Saturday, September 24, 2016

Night Stalker Rex Part I: Sue Is Built Like A Brickhouse

Here it is folks - the king of all kings, the prize fighter of antiquity, the grand poobah, good ol' sexy rexy himself - as you have never imagined before. Some say that we know more about Tyrannosaurus rex than other extinct animals - or even extant ones - and it is high time other lesser known animals should receive attention. Until recently I was of the same notion. However a new flurry of T.  rex research - highlighting it as an intelligent "super senses" endowed predator - has stirred a quiet but growing mumbling on what it really meant to be the tyrant lizard king. Emerging from the haze of past interpretations a more resolute and astonishing picture of T. rex has been forming in my mind. It is a startling beast quite distinct from past visages but not without a newfound ghoulish and nightmarish quality all of its own. Ladies and gentleman I give you Night Stalker Rex.

credit Robin Liesens


"Now wait a second here" you might be thinking "I have heard about likely nocturnal behavior in T. rex and other theropods before - nothing new here." And you are right, it has been suggested before. Indeed a central tenet of this hypothesis - the specialized detection and predation of sleeping dinosaurs by T. rex - has been at least cursorily mentioned at least once on the interwebz.  But you will not find the idea sold to you with quite the same zest and zeal that you will get from me. And you will not find several new and compelling lines of evidence put forth by me that further leverage and embellish the night stalker rex hypothesis.

To get the cognitive gears pumping I want to address a potential role of nocturnal hunting in a modern day T. rex sized predator - the killer whale (Orca orcinus).


This video is astonishing. What really captures my interest is not so much the explosive action and graphic violence but the more economic questions of foraging efficiency. The pod of orcas has obviously positioned themselves at depth - maybe even partially concealed by features we can't discern from the surface - and execute a precise ambush on a large pod of common dolphins. The orcas - which combined as a predatory arsenal must weigh several dozens of tons with appetites to match - expend much energy, foresight, and effort to snatch up just one small dolphin. For what amounts to basically a hard won "snack" for one orca much less for the whole pod the question arises "how efficient is such a foraging tactic for these massive, hot blooded, predators?" Now that the pod of dolphins is aware of the orcas seems the chance for another capture is remote... Not a great optimal foraging capability for the oceans top predator... Or are we looking at how orcas hunt - or more precisely "when" they hunt - in totally the wrong manner? Ask yourself this: for a predator that utilizes stealth to hunt a very agile and intelligent prey would it not be even more efficient for orcas to utilize the cloak of darkness for better tactical advantage? In diurnal predatory events in orca are we not in fact witnessing the exception to the rule of a generally nocturnal predator?

It is paramount to take heed of the obvious bias in wild orca research. Humans are diurnal. Humans are not marine. Most studies of wild orca will be conducted during the day for obvious reasons of practicality, safety, and ease of observation. Working from a research boat on difficult seas it is patently obvious why the overwhelming majority of wild orca research would occur during the day. Because wild orca are mainly observed during the day and because predation events are therefore only observed during the day the emerging bias becomes reinforcing - Orca are diurnal and do their hunting during the day.

However all may not be as it seems with the blackfish and an emerging trickle of data might in fact point to a more nocturnally active hunter than previously suspected.

By recording vocal activity at night at St. Paul island in the Bering Sea researchers K. Newman and A.M. Springer were able to elucidate not only heightened vocal activity at night in transient marine mammal hunting killer whales but they attributed this to predation events. Not only were calls recorded during the night but vocalizations peaked 1 hour after sunset and were more common from midnight to noon than noon to midnight (keep in mind the long days of the northern summer). Although transient killer whales remain silent during the hunt, after a chase or kill is initiated a flurry of calls commences - which when combined with the nocturnal foraging of the prey animal (northern fur seal) - led the researchers to conclude that nocturnal foraging was very important for these transient killer whales.

Could we be on the cusp of recognizing the importance of nocturnal foraging in many or even most populations of killer whales? The obvious utility of ambush technique shown in the above video suggests that such tactics would only be more adventitious at night, especially for an acoustically sensitive predator. The possibility is certainly there and it is easy to see why a bias of diurnal activity in these animals took root. Such a reversal in our thinking is certainly not without precedent.

Nocturnal hunting in great white sharks was only recently documented overturning the diurnal dogma that afflicted the nature of these fish; spotted hyenas long assumed to be solely scavengers from day time observations but long term studies including night time observation elucidated their predatory nature; and the king of beasts has long been known to be a primarily nocturnal hunter - what is less appreciated is that the male lion - long regarded as the inferior hunter compared to females - can actually hold his own as a nocturnal ambush predator of thick brush.

from here
Game cameras are revealing startling and explosive predatory action that occurs under the cloak of night such as this wild chase and capture of feral pigs by an American black bear - an animal not usually noted for stalk, ambush, and predatory technique.



Hunting dangerous, elusive, and quick quarry by predators under the cloak of night is a time honored tradition. Intuitively this makes sense, better to stalk and ambush prey from darkness. Nothing particularly revelatory about that. However there is a seldom mentioned facet of nocturnal predation that - when your prey is herbivorous - consistently tips the balance of power in favor of the predator. It is an inherent advantage the predator has that the herbivore can really do nothing about. One has to ask the question before one can come to an answer: "Why, if nocturnal vision is so advantageous for nocturnal predators, have not herbivorous prey answered the evolutionary arms race by evolving excellent nocturnal vision themselves? I mean, its not like they have not had enough time to evolve excellent night vision as this nocturnal depredation has been going on for some time likely. Darwinian evolution would almost predict such an advantageous adaptation arising."

The answer to be blunt is that herbivores can not, and likely never have, equalled the superior night vision of their predators because they can't. And it's because of what they eat.

