September 22, 2016:

by Duane Nash

Pretty much ALL aspects of Microraptor have achieved special attention and thought. In this post/article - which is really a continuation of my take (and others too) on Microraptor as an especially diabolical and efficient predatory terrorist of all things small - I am going to highlight an aspect that has been startling, controversial, and plagued with misunderstanding, coloration. Ahem, not just coloration but structural coloration i.e. iridescence (Li, 2012). More specifically that iridescence does not in fact preclude a nocturnal lifestyle.

Reader beware and tread carefully. Many of the arguments and thoughts I will bring forth stand on necessarily shaky ground. That is because issues of coloration, roosting behavior, iridescence, cryptic coloration, "cloaking" and other aspects of modern animals are woefully understudied and really on the cutting edge of modern animal research much less in long gone paravians. That does not mean I am not going to "go there" and in the end create a startling and believable lifestyle portrait of Microraptor gui.

For this piece I am going with the notion that the coloration of Microraptor is indeed black. Although arguments for degredation of color fibers etc etc. have been put forth I don't find them especially compelling. I might be wrong, but for this piece I am going with all black. Which really is a good policy for attire anyways, when in doubt go black. Or even better go with all black and iridescent if you really want to make a statement... For this piece I will address the two issues of black coloration and iridescence separately at first and then bring them together at the end.

For as much as we might be tempted to or find it "nice and tidy" to leverage arguments about what Microraptor did based on what modern birds do let us remind ourselves that Microraptor was not a bird nor was it ancestral to birds. It was its own thing. More so than that there is a tremendous gulf of time between Microraptor and modern birds - especially advanced "blackbird" type passerines. The environment is exceptionally different between what Microraptor inhabited and where modern "blackbird" gestalt birds generally thrive. Indeed a cool temperate rain forest dominated by giant old growth gymnosperm trees and lakes is about as far from the prairie like, open grassland type habitats that many "blackbird" type birds thrive in. Not to mention the differences in ecology between Microraptor - as yet only known to have eaten flesh - and the more granivorous or omnivorous tendencies of grackles, corvids and other passerine grassland thriving "blackbird" types.

I must ask some questions concerning the repeated occurrence of "blackbird" type birds in open grassland dominated habitats namely, "Why evolve a dark conspicuous coloration that highlights your presence to potentially other color vision attuned avian predators during the day?" You know it's true. Just look at a flock of crows foraging in a vacant lot or a flock of any of a number of blackbirds, whether the vegetation is flush with new growth and is bright green or it is brown and yellow after months of little rain these birds stick out like sore thumbs. Seems a little maladaptive does it not? Unless of course it is not actually maladaptive but exquisitely adaptive actually...

There is another strategy that links group foraging "blackbird" type birds of open habitats and that is communal roosting in trees. Again here we see that the black or darkened coloration of these open habitat communal foraging birds plays a dual function at night. The dark coloration of these birds helps "cloak" them at night by, well, essentially mimicking the dark night sky. Even dark colored American coots may benefit from this as they also roost together at night next to bodies of water.

The defensive tactics of communal roosting birds is woefully understudied. Indeed what goes on at night in communal roosts is hardly looked at. I suspect there is a lot more going on than just sleep in some of the absolutely massive roosts that many birds engage in. The smell, noise, and general ruckus a large roost creates surely must act as a beacon to predators. Normally diurnal raptors might try to pick off roosting birds in early morning or late evening but black birds might just sort of disappear in darkness. Owls you would think could pick off roosting birds with ease but the predatory pressure that they put on roosting birds does not seem to be enough to dissuade the behavior. Likewise felids with their night vision would seem to be ideal candidates to depredate large roosts but cats are fairly near sighted and - when looking up at a dark colored roosting bird from the ground - the birds might just kind of disappear into the night sky. Check out this piece on what cats actually "see". I have done a little of my own field research into night roosting crows on Milpas street in Santa Barbara. A busy street during the day the crows gather in the large English laurels at night. When I checked out what was going on I could hear lots of activity actually. The crows engaged in subdued chatter, there was some movement and flight occasionally. But when my eyes tried to focus on any individual crow it was very difficult. I could kinda tell where they were at but they looked less like discernible objects and more like... negative space. Almost as if they were just black voids in the tree. They did not reflect the street lights - this roost is by a brightly lit gas station - but appeared to just suck up all the light that hit them.

"And my soul from out that shadow that lies floating on the floor / Shall be lifted - nevermore!!" Edgar Allan Poe "The Raven"

But what works for communal nigh roosting blackbirds today in terms of cryptic coloration at night to avoid predators could work equally as well for a cryptic predator at night seeking to hide itself from prey... take home message: there is an argument to be made that black coloration in Microraptor is a useful element for a nocturnal stealth predator.

And now on to the iridescence.

