn’t find definitive evidence that the red colour is from pigmentation or from blood vessels. Perhaps it is a combination of both.
Now I’m pretty interested in finding out just what protection, if any, birds have in their skin to shield them from UVB rays. Perhaps the protection just isn’t as important for birds, with a physical barrier of feathers and a relatively short life-span, as it is for longer-lived pale-skinned humans ((In a sense, skin cancer is a disease of our lifestyle. Most likely, humans living 25,000 years ago died of other things well before skin cancer got them.))
Associated is what kinds of skin cancer, if any, birds suffer. Anecdotal evidence says yes, but is the anecdotal cancer really a cancer of the integument?
And finally, if birds do suffer radiation damage from the sun, what do the symptoms look like?