Luckily enough through the power of google search I was able to source this little free preview snippet which I will provide below from Essential Fatty Acids and Eicosanoids: Invited Papers from the Third International Congress (ed Sinclair & Gibson 1992) from none other than the American Oil Chemist's Society:


Vitamin A is concentrated in animal tissue but scarce in plants. It is essential for night vision and because predators have a ready and pre-made form of it they will always have a greater capacity for night vision relative to herbivores. Not because herbivores would not benefit from good night vision but because of biochemistry. Yeah science!! If the present is the key to the past and the same unequal playing field occurred in dinosaurs (no reason to think that it didn't) there was likely a high bias of nocturnal theropods stalking the Mesozoic nights. T. rex - as an obligate hunter - certainly slots in nicely to this realm.

The question then becomes "well if T. rex was a nocturnal hunter what type of hunting strategy did it use?". Various methods could and likely did take place such as ambush  at known prey "hot spots", stampeding prey into confusion, stalking of prey in dense foliage, pursuit etc etc. Long story short I think all of these tactics were utilized during the different ontogenetic stages of T. rex. The more light and leggy youngsters utilizing more athletic, running pursuit strategies morphing into a more stealthy, ambush style predatory tactic with the onset of robust build and large mass.

For this hypothesis I want to concentrate on the onto-morph of the adult T. rex. We are talking about Sue sized rex here. And this is an important distinction because a tenet of this hypothesis is that the hunting strategy of other, smaller  tyrant lizard species was encapsulated in the ontogenetic history of T. rex. Essentially in moving up in size through sprightly large coelurosaur sized juveniles, to Albertosaurus like teenagers, to Daspletosaurus sized subadults the behavioral ecology of these respective tyrants was mimicked. But by the time we get to "Sue" sized adults T. rex was playing a different predatory ball game altogether.

Sue Is A Brickhouse - Built Like an Amazon



First of all time to talk about the elephant in the room when it comes to T. rex. And I literally mean elephant in the room. Sue not only was big, she was a certifiable fatty. Sue was not just a tad bit on the hefty side, she would have in life appeared ponderous and round to an almost cumbersome degree. Seriously T. rex as depicted in paleoart is probably the most shrink wrapped, trimmed up, and "wishfully" sveltely depicted  prehistoric animal of all time. Its like there is a collective denial of T.  rex's true body type. T. rex paleoart is the equivalent of gaining a few pounds (or a lot) and keeping that selfie around on social media from when you were trim. And I am not just discussing fan art or deviantart renderings of T. rex - I am talking about the big name "world renowned" paleoartists. You can take your pick, I say that they all underestimate the genuine "girthiness" of ol' sexy rexy. Not by a little, but by a lot mind you. It's time we embrace the big rex and stop the body shaming denial. Big is beautiful!!

If you want to move towards a more realistic countenance of T. rex draw an animal fatter and more rounded than pretty much all other depictions. Now make that animal 20% larger still!!



Have your doubts? Remember there was that little study published a bit ago by Hutchinson & Makovicky? They found that previous estimates were substantially low and their computer modelling suggested an increase of about 30% pushing Sue up to about 18,000 lbs or 9 tons - and they call this size on the conservative range!!



I got a chance to visit with Sue and talk to her about her self image and eating issues as she recently stayed at the Santa Barbara Museum of Natural History. Let me tell you that song "Brickhouse" (She's a brickhouse, just letting it all hang out") does not even come close to doing her justice.

"Sue" credit Duane Nash

When you really look at Sue - and to a lesser extent smaller adult rexes - once you get past the huge maw, lethal bananas, and overall size - you have to be impressed with that barrel chest. I mean come on now, if you take the perspective of the above photo and add on even just a smattering of integument, skin, muscle, and fat to the torso you quite literally would not see the hips from behind that thick barrel chest!!

"Sue" torso credit Duane Nash

I mean really now, it's just ridiculous. Especially when you compare the torso against other slab chested theropods or even other herbivorous dinosaurs for crying out loud. Go look at the various museum mounts of T. rex mounted in pursuit of herbivorous prey - the degree of roundedness in the torso of rex even crushes the giant, rear fermenting sauropods, ceratopsids, and hadrosaurs it was hunting. While not as wide as ankylosaurids T. rex certainly had a deeper chest than they did. Also compared against the torso of canids, felids, and even ursids T. rex looks unambiguously ahead of the curve in terms of a massive torso.

The real elephant in the room is not that T. rex had a massive barrel chest - that has been known and commented upon for some time - the issue is that no other terrestrial tetrapod predator has such a barrel chest!! You really have to go into the aquatic realm to find such girthy predators, animals that have escaped the burdens of gravity. Obviously T. rex is no whale or aquatic animal - it still had to operate under the confines of 1 G - but I do think it had escaped the traditional limits of what it means to be an agile, cursorial predator. No longer hemmed in by ecological and functional constraints of maintaining high degrees of speed and agility other evolutionary pressures dictated an increase in general size, robusticity, and overall swelling of the tyrannosaurid bauplan. These evolutionary pressures included territorial defense, intimidation of rivals, garnering mating privileges, and storage of fat for lean times. All of these Darwinian benefits would dictate and be benefited by increase in size/girth but only after the evolutionary pressures that necessitated speed & agility were lifted. In other words T. rex only could become the T. rex we know and love after it developed uncanny predatory technique as a cryptic, nocturnal, super senses equipped, arch predator specializing in detecting, infiltrating, and apprehending sleeping dinosaurs.

So how fast was T. rex

Obviously this is not a full on review of the research into tyrannosaurid speed. But it is my personal reconciliation of the data and not at all incongruent with accruing data and more coming down the pike suggesting that for T. rex (and probably many gigantic theropods) they were NOT SO FAST (or more importantly agile).