"All modern iridescent birds are diurnal and therefore Microraptor was not nocturnal"

There it is, that stock line that in one fell swoop took the wind out of the sails in terms of nocturnal behavior (Schmitz, 2011) in Microraptor in many people's minds. Except such reasoning is not as ironclad as some might suggest.

And here is where I have to get all weird, paramilitary weapons technology on ya' all. Iridescence in many animals and plants might have a lot more to do with escaping - or possibly bewildering - detection rather than attracting it. So to prime you for it best check out a clip from one of the best sci-fi action films of all time and one of the best creature designs of all time: 1987's Predator:

The alien predator uses a technologically devised cloaking structural coloration to in fact mimic and blend into its surroundings. This ingenious camouflage was put to excellent use in the movie and the comparison to Microraptor in terms of an agile and multifaceted predator that takes advantage of 3-dimensional space - remember when Dutch (Arnold Schwarzenegger) realizes "It's using the trees."- and structural coloration should not go unnoticed. I realize some may have just checked out with that statement but yeah, I just kind of made the argument that a game hunting, cloaking alien from a Schwarzenegger movie is an analogy to take heed of when thinking about Microraptor.

Still with me? Ok time for a quick primer and some links to structural coloration of which iridescence is one of several types. Structural coloration is any microscopically structured surface fine enough to interfere with visible light. Visible light is not one color as we see it but contains all the colors we can and can not perceive. These "colors" have wavelengths and the microscopic structures in animals can alter these "colors" through wave interference. Based on the structures at hand wave construction can occur where colors are enhanced or wave destruction can occur where colors are diminished. The geometry of the material also plays a significant role as light both enters and refracts off of the structural material. This is why iridescent animals change colors depending on the angle you view them at as does soap bubbles... Still confused? Check out this primer on Iridescence "causes of color" and browse through the other sections.

Back to the question at hand which in this post is deconstructing the statement "Microraptor was not nocturnal because no modern day iridescent birds are nocturnal". My main lines of attack are two pronged and that there are two inherent assumptions implicit in the above statement 1) that iridescence is wholly and totally about "display" and therefore implies diurnal habits and 2) that Microraptor compares ecologically compatible with modern aves. Obviously I disagree with both assumptions and therefore strongly disagree with the logic of the argument itself.

I think one question that begs asking "Is iridescence totally or even mostly utilized in display in modern birds?" Of course in bird like a peacock the answer is an obvious YES!! But peacocks, like the various "blackbirds" I discussed earlier - at least the male - is a nocturnal tree rooster. Under the diminished light of the moon does the iridescence of the peacock perhaps play another subtle role in evading detection?

And what of the common occurrence of not only a dark or black coloration in many communal roosting birds combined with iridescence on one or both of the sexes to various degrees? One example that suggests more of a display trait in iridescence is grackles in which the male is the more iridescent example but starlings confuse the situation because both sexes are iridescent. This could be a case of mutual sexual selection but I suspect that nocturnal evasion aided by iridescent wave interference might be a hypothesis worth exploring. Is the iridescence of starlings totally about display? Or is there a dual functionality going on?

Again, what goes on in nocturnal bird roosts? what tactics do they use to avoid predation? etc. etc., these are all areas of high speculation and little study. But the common occurrence of dark coloration, iridescence on one or both sexes, and nocturnal roosting is a topic just begging for more study. As I stipulated earlier there is much extrapolation and speculation inherent in this text and argument but I believe (pun intended) there is more than meets the eye with regards to iridescence.

I am not completely sold on the notion that iridescence in modern birds is >completely< about display and I move to extend out arguments in terms of uses for iridescence outside of the rather arbitrary context of the restrictive phylogenetic bracket to include other organisms that utilize iridescence. Namely I want to focus on organisms that inhabit forested environments as deep forests serve as the most likely habitat of Microraptor. It is in the adaptational context - or adaptational bracket if you will - that we find not only the highest signal for iridescence in modern animals but also our most compelling arguments for iridescence as an aide in camouflage.

I start with the beetles. Shiny, iridescent, jewels of the animal kingdom. Yet their bright iridescent coloration shining and beckoning forth in the full light of day takes on a more muted, cryptic quality in the filtered light and shadow of deep forest canopy. Yet beetles do not use sexo-social display coloration as birds do but rely on chemical attractants. The high incidence of iridescence in beetles of the tropical forest just begs for an explanation in terms of camouflage and indeed this is the most compelling idea put forth.

Next I want to highlight animals closer both phylogenetically and ecologically to Microraptor which also sport iridescent coloration: snakes, but especially species that offers much utility in terms of understanding an iridescent forest hunting specialist of small things - the rainbow boa (Epicrates centria) of Central & South America and the Boelen python (Morelia boeleni) of Papua New Guinea.