People love to ask this question, and paleontologists love to give eternally "sitting on the fence type answers" - as they should because we really don't know. Not only that but solid, concrete speed numbers on most extant animals is lacking. What I will say is this. Unless Usain Bolt is a secret paleo fan and reader of antediluvian salad, T. rex is probably faster than anyone reading this blog. Now, one statement I hear again and again is that "it does not really matter how fast T. rex was as long as it was faster than its prey". Which on the face of it seems like a reasonable answer, if you are assuming that T. rex was a bit of a pursuit predator and that leg length is a prime determinant of speed. But I am not quite so sold on this line of thinking because there are some notable exceptions - chiefly bears - which constantly fly in the face of the dogma that dictate long lower legs equal high speed.

Based on relative leg length we should expect camels to thoroughly smash bears when it comes to speed...




If both bears and camels were extinct based on comparing lower leg elements the camel would be asserted to be faster. The camel can indeed move pretty fast - indeed I was astonished when actually seeing them get into a full on gallop in the video below - but I would not by any means be confident that in short bursts bears are not just as fast as the longer legged camels.





When it comes to speed in T. rex, its likely prey base, and what it means to be a good & efficient predator I think the questions we ask play a big role in the solutions we seek. When we ask "how fast was T. rex?" implied and embedded in this question is that pursuit was important for T. rex and however fast it was it had to have been faster than its prey base in order to make a living. Instead of trying to get to answers based on the question of speed - which may in fact be a wash when compared against its prey base because of bears and how they break the rules - let us instead ask a more incisive and telling question: "how agile was T. rex?" This line of thinking has more merit to it than simply asking "how fast?" because when you couple the agility of T. rex with the attributes of its prey base there are some significant conclusions to be drawn.

T. rex was horrendously not agile. Indeed it is hard to imagine nature coming up with a design less equipped to handle tight turning. A tall biped, long and heavy all over. Once it gets a head of steam going in one direction it has horrible turning ability. Kind of like running while carrying a big heavy, long log and being asked to twist and turn. Hutchinson (yes the same Hutchinson form the revised mass estimate paper) came to this conclusion when he looked at the turning ability of rex. Ultimately the study concluded that T. rex took a full 1-2 seconds to make a quarter turn (45 degrees).

Past assertions have all resorted to the stock answer that T. rex was "just good enough" to chase down its gigantic prey base are lacking. First of all we don't have reliable speed indexes of modern animals much less extinct ones. The ability of short legged bears to sprint at the speed of long legged ungulates casts doubt on the mere long legged argument to infer high speed. We can't be confident that T. rex was faster than Edmontosaurus, Triceratops, or hell, even Ankylosaurus!?! Short legs, if well muscled and full of bouncy tendon can still do the same job of long legs. But one thing we can be certain of - more than the relative speed argument - is the relative agility index between T. rex and its presumed prey. And here we can see - unequivocally - that T. rex is not on a level playing field with its prey, all of which were quadrupedal with a lower center of gravity and better turning ability. If we combine this with the possibility that some of these herbivores were as fast or even faster (da' bears) than ol' sexy rexy we come to the distinct possibility that these animals could run circles around ol' rexy. Indeed if T. rex was a pursuit predator of gigantic and dangerous prey that could easily outrun, outmanoeuvre, and kill it T. rex would have quickly become erased from the fossil record. An obsolete and inefficient design with glaring flaws that can not be ignored or explained away.


Quite a predicament for rexy to be in if it somehow had to make a living off of catching these animals and - especially in the case of gigantic hadrosaurds, ceratopsians and ankylosaurids - these animals could out manoeuvre you and potentially mortally wound you too!! What is a T. rex to do?


The answer is that (adult morph) T. rex did not chase much of anything down except maybe another T. rex. Ol' sexy rexy eschewed the whole speed game altogether and in doing so achieved both great hunting prowess and great size. It snuck up on dinosaurs both awake and asleep - although sleeping dinosaurs became more of a specialization in larger rexes - under cloak of darkness. The average "chase" was measured in just a few meters or even centimeters. This opened up rexy to exploit not just the large ceratopsians, hadrosaurids, and ankylosaurids that it shared its habitat with but rex could now exploit all the smaller and even more agile dinosaurs it shared its habitat with. A true ruler - a tyrannical rex in every sense of the word - that by highlighting stealth and negating speed it could demand caloric tribute from every underling in its kingdom. Literally nothing was safe in the kingdom of rex - everything from armored ankylosaurids to speedy ornithomimids - could and did end up in the belly of the tyrant ruler king.

I guess I was a little naive in imagining I could get through this hypothesis in just one post. In this post I wanted to concentrate on the unparalleled girth of rex, highlight the fundamental underestimate of rex's size both in both paleoart and technical literature, and show why, if there is any sort of consensus on rex speed & agility, it should be moving in a direction of caution with regards to extremes in both of these dimensions. 

Up next I want to delve into the super senses of T. rex, how to be a giant stalker, and why T. rex had a "childhood".





Refs

Hutchinson JR, Ng-Thow-Hing V, Anderson FC. A 3-D interactive method for estimating body segmental parameters in animals: application to the turning and running performance of Tyrannosaurus rex (2007) Journal of Theoretical Biology vol 246 Issue 4. 21 June 2007 abstract

Hutchinson JR, Bates KT, Molnar J, Allen V, Makovicky PJ (2011) A Computational Analysis of Limb and Body Dimensions in Tyrannosaurus rex with Implications for Locomotion, Ontogeny, and Growth. PLoS ONE 6(10): e26037. doi:10.1371/journal.pone.0026037

Newman, K, Springer, AM. Nocturnal activity by mammal-eating killer whales at a predation hot spot  in the Bering Sea (2008). Marine Mammal Science, 24(4): 990-999 (October 2008)

Zelenitsky, DK, Therrien, F, Yoshitsugi, K. Olfactory acuitty in theropods: paleobiological and evolutionary insights (2009) Proceedings of the Royal Society of Biological Sciences. link



Wednesday, September 14, 2016

Microraptor Shenanigans Part II: Like a Shadow in the Night

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.

credit George Hodan. public domain

Working from an all black plumage in Microraptor the mind necessarily thinks about modern day aves that don an all black attire. Various corvids come to mind - especially the common crow and raven - as well as grackles, and other various birds colloquially known as "blackbirds" in the passerine group. And this is where the confusion and typological thinking really mires us down in our interpretation of Microraptor.

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...

Blackbirds & Starlings in field credit USDA
And here I have to venture forth a hypothesis - a reasonable enough one I think - that the black or darkened coloration of many open habitat communal foraging birds is highly contrasting with the environment for a reason and that is in order to achieve group cohesion. I honestly don't know if this has been suggested before ;') . Birds can detect other foraging birds from distance and therefore keep social unity. More individuals in one place will actually lower the predator risk to any one individual at the same time allow more eyes for predatory vigilance. Having a dark black coloration would also be selected for as it allows birds to spot other birds foraging in food rich areas. That many of the birds engage in predatory mobbing type behavior plays into this.

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.

crows at roost credit (c) Daniel Hoherd
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.

credit Gustave Dore "Not the least obeisance made he" public domain
"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?

Sturnus vulgaris credit Tim Felke. CC2.0

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.

Murmuration of Starlings preparing to roost. Scotland credit Walter Baxter CC2.0
Throwing another wrinkle in the equation what about the dark colored, iridescent cormorants? These are predators and like Microraptor inhabit and hunt in a dark, shadowy realm...

credit birds of the world

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.



What manner of beguiling tricks of light and shadow do these predators play on their prey in the dark cracks and crevices of their deep forested environment? It should not go unstated that both species are nocturnal...

Boelini Python credit Marc A. Spotoro

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.

credit Marc A. Spotoro
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.

S. willdenowii. credit Andre Cardosa
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


The authors are keenly aware that what can be used to hide prey from predators can also be utilized to disguise predator from prey:



Militaries Study Animals for Cutting - Edge Camouflage

credit LeeRobertsMe CC2.0

The males of the blue morpho family of butterflies use blue iridescence displays that can be seen from up to a 1 km away announce territory. Peter Vukusic, a physicist at Exeter University of England has been studying how the wing scales of these insects influences light waves. He posits that understanding how these scales work is the first step in creating a revolutionary style of camouflage.

"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. 




And so concludes my little miniseries on Microraptor. A strong case can be made for a highly efficient, nocturnal, cryptic, multi-dimensional predator of small things. Being a predator of small things is no easy feet. Not only are you yourself a prey animal so every bit of time you spend foraging you are exposed to predation but the opportunities for facultative scavenging and usurping easy meals from other animals are diminished. Small bodies lose heat quicker than big ones so that small hot blooded predators need to eat a lot. A small animal predator can't afford to be inefficient. If small felids and mustelids worked with the same efficiency of killing as their larger cohorts they would be soon out of a job. A Microraptor that was a poor runner, poor flyer, and poor climber just would not make it. Instead it was fairly competent in all these realms - and maybe even swimming too (like a dipper). We sometimes speak of the large arch-predators as nature's perfect killing machines but it is really the small killers that outshine the larger ones in just about every metric of killing efficiency. No reason to suspect diminutive dinosaurian predators were any less savvy, efficient, and voracious than their modern ecological equivalents.

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", Science335: 1215–1219, doi:10.1126/science.1213780PMID 22403389

Schmitz L, Motani R (2011). "Nocturnality in dinosaurs inferred from scleral ring and orbit morphology". Science332 (6030): 705–8. doi:10.1126/science.1200043PMID 21493820


"A Long habit of not thinking a thing wrong, gives it a superficial appearance of being right, and raises at first a formidable outcry in defense of custom". Thomas Paine


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Friday, August 26, 2016

Microraptor Shenanigans Part I: Hind Wings & Stealth

Boy I really stepped in it now...  Microraptor. For a long time I avoided paraves, Microraptor, the whole dino-bird subject because; there is a lot of talk about maniraptorans on the web already; I enjoy focusing on "meatosaurs" (and still do); and, well, quite literally this whole backwater of quasi dinosaur, quasi bird conundrums literally ruins people. I mean it is a war zone out there folks. Seriously now, academic career types - not just free spirited blogger/idea spinners like yours truly - have gotten their heels so dug in on various sides of the debate it is quite astonishing. The stakes are high in this subject of birdy origins and history will see winners and losers... So why not dive in with Microraptor probably one of the most celebrated, controversial, and perplexing dinosaurs and certainly maniraptorans of the last 25 years? I would definitely put this animal in an unholy triumvirate with Deinonychus and Archaeopteryx in terms of paradigm shifting, scratching your head, game changing iconoclastic status. It really is an odd bird and one we still have loads of unanswered questions about.

closeup cast credit Hiroshi Nishimoto CC 2.0

So for this piece I am mainly going to side step questions about the aerodynamic efficiency of Microraptor. I don't have a wind tunnel, nor a model to test it in, nor an advanced understanding of aerodynamics. To summarize my current opinion on Microraptor's flight capability I am hedging towards rather limited flight capability - possibly more of a burst flyer of short distances, great maneuverability, with some gliding ability though far from ideal due to all that drag it incurs. I think that the biplane model with the legs spread wide is far from established as I still have questions about ,well, could it anatomically splay its legs? So for now I am going to go with a base line of fairly limited flight capability with pretty good maneuverability and pitch control via the tail and hind wings. I am not wedded to this interpretation and am open to new insight.