As is the case with iridescent beetles we can dismiss sexo-social display as an explanation for iridescence in these snakes as they don't actively display these structures and are more inclined towards scent and chemical cues rather than visual display. Microraptor, though displaced phylogenetically from these two boids, has much more in common with them ecologically and environmentally than the grackles and other grassland feeding birds often compared to it. I would offer that whatever benefits iridescence plays in the lifestyle of these two snakes Microraptor enjoyed as well.

I would be remiss not to mention that there are several largely subterranean snakes that feature iridescence as well as the pertinent fact that the vast majority of cryptic, stealthy forest understory snakes don't feature iridescence. These observations create the argument that iridescence serves potentially no function at all. Perhaps some type of evolutionary byproduct of other happenings - one of Stephen Jay Gould's elusive "spandrels o fevolution". Personally I don't think that is the case - the ubiquity of iridescence in numerous understory reptiles, birds, insects and plants points to some adaptive value.

And yes understory plants feature iridescence. Here we revisit an old friend of the blog the lycopsid genus Selaginella species name willdenowii. Discussed in this blog post and in this research paper this clubmoss (not a fern!!) shows striking blue iridescence and lives in deep shade.

Many hypotheses have been put forth trying to explain the blue iridescence of S. willdenowii and other deep shade blue iridescent plants. Ideas centered on better absorption of red color wavelength light in deep shade but this hypothesis was not shown by Thomas et al. (2010). Instead blue iridescence might offer several adaptive benefits; a photoprotective mechanism that shielded deep shade plants from sudden exposure to full sun; a visual defense against herbivores. Obviously the second hypothesis is of our interest and I will cut and paste the explanation of Thomas et al. (2010).

This adds another interesting wrinkly to the notion of structural coloration used to evade detection. As iridescence can change depending on the angle of the viewer another potential role for iridescence is confusion i.e. what the authors refer to as "making it harder for insects to form a search image". Note that this shimmering - alternatively blending and contrasting with the background - might offer a potential use for a predator as well by confusing and confounding prey - bedazzling them.

Imagine you are an enantiornithine sleeping high up in your roost at night in a large ginkgo. You are among many other birds. Suddenly you wake up detecting movement coming towards you. It is an animal of the same size as the other birds but it alternatively comes into and out of view in a transfixing beguilement of moon reflection and blackness. Your curiosity and inability to form a solid image of the animal is all the hesitation the predator needs. Just as you tense up to fly it is upon you, jaws and killing claws piercing your vitals.

As I mentioned earlier iridescence and what it means for living organisms is very much at the cutting edge of science for things much less long dead ones. But if the present is the key to past...

Here is a cut-out form a very pertinent review article titled Iridescence; a functional perspective

"If you know how to manipulate the way light moves and reflects then you can make a surface brighter or darker."

Vukusic sees the potential for developing a type of camouflage that mimics the color of the surrounding environment - the obvious paramilitary advantage needs not be overstated. Vukusic sees tremendous potential in moths, several species of which he states have developed perfect nocturnal crypsis.

Moths have evolved modifications in their wing scales that swallow up light so their wings appear black:

"Instead of multilayers, some types of moth wings have thousands of tiny, nipple shaped structures arranged in a hexagonal array. If they are really small, half the wavelength of light, then their effect is to reduce reflection from the wings surface" Vukusic says.

Want to go deeper down the wormhole of biomimicry inspired paramilitary technology? How to disappear completely.

A little unsure about what Microraptor was really up to with its iridescence? Or why so many diurnal active nocturnal tree roosting birds are painted black and iridescent? What purpose does that serve to be all black and stand out during the day? Why is iridescence so common in understory plants and animals? Do iridescent animals shine and shimmer when under dense canopy and/or the pale moonlight? Or does their brilliant iridescence take on another more subdued, cryptic usage under dull light? Feeling a little off balance? A little wobbly? A little less than certain at what you are really looking at in Microraptor through the obfuscating lens of millions of years of subsequent avian evolution, advanced passerine mobbing defense, and open grassland ecosystems? If you are feeling a little wobbly, a little confused, a little bedazzled at what you are actually looking at in Microraptor then good - that is what Microraptor wants you to feel before it snatches your life's breath away.

Up next I will be looking at a theropod that you may have heard about. A theropod on the complete opposite side of the size spectrum. I will be elaborating on how it was a predator like none we have seen before or since. Consider yourself put on notice.

Papers

Doucet, S., Meadows, M.G. (2009) Iridescence: A Functional Perspective. Royal Society Publishing 2009: April 6. online here

Li, Quanguo (9 March 2012), "Reconstruction of Microraptor and the Evolution of Iridescent Plumage", Science, 335: 1215–1219, doi:10.1126/science.1213780, PMID 22403389

Schmitz L, Motani R (2011). "Nocturnality in dinosaurs inferred from scleral ring and orbit morphology". Science. 332 (6030): 705–8. doi:10.1126/science.1200043. PMID 21493820