I will be working from this point in my attempt at a reasonable lifestyle analysis of this critter. I think it fairly conservative and in line with much of the research into this animal.

William Beebes' prescient 1915 "Tetrapteryx" 1915 public domain

Now onto another very contentious issue: terrestrial or arboreal? In order to untangle why this issue is so contentious - I don't think it need be actually - it is worth looking at this question through another lens. The issue of terrestrial versus arboreal in Microraptor has double implications because whether or not this animal was terrestrial or arboreal provides a robust layer of argument to the issue of the competing ground up or trees down hypotheses in the origin of flight in aves proper. And we all know how contentious that issue is and how various factions have their heels dug in on either side of the debate.

The answer to the question of arboreal versus terrestrial habits in Microraptor is quite literally that there is no question of arboreal versus terrestrial in Microraptor's head. And it is in the headspace of Microraptor that we need to get into here - not the head of a tenured professor with a life times' academic stake in the question of ground up or trees down origin of flight. That is quite literally the last head we need to get into in this question. We need to get into the head of a 1 to 2 kilogram predator of small things. An animal that goes recklessly and relentlessly where its prey is. If that prey is roosting 20 meters up a giant old growth ginkgo tree by golly that is where Microraptor is going. If prey is swimming and swarming in the shallows of a lake Microraptor is having a go at that prey. If prey is scurrying from the safety of a burrow in the still of the night Microraptor is right there to greet them. If prey is secreted away deep inside the hollow of fallen tree trunk, Microraptor is sniffing out that quarry and going in that rotten log and making their life hell. Microraptor is kicking ass and taking names and if you are a small swarming, swimming, flying, or scurrying thing and living in the same neighborhood as Microraptor you better watch your back because Microraptor is coming.



"Lock up your children the axeman is coming"

Yes in the trees, in the shallows, in the little cracks and crevices of the environment where the small things hide out Microraptor is going there. If Microraptor could evolve a human intelligence and humour and knew of our debates concerning it Microraptor would be laughing at the pedantic back & forth of terrestrial versus arboreal because Microraptor goes where Microraptor wants to go. Microraptor does not see a sharp division of terrestrial versus arboreal realms. Microraptor sees a 3 dimensional world where the subterranean, aquatic, terrestrial, arboreal, and aerial bleed into one another and Microraptor bleeds into them and exploits them all as it sees fit.

Seeing the world in such a 3 dimensional matrix of positional vortexes is not at all dissonant with how modern supple fingered, clawed, predatory, and small bodied tetrapods exploit and utilize their environment. We see echoes of this multi-functionality in mustelids, varanids, viverids, and probably most intuitively felids, especially our own house cats. I think it was Thomas Holtz who compared microraptors to small felids and I think the comparison is very apt. If you have ever wondered why your house cat has a penchant for seeking high ground - both to hide from threats and survey potential prey - you are seeing echoes of Microraptor in fluffy. I can also recall Darren Naish mentioning in several blog posts that although microraptorines (and other dromies) did not have highly specialized arboreal capabilities that does not mean we should exclude them from scrambling around in the trees, bushes, i.e. exploiting the >shrubbery<.

Of course the bauplan Microraptor started with is different from mammals and lizards. Lacking the low body plan as well as the supple and flexible limbs and torso that allow other tetrapod predators such as lizards and mammals to excel at this particular eco-niche microraptorines evolved alternative solutions to the problems of navigating a cluttered, three dimensional eco-space. The wings on the forearms served to break falls that occurred at elevation, aide in terrestrial balance and pursuit, and provide gliding and flight. Note that I am not concerned with the question of >what< stimulated the evolution of the wings in microraptorines merely how such wings would be useful for a theropod navigating three dimensional space in the pursuit of small game. The >what< of this question (i.e. what caused evolution of wings on the forelimbs) may never be boiled down into a single causal factor but a multiplicity.

Getting back down to earth, literally, I want to address a bit of non-truth that has consistently swirled around microraptorines for a while now. That piece of misinformation - a truism that has always been asserted but never actually demonstrated - is that their "hind-wings" severely hampered terrestrial movement. I see this assertion upheld again and again in both scientific and popular discussions. The idea being presumably that such flight feathers would be a hindrance in locomotion, get caught up in vegetation , and generally cause a kerflunkle for any microraptor trying to just... walk around.  That animals with relatively long cursorial looking rear legs; that plot out very much like ground birds morpho-metrically; that this animal could not run or even walk well points to a very mal-adapted and cumbersome beast. Caught in sort of an evolutionary purgatory - not yet an accomplished flyer, glider or climber as well as a cumbersome terrestrial mover - such an animal begs for a quick extinction. Seriously this notion of a terrestrial limited Microraptor should have always have been a non-starter. Kind of like sauropods that could not walk on land. Or giant pterosaurs that could not fly.  The kind of "evolutionary experiment" that paleontologists like to posit but can't find any examples of in the real world. Because in the real world their overly cumbersome traits would select them for Darwinian elimination before they even had a statistical chance of entering the fossil record.

Again, we need to get back to what it means to be a predatory small bodied, hot blooded terrestrial tetrapod. You are not just an OKish hunter that sort of blunders through the environment hoping to get lucky enough to snag a meal or two. Such an animal would be selected for extinction quite readily. Nope you are an efficient, ruthless, and relentless hunter. Just look at the pedigree of hunting prowess in small felids and especially small mustelids (i.e. "weasel" types). The true acme of hunting prowess in mammals is not to be found in large canids or felids - they fail all the time. Small bodied predators have to catch a lot more due to their relatively higher metabolism and the relatively small size of their prey. They can't afford to only have a 5% success rate. They are hungry and they are coming. No reason to assume among predatory theropods microraptorines were not the acme of theropod hunting prowess. In fact evolutionary pressure and comparison to modern small bodied tetrapod hunters would dictate that they are.

I mean, come on now, just look at those legs.

Specimens by Jaime Headden CC3.0
 (User:Qilong) - http://qilong.deviantart.com/art/Microraptor-Skeletons-27326378

Microraptorines could move and move around well on the ground. Assertions that the feathers on the legs would cause undue hardship and toil are simply not meted out when viewed against the panoply of abuse modern birds put their flight feathers through. I was recently watching the flock of California condors that are maintained at the Santa Barbara Zoological Gardens. Splendid birds and you really get a sense for the nuanced social dynamics at play when you watch them for some amount of time. One of the birds sort of crash landed into the branches of a large oak tree in the enclosure after a small skirmish. Mind you this was a California oak (not sure the species) and it had the harsh, spiky leaves of a oak adapted to the drought prone, semi-arid climate of California quite unlike the soft rounded leaves you might encounter on oak leaves in more verdant climes. Anyways, this was no soft landing that the condor made into the tree and it was totally caught up in the brittle, spiky canopy. The bird (second heaviest flying bird in North America) then proceeded to use its wings (e.g. its flight feathers) to literally "swim fly" through the brambly and spiky oak foliage until it could get up to a solid enough limb to perch upon. I got the feeling this was no extraordinary ordeal for the bird and had probably done it a number of times. Not a flight feather lost or even seemingly damaged. Long story short flight feathers - whether on the arm or leg - are not the wimpy structures sometimes assumed to be and are  deeply anchored within the dermis or even into the bone and can both receive and deliver substantial abuse and punishment. Microraptorine leg feathers were doubtless anchored well into the leg . Furthermore there is no reason that the feathers on the leg would not have been able to fold up against one another as the leg moved or crouched. It is also entirely possible that such feathers could be moved so as not to scrape against the ground. This is no different than what the flight feathers on the arms of birds and maniraptorans presumably are able to do.

Anyone who argues that bird feathers - especially the pennaceous "flight" feathers whether on the legs or arms - are prone to falling off, getting damaged, or just inhibiting to violent, physical interactions just has not been paying attention to what birds actually do.



Now that we put that whole notion of "could not walk well" to bed for maniraptorines I want to dive a bit deeper into potential uses for the leg feathers. I want to stipulate that these ideas are not mutually exclusive from any of the flight scenarios that people have posited for these animals. Remember that I am not concerned with could or how well Microraptor flew but creating a lifestyle portrait of a small animal killer. Being a specialized small animal killer is an easy inference given the diversity of, well, small animals found in the guts of Microraptor.

To make my case I first want to bracket Microraptor between several other theropods that sported leg feathers. Yes leg feathers were quite en vogue look for several theropods back in the day and not all of them were microraptorines.

Pedopenna daohugouensis. Remember this guy? Yeah I don't either. Just another in the long and confusing line of maniraptorines getting pulled out of the ground in China, I would be lying to you if I don't constantly have to 2x check the names of these guys. Except this species is one that you should take note of. First of all it predates Microraptor and the other Liaoning/Jehol fauna as it dates to somewhere in the mid-late Jurassic (age of the Daohugou beds are debated) but more importantly for our purposes here the legs are feathered - they had hind wings!! The legs are all we really know of this animal and according to wiki: "the long pennaceous feathers of the foot... differ from those of animals like Microraptor. Pedopenna hind wings were smaller and more rounded in shape. The longest feathers were slightly shorter than the metatarsus at about 55 mm (2 in.) long. Additionally, the feathers of Pedopenna were symmetrical, unlike the asymmetrical feathers of some deinonychosaurs (including Microraptor) and modern birds."

The symmetrical feathers of the hind wings in Pedopenna (and Anchiornis btw) are interesting as this suggests that they did not have a use aerodynamically. The hind wings are potentially vestigial and point to a flighted or at least gliding ancestor for Pedopenna (Xu & Zhang, 2005). This invokes the strong potential for "microraptorine" style flyer/gliders going back into the mid-Jurassic - very cool. This also suggests that flight could have evolved, got lost, got regained, and evolved in multiple parallel branches over a fairly long span of time. However I am not quite so sold on Pedopenna and Anchiornis being secondarily flightless. If this was true why would they so quickly lose  asymmetrical feathers? Would not asymmetrical feathers still be of use in short glides and other such aerial ventures? I mean, these were not large animals so retention of some aerial ability would still be of use in gliding one would presume?

Time to highlight the seldom mentioned fact that the giant compsognathid Sinocalliopteryx also is noted for non-pennaceous feathers along the back of the metarsus and longer ones on the back of the thigh. These feathers did not form a hind wing nor are compsognathids considered to be on their way to flightedness or secondarily flightless. Yet there was some congruence in evolving feathers along the trailing edge of the lower legs in these disparate animals.

To add a further layer to the enigma of hind wings and lower leg feathers is the amazing microraptor on steroids, Changyuraptor a turkey sized double winged and exceptionally long tailed microraptorine that, despite its size, still points towards being somewhat flight capable or at least gliding.

turkey sized microraptor Changyuraraptor credit Emily Willoughby CC4.0
Although I have my doubts about how long this animal could sustain powered flight, it is always good to remember that wild turkeys and peacocks do fly and they also go into trees as well.



I do think it pays to take a look at these flying large galliforme videos. Notice the seemless transition from an arboreal start in the trees to a short distance flighted phase effortlessly transitioning to an efficient terrestrial phase. Even at this size a turkey sees, interprets, and inhabits its world in a 3-dimensional matrix. I guess this "ground bird" with no obvious arboreal adaptations never got the message that it could not use the trees as it sees fit. Not impressed with flying turkey how about flying peacocks - another "ground bird" that goes where it wants to.


And watch these peacocks in China seemingly sail down en mass from elevation. Reminds me a lot of those turkeys sailing in from the tree tops in the earlier video. Very cool.


We can look at Microraptor not as an isolated hind wing animal but one that slots, or is bracketed in a sense not just phylogenetically but behaviorally,  between a number of hind wing or at least feathered leg/metatarsal having theropods. And I think it important to speak towards ecological and behavioral congruity in these theropods because, irrespective of phylogeny, similar lifestyles often confer similar adaptations by convergence.

And the unifying behavioral characteristic that unified microraptorines, Anchiornis, Pedopenna, and Sinocalliopteryx is that they were all terrorists of small game. I have actually discussed Sinocalliopteryx gigas before here. Sinocalliopteryx was obviously not arboreal and it was not flighted, near flighted, or secondarily flightless. But it was really good at snacking on animals that were flighted and/or more inclined towards aerial/arboreal capabilities than it was hence the belly full of dromie leg & enantiornithines. Because of this the authors suggest that it was a stealth hunter, a notion that is hard to dismiss.

Unpacking what it means to be a stealth hunter with a theropod bauplan I believe is key to illuminating what stimulated the evolution of hind wings.

When you are a hunter of small things that means that you have to go to where the small things are at. That implies negotiating tight spaces and deep, dense vegetation. If you are negotiating this eco-morphospace as a small mammal, lizard, or better yet snake - no problem you are already low to the ground with supple legs and torso. But if you have the stiff torso and the tall stiff action limbs of a theropod there is less of an exaptation for maneuvering in such tight places which means there is even more impetus on stealth. A theropod stealthy stalker of small game has to get as close as it can as quietly as it can because whatever you are stalking - chances are that once it detects you it has the better capabilities in navigating tight spaces (or just flying or swimming away). And so imagine the stealthy stalking ability of modern herons but imagine it occurring not while wading but while in deep vegetation. One of the problems such an animal would encounter is curtailing or muffling the disturbance and noise that would occur by walking on and brushing against vegetation/leaf litter/brush with those long, stiff action hind legs. A soft, padded foot would help with muffling footfalls. Likewise a trailing line of feathers along the back of the leg and going down the metatarsus to the foot would potentially muffle or curtail the audible "snapback" of brush that occurs when a leg moves through vegetation - especially dried and brittle twigs, stems etc. etc. Anyone who has done any significant off trail bushwhacking knows what I am talking about. As a tall biped we humans are especially noisy, cumbersome, and just maladapted for stealthy stalking through dense brush. In fact if you are going through dense brush in a group one will often find it necessary to grab a branch or stem that might in fact "snapback" and hit the person following you!! Theropods - which share with us a relatively tall, bipedal build - would face a similar dilemma of "noise pollution" as they moved through dense brush. Having a rearward guard of feathers along the back of the leg and foot would allow vegetation to be "caught" and eased back as the leg stealthily moved through it.

That modern ground birds lack such hind wings or even feathers on the back of their legs may in fact have more to do with a lack of birds engaging in cryptic, stealthy stalking of small game in dense brush. An eco-morph today dominated by mammals, snakes, and lizards.

So here we can draw a rough outline for the impetus and evolution of hind wings - appreciating their import in stealth - that is in fact congruent with what is seen in the fossil record.

A Rough Outline on the Evolution of Hind Wings

I Sinocalliopteryx gigas shows what a theropod with only the barest, incipient stages of this evolution would have. Just simple and small trailing lines of non-pennaceous feathers that assisted in stealthy movement through thick brush while stalking small and agile prey. Coelurosaurs had bigger and more audacious plans in their future so the hind-wing evolution and flight never became more realized in this group.

II Pedopenna, Xiaotingia & Anchiornis. From something similar to Sinocallipoteryx ecologically (albeit dromaeosaurine & smaller) animals like Pedopenna & Anchiornis would arise. The hind wing feathers are now pennaceous although they are not yet flight adapted as they are symmetrical (on both arm & leg wings). However these are true small game hunters of the underbrush as such they capably explored a more three dimensional realization of their environment, certainly more so than the relatively giant Sinocalliopteryx.  Although not exquisitely adapted for arboreal life they certainly found themselves up in the trees on occasion. The occasional fall was not an issue as both the arm wings and hind wings would serve as safety nets.

Credit Jaime Headden CC3.0 http://qilong.deviantart.com/art/The-Essence-of-Huxley-160904197

Despite consistent assertions to the contrary Anchironis and other hind-winged microraptorines were excellent terrestrial runners. Arguments against efficient terrestrial running have centered on the lack of leg feathers on modern ground birds. However these arguments have not accounted for a scientific control in terms of behavioral ecology. Big herbivorous ground birds are simply not ecologically comparable to stealthy, small game paravian hunters of dense vegetation. When stealthy hunting in deep vegetation is at a premium a trailing edge of feathers along the back of the leg will often develop in small theropods.


The merit of a strong hypothesis, or theory, is that predictions can be made and then tested. One such prediction of hind wings being intimately linked to a behavioral ecology of predatory stealth is that such hind wings should not occur in paravians with diminished predatory behavior i.e. omnivory/frugivory/herbivory. So far the prediction I can make holds up in light of the fossil evidence that we have accrued. Hind wings are only found on predaceous paravians and are completely lacking in the groups suspected of more omnivorous/herbivorous inclinations such as caudipterids/enantiornithines/therizinosaurids/oviraptorids etc. etc. 

That we have a diverse and well sampled inventory of lifestyles in paravians and it is only the predatory ones that show up with hind wings is an observation that one should keep in their back pockets.


Is there a test for my argument that hind-wings & leg feathers are intimately associated with both stealth and a small game carnivorous lifestyle? Such a paravian would have to be showing a roughly comparable level of flight efficiency to microraptorines with one exception in that it was not a hunter of small game but an omnivore/insectivore/herbivore i.e. stealth was not a critical aspect of its lifestyle.

Turns out the fossil record does provide such a test and it's name Eosinopteryx brevipenna. Eosinopteryx  comes from the same time and place as Anchiornis and Brian Switek gave it an excellent summary article here. Personally I think Switek was very prescient in observing how the glut of feathered dino-bird things coming down the pike in last couple of decades has caused a bit of a "numbing" effect. We are not as excited about them as we once were. Switek maintains that specimens like Eosinopteryx might yet have special stories to tell. Eosinopteryx does indeed have a special story to tell I suggest. Eosinopteryx does not have hind wings and it shows a trend indicating decreasing predatory habits. Loss of predatory features includes the lack of a toe claw or even highly recurved foot  claws at all and very foreshortened snout with reduced dentition.


Now granted Eosinopteryx is not especially flighted, in fact the arms have been argued to be especially lacking in the mobility needed for flight. But that does not imply it could not have clambered into trees - as other contemporary paravians that did have hind wings likely did - and that it could not have used the arm-wings as a safety net for falls or gliding. Yet it lacked the hind wings of the contemporary but decidedly more predatory animals like Anchiornis. Anchiornis lacked asymmetrical flight feathers itself and, if flighted, was hardly a good pilot. The main deciding trait that separated Eosinopteryx from hind winged paravians I suggest is diminished predatory habits.

*Update 8/26 I had it pointed out to me via Matt Martyniuk that Sapeornis has some hind wing action going on as well as several other enantiornithines such as those from the new Crato formation: link
Although the hypothesis is not as robust as I constructed it do to these exceptions, it is still possible that a use in stealthy predation was still coopted into the ability to fly with better maneuverability with enhanced hind wings...

III Microraptor. From animals similar to Anchiornis Microraptor likely evolved. Microraptor kept the long hind legs and strong cursorial ability of these animals but improved upon their condition by improving utilization of 3-dimensional space. This revolution was chiefly executed via more efficient flighted ability and true asymmetrical flight feathers on the fore & hind wings. The hind wing feathers still allowed for stealth movement through thick vegetation but were now co opted for aerodynamic purposes.

Hind wings in Microraptor allowed two very important facets of predatory behavior. 1) They allowed for increased maneuverability in the air. 2) They allowed for a more stealth approach in the air.

Microraptor 4 winged flight credit David Krentz
Point 1 is strongly supported via the studies of Hall & Habib et al. (2012) A strong take home message of the work of Hall & Habib is the concept of a "drag tax" in Microraptor's flight. All of the surface areas that increased maneuverability in Microraptor incurred a consequence in terms of speed. As Microraptor negotiated a very cluttered and complex forest ecosystem such compromise was necessary.   And as I will argue further in a future post speed was not at the crux of Microraptor's predatory arsenal - maneuverability and stealth were its chief aims.

Point 2 is a new aspect I want to highlight that will be further embellished in my next post. The hind wing of  Microraptor - which I argue is an exaptation co opted by small dromaeosaurines (like Anchiornis) that enabled stealth stalking through thick vegetation - now in the flighted form allowed for an appreciable degree of sound muffling while the leg moved through air.

Swing any long cylindrical object through the air with speed - a bat, a twig, or a leg - and you will get an audible "whoosh" as the air rushes around the cylindrically shaped object and collides together on the rear side. But put a trailing edge of feathers on such an object you will appreciably diminish this said "whooshing" effect. Owls have taken this aspect of stealth approach to the extreme via the sound damping effects of their feathers. I am not suggesting that Microraptor had such capabilities as an owl but the concept remains true to point out. And any such advantage a predator can use, even it only incrementally increases its odds, is still a useful and tactical one.

To restate the angle I am taking - how to figure out how a small theropod bauplan most effiectively operated as a small game hunting animal - let me get back to brass tacks. Microraptor hunted small enantironithines - flighted "birds" that were well ahead of Micraptor in terms of flight adaptations. The superiority of enantiornithine flight speed was no hindrance to the success of Microraptor's predation on them. Because Microraptor was a predator and predators use every advantage that they can garner, Microraptor simply honored the time honored tradition of predators immemorial. It struck in the dark.

Why Microraptor was in fact a nocturnal stealth predator of the utmost capability will be addressed on my next post...

Night Hunting Microraptor credit Robin Liesens


papers

Godefroit, P., Demuynck, H., Dyke, G., Hu, D., Escuillie, F., Claeys, P. 2013.Reduced plumage and flight ability of a new Jurassic paravian theropod from ChinaNature Communications. 4, 1394. doi: 10.1038/ncomms2389

Hall, JT, Habib, MB, Hone, DWE, Chiappe, LM. Hindwing function in four winged feathered dinosaurs. (2012) online

Ji, S., Ji, Q., Lu J., and Yuan, C. (2007). "A new giant compsognathid dinosaur with long filamentous integuments from Lower Cretaceous of Northeastern China." Acta Geologica Sinica81(1): 8-15


Xing L, Bell PR, Persons WS IV, Ji S, Miyashita T, et al. (2012) Abdominal Contents from Two Large Early Cretaceous Compsognathids (Dinosauria: Theropoda) Demonstrate Feeding on Confuciusornithids and Dromaeosaurids. PLoS ONE 7(8): e44012.doi:10.1371/journal.pone.0044012



Xu, X. & Zhang, F. (2005). "A new maniraptoran dinosaur from China with long feathers on the metatarsus". Naturwissenschaften92 (4): 173–177. Bibcode:2005NW.....92..173Xdoi:10.1007/s00114-004-0604-yPMID 15685441.


"A Long habit of not thinking a thing wrong, gives it a superficial appearance of being right, and raises at first a formidable outcry in defense of custom". Thomas Paine

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